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3D Probe for Magnetic Imaging and Non-destructive
Fawaz Hadadeh
To cite this version:
Fawaz Hadadeh. 3D Probe for Magnetic Imaging and Non-destructive Testing. Instrumentation and
Detectors [physics.ins-det]. Université Paris-Saclay, 2018. English. �NNT : 2018SACLS421�. �tel-
02073586v2�
HAL Id: tel-02073586
https://tel.archives-ouvertes.fr/tel-02073586v2
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3D probes based on
magnetoresistive sensors for
NNT : 2018SACLS421
magnetic micro-imaging and
Thèse de doctorat de l'Université Paris-Saclay préparée au Service de
Physique de l'Etat Condensé, et au Département imagerie simulation
pour le contrôle, CEA
École doctorale n°564 Physique de l'Ile de France
Spécialité de doctorat : Physique
Thèse présentée et soutenue à Gif-sur-Yvette, le 14 Novembre 2018, par
Composition du Jury :
M. Philippe Lecoeur,
Laboratoire C2N, Marcoussis Président
Mme Claire Baraduc
CEA-Spintec, Grenoble Rapporteur
M. Jerome Gattacceca,
Technopole Environnement Arbois-Méditerranée,
Aix en PROVENCE Rapporteur
M. Hervé Tretout
Dassault Aviation, Argenteuil Examinateur
M. Nicolas Poulakis
Technological Educational Institution of West Macedonia Examinateur
M.Claude Fermon
CEA-SPEC, Gif-sur-yvette Directeur de thèse
Natalia Sergeeva-Chollet
CEA- LIST, Gif-sur-yvette Encadrante
I. RELATED PUBLICATIONS ................................................................................................................ V
II. LIST OF FIGURES ............................................................................................................................... VI
III. LIST OF ABBREVIATIONS........................................................................................................... XII
IV. INTRODUCTION ........................................................................................................................... XIV
PART 1: LITERATURE REVIEW.............................................................................................................................. 1
CHAPTER 1. MAGNETIC SENSORS ............................................................................................................... 2
1.1 INTRODUCTION ....................................................................................................................................... 2
1.2 MAGNETIC SENSORS ............................................................................................................................... 2
1.3 SENSITIVITY, DETECTIVITY AND NOISE. .................................................................................................. 3
1.3.1 Sensitivity .......................................................................................................................................... 3
1.3.2 Noise.................................................................................................................................................. 3
1.3.3 Detectivity. ........................................................................................................................................ 3
1.4 NOISE IN MAGNETIC SENSORS................................................................................................................. 4
1.5 INDUCTIVE MAGNETIC SENSORS ............................................................................................................. 6
1.6 FLUXGATES .......................................................................................................................................... 10
1.7 GIANT MAGNETO-IMPEDANCES ............................................................................................................ 12
1.8 HALL EFFECT SENSOR .......................................................................................................................... 14
1.9 MAGNETORESISTIVE SENSORS .............................................................................................................. 15
1.9.1 Anisotropic Magnetoresistance (AMR) ........................................................................................... 16
1.9.2 Giant magnetoresistances (GMR) ................................................................................................... 17
1.9.2.1 GMR with spin valve structure ............................................................................................................... 19
1.9.2.2 GMR effect ............................................................................................................................................. 20
1.9.2.3 GMR sensitivity ..................................................................................................................................... 21
1.9.3 Tunnel magnetoresistance (TMR) ................................................................................................... 23
1.10 CONCLUSION ........................................................................................................................................ 24
CHAPTER 2. NON-DESTRUCTIVE TESTING ................................................................................................. 29
2.1 INTRODUCTION ..................................................................................................................................... 29
2.2 NON-DESTRUCTIVE TESTING ................................................................................................................ 29
2.3 OPTICAL METHODS ............................................................................................................................... 30
2.4 PENETRANT TESTING ........................................................................................................................... 31
2.5 MAGNETIC PARTICLE INSPECTION (MPI) .............................................................................................. 31
2.6 ULTRASONIC TESTING .......................................................................................................................... 32
2.7 RADIOGRAPHY ..................................................................................................................................... 33
2.8 MAGNETIC FLUX LEAKAGE ................................................................................................................... 34
2.9 EDDY CURRENT TESTING ...................................................................................................................... 34
CHAPTER 3. MAGNETIC IMAGING ............................................................................................................. 41
3.2 MICROSCOPE FOR MAGNETIC IMAGING................................................................................................. 41
3.3 TRANSMISSION ELECTRON MICROSCOPES (TEM) ................................................................................. 42
3.4 LOCAL PROBE MICROSCOPY ................................................................................................................. 43
3.4.1 Scanning Tunneling Microscopy (STM) .......................................................................................... 44
3.4.2 Magnetic force microscopy (MFM)................................................................................................. 45
3.4.3 Scanning magnetoresistance microscopy (SMRM) ......................................................................... 47
3.4.4 Nitrogen Vacancy (NV) centers ....................................................................................................... 47
3.5 OPTICAL MICROSCOPE ...................................................................................................................... 48
3.5.1 Magneto-optical devices based on garnets films ............................................................................. 49
3.6 CONCLUSION ....................................................................................................................................... 51
PART 2: 3D PROBE AND APPLICATIONS FIELDS .................................................................................................. 52
CHAPTER 4. 3D PROBE .............................................................................................................................. 53
4.2 ELECTRONIC CIRCUIT ........................................................................................................................... 54
4.3 EXPERIMENTAL SET-UP ........................................................................................................................ 56
4.3.1 Experimental set-up for DC signal .................................................................................................. 56
4.3.2 Experimental set-up for AC signal .................................................................................................. 57
4.4 DESCRIPTION OF 3D PROBE FORMED BY FOUR GMR ............................................................................ 57
4.5 GMR SENSORS USED FOR 3D PROBE DEVELOPMENT ............................................................................ 60
4.5.1 Type 1 GMR sensors ....................................................................................................................... 61
4.6 CONCLUSION ........................................................................................................................................ 79
CHAPTER 5. 3D PROBE IN MAGNETIC IMAGING........................................................................................ 81
5.2 SET-UPS ................................................................................................................................................ 81
5.3 STUDY OF CURRENT LINE ..................................................................................................................... 83
5.4 MAGNETIC IMAGING OF DOLLAR BIN .................................................................................................... 86
5.5 IMAGING OF SOFT STEEL OBJECTS ......................................................................................................... 89
5.6 SUSCEPTIBILITY MEASUREMENT .......................................................................................................... 92
CHAPTER 6. 3D PROBE IN NON-DESTRUCTIVE TESTING ............................................................................ 95
6.2 OPTIMIZATION OF EC PROBE ................................................................................................................ 95
6.3 3D EC PROBE ....................................................................................................................................... 99
6.3.1 Simulation results with 3D EC probe .............................................................................................. 99
6.3.2 Experimental Results ..................................................................................................................... 101
6.3.2.1 Subsurface defects ................................................................................................................................ 101
6.3.2.2 Surface defects ..................................................................................................................................... 105
6.4 CONCLUSION ...................................................................................................................................... 109
CONCLUSION AND FUTURE WORKS .................................................................................................. 112
REFERENCES.................................................................................................................................................. 114
I. Related Publications
1. Article:
 F. Hadadeh, A.Solignac, N. Sergeeva-Chollet, M. Pannetier-Lecoeur et C.
Fermon "3D magnetic imaging with GMR sensors". (submitted).
2. Conferences:
 F. Hadadeh, N. Sergeeva-Chollet, E. Paul, M.Pannetier-Lecoeur et C. Fermon
" GMR probes for Magnetic Micro-Imaging and Non-Destructive Evaluation
", Joint European Magnetic Symposium, August 2016, Scottish Exhibition
and Conference Centre (SECC), Glasgow, UK
 F. Hadadeh, N. Sergeeva-Chollet, E. Paul, M. Pannetier-Lecoeur et C.
Fermon "Développement de la sonde CF à base des capteurs GMR, pour la
détection des trois composantes du champ", COFREND, Mai 2017,
Strasbourg France
Fermon "Probe based on magnetoresistive Sensors for magnetic micro-
imaging and NDT", Workshop on Electromagnetic Non Destructive
Evaluation, Septembre 2017, Saclay France
Fermon "La détection des trois composantes du champ par une sonde CF à
base des capteurs GMR", Colloque Louis Néel, Septembre 2017, Paris
 F. Hadadeh, B. Esteves, N. Sergeeva-Chollet, A. Skarlatos, A. Solignac, M.
Pannetier-Lecoeur et C. Fermon "Local magnetic microscopy of
ferromagnetic steels based on magnetoresistive sensors", 12th European
Magnetic Sensors and Actuator, July 2018, Athens Greece
3. Patent
 A. Solignac, N. Sergeeva-Chollet, F. Hadadeh, M. Pannetier-Lecoeur,
C. Fermon "Dispositif et methode de cartographie de la susceptibilité
magnétique de surface de matériaux. " (In preparation)
II. List of Figures
Figure 1.1- Measurement diagram of magnetic field .............................................................................. 3
Figure 1.2-GMR characteristics as a function of the magnetic induction (left part), noise in a yoke-
type GMR (right part) of 5µm width ..................................................................................... 6
Figure 1.3-Equivalent circuit of induction sensor with load capacity 𝐶0 and load resistance 𝑅0. ........ 7
Figure 1.4-Square Coil............................................................................................................................ 8
Figure 1.5- Cylindrical Coil .................................................................................................................... 9
Figure 1.6-Principle diagram of fluxgate ............................................................................................. 10
Figure 1.7- Principle of pulse shift method ........................................................................................... 11
Figure 1.8- GMI sensor made of ferromagnetic wire crossed by alternating current and subjected to
an external field 𝐻𝑒𝑥 ........................................................................................................... 12
Figure 1.9- GMI sensor marketed by AICHI Company [37]................................................................. 13
Figure 1.10- Principle of Hall effect ..................................................................................................... 14
Figure 1.11- Photograph of the linearly integrated Hall sensors (LIHaS): 64 InSb Hall sensors [51] .
............................................................................................................................................. 15
Figure 1.12-Principe diagram of AMR sensor ...................................................................................... 16
Figure 1.13- The two discoverers of GMR ............................................................................................ 18
Figure 1.14- Left: GMR scanning element for magnetic field mapping. SEM picture of a 450µm
meteorite collected in Antarctica, Right: Magnetic mapping of the meteorite magnetic
dipole. Coll. Cerege – Jérôme Gattacceca [66]. ................................................................. 19
Figure 1.15- Example of spin valve GMR [73] ..................................................................................... 19
Figure 1.16- GMR structure: (a) Free layer, spacer and pinned layer Illustration of the spin-
dependent scattering in ferromagnetic materials (FM). The spacer is non-magnetic (NM);
(b) The equivalent electrical circuit with two conducting channels is shown for anti-
parallel magnetizations (red) and for parallel magnetizations (blue); (c) The figures at the
top illustrate the trajectory of the electrons in antiparallel configuration and illustrate at
the bottom the parallel configuration . ................................................................................ 20
Figure 1.17- Measurement scheme of GMR sensitivity. ........................................................................ 21
Figure 1.18- Example of GMR response as function of magnetic field applied along its sensitivity axis
Figure 1.19-Typical stack of TMR sensor on thickness of Fe diffusion barrier for double MgO-based
p-MTJ spin-valves with top 𝐶𝑜2𝐹𝑒6𝐵2 free layer [79]. ..................................................... 23
Figure 1.20- Comparison of the detectivity of magnetic sensors : coil, fluxgate, Hall, GMI, AMR,
GMR and TMR sensors ....................................................................................................... 26
Figure 2.1- Non-destructive testing in the aerospace industry [91]...................................................... 29
Figure 2.2- Optical instrument for surface deformations [93].............................................................. 30
Figure 2.3- The four steps of PT technique ........................................................................................... 31
Figure 2.4- Shema of Ultrasound Principe [35] ................................................................................... 32
Figure 2.5-Example of uses of Radiography method [98]..................................................................... 34
Figure 2.6- Principle of eddy current generation [35].......................................................................... 35
Figure 2.7- Evolution of current density as function of piece depth (z) in the case in the case of plane
wave excitation [103]. ......................................................................................................... 36
Figure 2.8- Evolution of skin depth against frequency for different materials ..................................... 37
Figure 2.9- Photo of the IMAGIC MR probe for buried flaws detection [106]..................................... 38
Figure 3.1- In situ TEM study of magnetization reversal in Co80Ir20 film: Fresnel images in a
hysteresis sequence on the hard axis in plane of the film [125]. ......................................... 43
Figure 3.2- Schematic view of an STM (left part) Schematic illustration of the SP-STM method [126]
(right part)........................................................................................................................... 45
Figure 3.3- Atomic force microscopy principle (a) Tip form (b) Long- and short-range (interatomic)
forces dependent of various distance (c) [130, 132]............................................................ 46
Figure 3.4- MFM images of Co sample with wire width 150 nm with gold electrodes on top, with I−
and I+ indicating the direction of current flow [137] ......................................................... 46
Figure 3.5- Example of NV-center Nanodiamond used for thermos dynamic application [151].......... 48
Figure 3.6- Schematic drawing of the Faraday effect. The sample is illuminated by linearly polarized
light. Polarizer and analyser are set in crossed position. The light enters the magneto-
optically active layer (MOL) and is reflected at the mirror layer. In areas where no flux is
present in the sample, no Faraday rotation takes place hence these regions stay dark in the
image. In all areas where flux is present, the Faraday rotation changes the polarization
plane so that this light is able to pass through the analyser thus leading to bright areas. In
the drawing, a reflection angle #O is shown lor clarity whereas in the experiment we have
perpendicular incident light. [153]...................................................................................... 49
Figure 3.7- Magneto-optical images used to calculate distribution of electric field at 4.2 K in the
superconductor. The image shows such distributions for a YBCO film with a grain
boundary [160] .................................................................................................................... 50
Figure 4.1-Electronic circuit ................................................................................................................. 54
Figure 4.2 Alimentation part of electronic circuit ................................................................................ 54
Figure 4.3- Full bridge circuit relied to INA103 .................................................................................. 55
Figure 4.4- 3D probe with electronic circuits ....................................................................................... 56
Figure 4.5- Experimental set-up for DC signal..................................................................................... 56
Figure 4.6- Experimental set-up for AC signal. .................................................................................... 57
Figure 4.7- Description of the probe (left part), Right: GMR sensors (right part) .............................. 58
Figure 4.8- GMR sensors position in three axis.................................................................................... 58
Figure 4.9- Distribution design of GMR sensors according to x-z plane (left part) and to y-z plane
Figure 4.10- GMR sensor in a yoke-shape before cutting .................................................................... 60
Figure 4.11- Distribution of magnetic moments in free layer with a yoke shape structure. ................. 61
Figure 4.12- Spin valve stack of type1 GMR sensors. The thicknesses (X) are given in nm ................. 61
Figure 4.13 – The response of four type 1 GMR sensors as function of magnetic field applied along
their z-axis ........................................................................................................................... 63
Figure 4.14- The response of four type 1 GMR sensors as function of magnetic field applied along
their y-axis. .......................................................................................................................... 64
Figure 4.15- The response of four type 1 GMR sensors as function of magnetic field applied along its
x-axis. .................................................................................................................................. 65
Figure 4.16- Voltage noise of four type 1 sensors with 𝑉𝐺𝑀𝑅= 0 mV, 100 mV, 300 mV and 1V ........ 67
Figure 4.17-Noise spectral density in V ⁄ √Hz of four type 1 sensors with 𝑉𝐺𝑀𝑅= 0 mV, 100 mV, 300
mV and 1V. .......................................................................................................................... 68
Figure 4.18-Field equivalent noise in T⁄√Hz of fourtype 1 sensors according to z-axis with 𝑉𝐺𝑀𝑅=
100 mV................................................................................................................................. 69
Figure 4.19- Spin valve stack of type 2 GMR sensors. The thicknesses (X) are given in nm ................ 70
Figure 4.20 – The response of four type 2 GMR sensors as function of magnetic field applied along its
z-axis.................................................................................................................................... 72
y-axis ................................................................................................................................... 73
x-axis ................................................................................................................................... 74
Figure 4.23- Low frequency noise in µV of four type 2 sensors with 𝑉𝐺𝑀𝑅= 0 mV, 100 mV, 300 mV
and 1V ................................................................................................................................. 76
Figure 4.24- Noise spectral density in V⁄√Hz of four type 2 sensors with 𝑉𝐺𝑀𝑅= 0 mV, 100 mV, 300
mV and 1V ........................................................................................................................... 77
Figure 4.25- Noise spectral density in T⁄√Hz of four type 2 sensors according to z-axis with 𝑉𝐺𝑀𝑅=
Figure 5.1- Experimental set-up at CEA-LIST ...................................................................................... 82
Figure 5.2- Experimental set-up at CEA-SPEC .................................................................................... 82
Figure 5.3- Measurement scheme of current line ................................................................................. 83
Figure 5.4- One dimensional (top) and two dimensional scans (bottom) of the magnetic field of a line
current according to x-axis; simulation results of Hx. component (left part) reconstruction
of Hx component measured by 3D probe. The maximum amplitude is equal to 1.2 μT with
SNR of 8.2 dB (right part) ................................................................................................... 84
current according to x-axis; simulation results of Hy component (left part) reconstruction
of Hy component measured by 3D probe.. The amplitude of Hy is almost equal to zero
current according to x-axis; simulation results of Hz component (left part) reconstruction
of Hz component measured by 3D probe.. The maximum amplitude is equal to 1.3 μT with
Figure 5.7- Measurement results of George Washington's face in the case of Hz component detection
obtained by Kletetschka [172] (left part). Picture of part of Washington's face in one dollar
bill (center part) and measurement results obtained by developed 3D probe (right part) . 87
Figure 5.8- Picture of Washington's face in one dollar bill (a) Magnetic image of a portion of a $1.00
bill scanned at a working distance of height of 400 µm, Hz component.. The gray scale
varies from about −300 µT(downward, black) to +310 µT(upward, white) (b) Magnetic
image of Hx component. The gray scale varies from about −80 µT to +85 µT (c) Magnetic
image of Hy component. The gray scale varies from about −65 µT to +60 µT in the case of
Hy (d)................................................................................................................................... 88
Figure 5.9- Steel mock-up with different shape objects ........................................................................ 89
Figure 5.10- Steel objet that contains many shapes .............................................................................. 90
Figure 5.11- Hz component, the grey scale varies from −100 µT (downward, black) to +80 µT
(upward, white) ................................................................................................................... 91
Figure 5.12-Hx and Hy components obtained by 3D probe. The gray scale varies from about −25 µT
(downward, black) to +40 µT (upward, white) in the case of Hx (left part) the gray scale
varies from about −35 µT to +40 µT in the case of Hy (right part) ................................... 92
Figure 5.13- The grid composed of magnet strips. ...................................... Erreur ! Signet non défini.
Figure 5.14- Simulation results (left part) and experimental results (right part) in the case of Hx
component analysis .................................................................... Erreur ! Signet non défini.
Figure 5.15- Simulation results (left part) and experimental results (right part) in the case of Hy
component analysis. The maximum amplitude is equal to1.6 μT Erreur ! Signet non défini.
Figure 5.16- Simulation results (left part) and experimental results (right part) in the case of Hz
component analysis. The maximum amplitude is equal to 1.2 μTErreur ! Signet non
défini.
Figure 5.17- Experimental set-up for susceptibility measurement............... Erreur ! Signet non défini.
Figure 5.18- The three components of magnetic field in the case of susceptibility measurement. The
maximum amplitude values are equal to Hx = 1.5 μT, Hy = 1 μT, Hz = 0.8 μT...... Erreur !
Signet non défini.
Figure 6.1-3D Probe based on a GMR array receiver and two current foils emitters ........................... 96
Figure 6.2- The parameters of two current foils ................................................................................... 96
Figure 6.3- Schematic of opposite excitation in two current foils emitters ........................................... 97
Figure 6.4- Image of 1D z component distribution of magnetic induction reflected by aluminum mock-
up for the two forms of excitations: phase and opposition phase ....................................... 98
Figure 6.5- Distribution of z-component of magnetic induction reflected by aluminum mock-up at
different distance between the two current foils with the applied current of 100 mA (left
part), 1D z-zone distribution at fixed x and y positions (GMR position) (right part) ......... 98
Figure 6.6- Configuration realized in CIVA for 3D probe.................................................................... 99
Figure 6.7- CSCAN of the defect in aluminum mock-uobtained with receiver places according to x-
axis (left part), reconstruction of Hx component obtained with 3D EC probe (right part).
........................................................................................................................................... 100
Figure 6.8- CSCAN of the defect in aluminum mock-up obtained with receiver places according to y-
axis (left part), reconstruction of Hy component of magnetic field with 3D EC probe (right
part). .................................................................................................................................. 100
Figure 6.9- CSCAN of the defect in aluminum mock-up obtained with receiver places according to z-
axis (left part), reconstruction of Hz component of magnetic field with 3D EC probe (right
part) ................................................................................................................................... 101
Figure 6.10- Experimental set-up for EC measurements (left part) 3D probe installed (right part). 102
component measurement ................................................................................................... 102
Figure 6.13- Simulation results (left part), and experimental results (right part) in the case of Hz
component measurement. .................................................................................................. 103
Figure 6.15- Simulation result's (left part), and experimental results (right part) in the case of Hy
Figure 6.17- Aluminum mock-up......................................................................................................... 106
measurement...................................................................................................................... 109
III. List of abbreviations
ACFM Alternating current field measurement
AMR Anisotropic magnetoresistance
AFM Atomic Force Microscopy
GMI Giant magnetoimpedance
GMR Giant magnetoresistance
EC NDT Eddy current non-destructive testing
ECT Eddy current testing
EC Eddy current
FM Ferromagnetic materials
HNDT Holographic nondestructive testing
LPF Low-pass filter
MI Magneto Impedance
MPI Magnetic particle inspection
MFM Magnetic Force Microscopy
MO Magneto-optical
MR Magnetoresistance
NDT Non-destructive testing
NM Non-magnetic
NMR Nuclear Magnetic Resonance
PT Penetrant Testing
PEC NDT Pulsed eddy current non-destructive testing
RTN Random telegraph noise
SQUID Superconducting Quantum Interference Device
SMRM Scanning magnetoresistance microscope
SNOM Scanning Near-Field Optical Microscope
SNR Signal-to-noise ratio
STM Scanning Tunneling Microscope
TEM Transmission Electron Microscope
TMR Tunnel MagnetoResistance
UT Ultrasonic testing
IV. Introduction
The work presented in this thesis focuses on the design of probe allowing simultaneously
reconstruction of three components of magnetic field at the same measurement point (3D
probe), for non-destructive testing (NDT) and magnetic imaging applications.
NDT is used in order to evaluate the properties of a wide variety of materials without causing
damage. Some of the common non-destructive techniques are eddy current, ultrasonic, optics,
radiography and liquid penetrant testing. In NDT applications, the eddy current technique
(ECT) is a widely technique used to detect and identify surface and sub-surface defects in
conductive materials such as aluminum, inconel or steel. The principle of the eddy current
technique is based on the interaction between magnetic field emitter to induce eddy currents
in a controlled piece and sensors to detect the magnetic field created by these eddy currents.
The sensors can detect the presence of defects in the controlled piece by monitoring changes
in the emitted field.
Magnetic imaging consists on measuring the magnetic stray field emitted by a magnetic
object. At micron scales, it allows to reconstruct the magnetic inhomogeneitites of the object,
which can be related to structural shape, material inhomogeneities or domain formation.
Magnetic imaging is also used to determine current flows on a material surface. DC
measurement gives the static configuration while AC measurement allows the determination
of either AC current flow, either local susceptibilities. Above a large use in condensed matter
physics, magnetic imaging is used in microelectronics for circuit failure determination, in
magnetic industry (storage and magnetic sensors) and in geophysics to determine properties
of rocks for example.
The probes developed in my PhD work are based on spin electronics sensors, which are based
on the use of the spin of electrons in conductive layer to manipulate their transport properties.
The giant magnetoresistance effect (GMR) discovered in 1988 allowed the fabrication of high
sensitivity and micron size magnetic sensors. The resistance of the GMR changes with the
application of external magnetic field. GMR with spin valves shape are a particular case of
multilayered structure where the resistance is varying as the angle of the applied external field
and an internal reference direction. GMR sensors have been first implemented in read heads
for hard disk drives and more recently in a wide variety of products for automotive, energy
monitoring or health applications. For NDT applications, GMR sensors overcome the limits
related to the fabrication of very small coils, while bringing a very good detectivity. The size
of GMRs allows also the realization of 3D probes.
Achievement of the Work
The main achievements of the research work can be summarized as follows:
 Literature survey that brought understanding and familiarity with magnetic sensors,
NDT techniques, and different types of microscopes for magnetic imaging.
 Design and construction of 3D probes based on four GMR sensors allowing
simultaneously reconstruction of three components of magnetic field at the same
measurement point, which includes hardware (mechanical support, electronic circuit,
emitter part for NDT application…) and software (code to reconstruct the three
components of magnetic field by the signals received from the four GMR sensors,
noise elimination…).
 Different types of GMR sensors have been developed for different types of
applications compromising detectivity and magnetic field range. Magnetic imaging of
model systems: current line, one-dollar bill, structured metallic object.
 Implementation of a magnetic imaging of susceptibility.
 Probe configuration and emitter dimensions for NDT have been optimized by
simulations in CIVA software.
 Detection of μm-sized and mm-sized defects in Aluminum, Inconel and Titanium
samples and the reconstruction of three magnetic components have been achieved.
Thesis Layout
Chapter 1 presents a non-exhaustive bibliographic collection of magnetic sensors, usable for
magnetic imaging and for NDT. This chapter highlights a summary of the main properties of
magnetic sensors, and details principle of the GMR (chosen for the development of the 3D
probe).
Chapter 2 reports a state of the art of different NDT methods : their application and their
performance. Review on the use of Eddy current testing and the key features of this method
for surface and sub-surface flaws inspection are also reported.
Chapter 3 describes non-exhaustive state of the art concerning microscopes using for
magnetic imaging. We will look at the advantages of the 3D probe based on GMR sensors for
micro-imaging application and compare them to other magnetic imaging microscopes.
Chapter 4 reports the design of the new 3D probe based on GMR sensors, which is comprised
of electronics and hardware. Two kinds of GMR used for the developing of the 3D probe have
been described: The type 1 GMR sensor has sensitivity of 19 V/V/T and has a maximal
working field up to ±15 Oe. The second type of GMR sensor has a larger working field (±200
Oe) than the type one sensor at the detriment of its sensitivity (3.5 V/V/T.) The characteristics
of these GMR sensors (stack, sensitivity, noise measurement and detectivity) are also
Chapter 5 introduces the experiments on the performance of the 3D probe for magnetic
imaging. Detection of three components of the field generated by a current line and one-dollar
bill are presented. Magnetic imaging of magnetic objects with arbitrary shapes in soft steel
object are discussed. Also we will show the performance of 3D probes for spacing between
close magnetic features and for susceptibility measurement.
Chapter 6 introduces the experiments on the performance of the 3D probe for NDT. Probe
configuration and emitter dimensions optimized by simulations have been discussed.
Detection of three components of magnetic field response caused by sub-surface and surface
defects (μm-sized) in Aluminum, Inconel and Titanium mock-ups are presented. Comparison
of the experimental results and simulation results are also presented.
Conclusions drawn from the research work and sets out recommendations for further work are
given at the end.
Part 1: Literature Review
Chapter 1. Magnetic sensors
The sensor in general is a device that transforms a physical quantity into a usable
quantity like current or voltage. Magnetic sensors described in this chapter are
measuring a magnetic field or a magnetic flux and produce a voltage output
proportional to that measured quantity. There are different types of magnetic sensors,
like inductive magnetic sensors (coils) [1], fluxgates [2], Hall devices [3], Giant
Magnetoimpedance (GMI) [4], anisotropic magnetoresistance (AMR) [5], giant
magnetoresistance (GMR) [6], tunnel magnetoresistance (TMR) [7], each one useful
for specific applications. This chapter presents first a non-exhaustive bibliographic
collection of these magnetic sensors, usable for magnetic imaging and for non-
destructive testing. There are other magnetic sensors with good performances in terms
of minimum detectable field and spatial resolution, such as SQUID (Superconducting
Quantum Interference Device) sensors [8] not presented here due to their conditions of
use with cryogenic cooling and their expensive cost which make them difficult to
operate in Non-destructive testing context. We will detail GMR sensors and their
characteristics more than the other sensors, because they are the base of developed
probes.
1.2 Magnetic sensors
A magnetic sensor is in general a part of measurement chain that transforms the
magnetic field detected into a usable quantity typically a voltage (Figure 1.1) [9]. The
main interest of the magnetic field is to freely propagate in space which allows non-
contact detection. For that reason, magnetic sensors are used in a wide range of
applications like distance, speed, current measurements, metal characterization or non-
destructive testing (NDT).
Figure 1.1- Measurement diagram of magnetic field
1.3 Sensitivity, detectivity and noise.
Here, we introduce some definitions extensively used in the document.
1.3.1 Sensitivity
The first one is sensitivity. Sensitivity is the output voltage obtained for a given field
applied on a sensor. That sensitivity is given in Volt(V)/Tesla(T) but depending of the
sensor technology other quantities can be used.
For example, magnetoresistances or Hall effect sensors where a resistance is varying
with the field, the output voltage is proportional to the bias voltage applied to the
sensor. Then the sensitivity is given in V/V/T.
As we will see later, inductive sensors have a sensitivity, given in V/T and it depends
on the frequency. Hence, sensitivity will be given at a specific frequency.
1.3.2 Noise.
The noise of sensors is as important as the sensitivity as performances will be given by
the signal to noise available. Noise is nearly always given in 𝑉/√𝐻𝑧 at a given
frequency. Sources of noise will be described later.
1.3.3 Detectivity.
The detectivity or field equivalent noise is the most suitable quantity to compare
different magnetic sensors. It is the field for which the signal to noise ratio is one.
Hence, it is equal to the noise in 𝑉/√𝐻𝑧 divided by the sensitivity. It is given in
𝑇/√𝐻𝑧. Detectivity is usually dependent on the frequency but also on the voltage
biasing, size etc... of each sensor.
1.4 Noise in magnetic sensors
The noise of magnetic sensors can be classified into two categories: white noise
independent of frequency (thermal noise and shot noise), and noise dependent of the
frequency such as: 1/f noise or random telegraphic noise (RTN). Also electronic and
instrumental noises influence on the level of magnetic sensors detection. The entire
measurement chain (including amplification and electronics circuit), gives noise signal
at their output, which limits the level value of useful signal measured. We introduce
noise directly related to magnetic sensors described in this chapter.
The first form of white noise is the thermal noise or Nyquist noise. Any resistance in
non-zero temperature is a source of electrical noise. The velocity of electrons is zero in
the absence of electromotive force. However, the resistance fluctuations is non-zero
due to Brownian motion. At a fixed temperature 𝑇, the voltage spectral density of
1 2⁄
thermal noise 𝑆𝑉,𝑡ℎ is given by the Nyquist formula (equation (1-1)), where 𝑘𝐵 is
Boltzmann constant, 𝑅 is the magnetic sensor resistance, and 𝑇 is the temperature [10]:
𝑆𝑉,𝑡ℎ = √4. 𝑘𝐵. 𝑇. 𝑅. (1-1)
The second form of white noise is the shot noise. It is an electric noise that can be
modeled by Poisson law. Shot noise reflects the discreet nature of carriers charge
(fluctuation of some number of carriers). Under the action of electric field, an electric
current is generated by the individual transport of charge carriers. This noise, unlike
the thermal noise, is directly related to electric current I and to the charge of carriers.
1⁄2
Its spectral noise density 𝑆𝑉,𝑆ℎ𝑜𝑡 is given in Volt, and can be calculated by the
following equation:
𝑆𝑉,𝑆ℎ𝑜𝑡 = √2. 𝑒. 𝐼 .̅ 𝑅 (1-2)
where 𝑒 is the charge of the electron and the coefficient 2 is coming from the fact that
we reason only in positive frequencies. This noise is present only in tunnel junctions,
and thus in TMR magnetic sensors which will be described later in this chapter.
1/f noise
The origin of 1/f noise is still a subject of discussion. This noise appears at low
frequencies and decreases roughly as 1/f. 1/f noise is observed in nearly all existing
phenomena ranging from earth quakes to brain activity but also in a simple resistance
wire. In metallic conductors this noise is related to a resistance noise and hence it is
revealed when a current is sent through the conductor. For that reason, it is particularly
important for magnetoresistances where the resistance is varying with magnetic field
Thus the 1/f noise could have an electrical and/or a magnetic origin in GMR sensors.
The spectral density of 1/f noise observed in various components is inversely
proportional to frequency (1/f γ ) with γ is about one. In 1969, F. N. Hooge proposed
phenomenological relationship through the collection of large number of experimental
results on semiconductors and homogeneous metals [11]:
𝛼𝐻 𝑉 2
𝑆𝑉,1⁄𝑓 (𝑓) = . (1-3)
𝑁𝑐 𝑓 𝛾
where 𝑉 is a potential difference that across the conductor, 𝑁𝑐 is the total number of
charge carriers in the material. The Hooge constant 𝛼𝐻 is non-dimensional parameter
that depends on the purity and on the number of defects present in the material [12].
The Hooge constant 𝛼𝐻 /𝑁𝑐 allows comparison of noise level between sensors. The 1/f
noise dominates the white noise (typically in GMR and TMR) below cut-off or
overlap frequency 𝑓𝑐 . The noise due to magnetic domain is very high in small
magnetic GMR sensors and is strongly related to structural properties and magnetic
configuration of the GMR. As this noise is inversely proportional to the number of
carriers, to increase the volume of the sensor reduces the 1/f noise. This can be done
by lengthening the yoke (will be presented in paragraph 4.5) or connecting several
GMR in series or in parallel.
Random Telegraphic Noise (RTN):
Random telegraph noise (RTN) is one of important dynamic variation sources in
digital circuits. RTN occurs due to the random trapping/detrapping fluctuations, it is
important to study the RTN, since it is often due to random fluctuations between the
metastable states of the magnetic domains of the free layer. This phenomenon
inducing undesirable variations in resistance is strongly dependent on working
conditions of the device, and on polarization current. The spectral density of RTN is
given in Eq 1-5
𝑆𝑉,𝑅𝑇𝑁 (0)
𝑆𝑉,𝑅𝑇𝑁 (𝑓) =
∆𝐸 2 ( ∆𝐸 ) + (2𝜋𝑓𝜏)2 ]
cosh( )
𝑘𝐵 𝑇 [𝑐𝑜𝑠ℎ 𝑘𝐵 𝑇
𝐸
Where 𝜏 −1 = ∑2𝑖=1 𝜏𝑖 −1 with 𝜏𝑖 = 𝜏𝑖,0 exp (𝑘 𝑖𝑇) , 𝐸𝑖 corresponds to energy level of
𝐵
state 'i'.
In magnetoresistive sensors like GMR and TMRs, RTN noise is mainly due to
magnetic domain fluctuations. It has been demonstrated that by a shape of the sensor
or by magnetic stabilization of the magnetic layers, it can be reduced or even
eliminated [13].
In Figure 1.2 results obtained by Trauchessec et al. [14] on a GMR sensor are shown.
The square root of the noise power spectral density (right part) exhibits two
components: thermal noise and the low-frequency noise; the thermal noise becomes
dominant above 1 kHz. The detectivity in the thermal noise regime of this sensor is
0.1 𝑛𝑇/√𝐻𝑧.
Figure 1.2-GMR characteristics as a function of the magnetic induction (left part), noise in a yoke-type GMR
(right part) of 5µm width
1.5 Inductive magnetic sensors
Inductive sensors are one of the oldest and most well-known type of magnetic sensors
[15]. Inductive sensors are coils with different geometries, flat or solenoid. Generally,
conventional coils are fabricated by winding a copper wire around a core.
Thanks to Lenz-Faraday induction phenomenon (Equation (1-5)), the magnetic field
variation into the inductive sensor creates a voltage. The transfer function of induced
voltage measured across the coil is given by V = f (B). This transfer function results
from the fundamental Faraday's law of induction:
𝑑Φ 𝑑𝐵 𝑑𝐻
𝑉 = −𝑛 . = −𝑛. 𝐴. = −μ0 . 𝑛. 𝐴. (1-5)
𝑑𝑡 𝑑𝑡 𝑑𝑡
where Φ is magnetic flux passing through a coil with an area A and a number of turns
n. The output signal of a coil, V, depends on the rate of change of flux density 𝑑𝐵/𝑑𝑡.
The sensitivity S of the coil is the ratio of amplitude V to the magnetic field B,
according to the Lenz-Faraday law, at the frequency f. It could be written as:
𝑉
𝑆 = | | = 𝑛. 2. 𝜋. 𝑓. 𝐴 (1-6)
This equation shows that the sensitivity of inductive magnetic sensor increases when
frequency, dimensions or number of turns increases. However, the sensitivity is
limited by the resonance frequency 𝑓0 (equation (1-7)). If the working frequency 𝑓 is
higher than the resonance frequency of the inductive sensor (𝑓 > 𝑓0 ), the sensitivity
starts to decrease. The equivalent circuit of inductive magnetic sensor is described in
Figure 1.3-Equivalent circuit of induction sensor with load capacity 𝐶0 and load
resistance 𝑅0.
This resonance frequency depends on the internal resistance R, inductance L and self-
capacitance C of coil sensor:
f0 = (1-7)
2. 𝜋. √𝐿. 𝐶
At frequency above the resonance frequency, the operation of inductive magnetic
sensor is dominated by the parasitic capacitance, and it becomes equivalent to a
capacitor. Thus, the inductive magnetic sensor will become unusable for NDT and
magnetic imaging applications. Above the resonance frequency, with 𝛼 = 𝑅⁄𝑅0 , 𝛽 =
𝑅. √𝐶 ⁄𝐿 , 𝑎𝑛𝑑 𝛾 = 𝑓⁄𝑓0 = 2. 𝜋. 𝑓. √𝐿. 𝐶 the sensitivity of inductive coil can be also
expressed by the following expression [16]:
𝑛. 2. 𝜋. 𝐴
𝑆=
𝛼2 (1-8)
√(1 + 𝛼)2 + (𝛽 2 + − 2) × 𝛾 2 + 𝛾 4
𝛽2
The inductive sensors could contain a ferromagnetic core in order to concentrate the
magnetic field and to increase the sensitivity [17].
According to equation (1-6) the sensitivity can be increased by increasing the coil
surface, but the counter part for NDT is the decrease of the spatial resolution. The
minimum magnetic field detectable by the coil is also limited by the intrinsic noise of
coil and by the noise of associated electronics. In addition, the intrinsic thermal noise
of the coil increases with its resistance, thus, the intrinsic thermal noise increases with
its dimensions and number of turns.
The performance of inductive sensors can be calculated analytically in case of coils
without ferrite core. Korepanov details it in [18].
Here we will take two examples, which will allow us to compare GMR/TMR sensors
and inductive coils performances for non-destructive evaluation.
The first example is a square coil that has the following characteristics: resistance of
3.5 Ohms, 8 layers, each layer has 9 mm2 of effective surface, and one turn (Figure
1.4).
Figure 1.4-Square Coil
According to the Nyquist formula, its spectral density of white noise 𝑆𝑉,𝑡ℎ is 0.24 𝑛𝑉/
√𝐻𝑧 . Equation (1-6) allows to calculate a sensitivity 𝑆 of 5.4 𝑉/ 𝑇 at 12 kHz. Its
detectivity is equal to 𝑆𝑉,𝑡ℎ /𝑆 = 44 𝑝𝑇/√𝐻𝑧 at 12 kHz, if the noise is limited by the
coil. If we consider a preamplifier with a typical voltage noise of 1 𝑛𝑉/√𝐻𝑧 this gives
a detectivity of 176 𝑝𝑇/√𝐻𝑧.
The second example is a micro coil that wound using 0.7 mm radius wire, 460 turns
and has a resistance of 32 Ohms.
Figure 1.5- Cylindrical Coil
1 2 ⁄
The cylindrical coil has white noise spectral density 𝑆𝑉,𝑡ℎ of 0.73 𝑛𝑉/√𝐻𝑧 . Equation
(1-6) allows to calculate its sensitivity 𝑆 = 53 𝑉/ 𝑇 at f = 12 kHz. Its magnetic noise is
equal to 13 𝑝𝑇/√𝐻𝑧 at f= 12 kHz. Mook presents in [19] the limit of miniaturized coil
dimension using a copper wire of 30 μm diameter: it has a width of 0.5 mm, length of
3 mm, 1000 turns.
Another type of coils is etched coils made on standard PCB or on flexible film of
Kapton type. Kapton is a thin film of polyimide, from ten to several hundreds of
micrometers, which has a very good flexibility. These flexible sensors allow to inspect
complex shapes objects. For example, an eddy current probe for NDT developed in
our laboratory is composed of 96 patterns of 2 micro-coils etched on a soft Kapton
film of 50 µm of thickness Woytasik presents in [20] an example of coils that have 13
turns in a volume of 30x18x100 𝜇𝑚3 . The micro-coils allow to innovate in biomedical
instrumentation such as Nuclear Magnetic Resonance (NMR) using for imaging or for
spectroscopy [21, 22]. Khelifa describes in [23] the manufacturing processes of
flexible rectangular micro-coil (500 × 1000 𝜇𝑚²) made on kapton substrate for
NMR. Micro-coils have a limited spatial resolution and a limited sensitivity at low
frequencies but are very efficient at high frequencies.
1.6 Fluxgates
Fluxgates sensors are composed of a ferromagnetic core and at least two coils. There
are several configurations of fluxgate sensor, which are described in [24, 25]. Figure
1.6 shows one of these configurations: an exciter winding surrounding the nucleus,
traversed by alternative current, creates an alternating magnetic field in the core,
locally parallel to the winding axis. The corresponding induction undergoes saturation,
symmetrical to zero. Another winding is arranged around the core, its axis being
rectilinear. The first coil is called the excitation coil, while the second one is called the
measurement coil or pick-up coil. These sensors exploit the saturation of
ferromagnetic material of core.
Figure 1.6-Principle diagram of fluxgate
The voltage at the output of the pick-up coil has a form of pulses. In the absence of
external magnetic field, the voltage is symmetric in time. In presence of external
magnetic field, the saturation of core is more rapid for one semi-period than another.
This asymmetry induces a time shift in the voltage at the output of pick-up coil. The
measurement of this time shift allows to estimate the value of the external magnetic
field. Figure 1.7 shows the principle of phase shift measurement at the output of pick-
up coil using to detect the presence of external magnetic field. We deduce external
magnetic field 𝐻𝑒𝑥 by the measurement of interval between the pulses.
Figure 1.7- Principle of pulse shift method
The excitation coil frequency for fluxgate is in general between 50 Hz and 10 kHz.
Fluxgates have a high sensitivity, especially for continuous magnetic fields. They are
developed initially for compasses, because they are able to detect the Earth's magnetic
field. They are able to measure the components of constant or varying magnetic field
vector in the range from 0.1 mT to 8 mT at frequencies up to several kHz. Pavel Ripka
describes in [26] a summary of fluxgates performances. Recently, printed circuit board
(PCB) technology has been used in the development of planar fluxgates for NDT
application [25]. Kejík has developed a 2D planar fluxgate composed of two
orthogonal plan coils and an amorphous ferromagnetic core in the shape of PCB ring
[27]. The authors report that the sensor is linear in a range of ±60 μT, and shows a
sensitivity of 55 V/mT. Also in biology application, a fluxgate sensors have been used
in order to determine the binding between Streptavidin proteins with Biotin [28] and to
study the release properties of hydrogel cylinders and microsphere [29].
Reducing of fluxgate dimensions decreases its performance. To prove this, we
consider two following examples: first, a sensor made by the Bartington Instrument
Company with a volume of 8 × 8 × 30 𝑚𝑚3 has white noise about 2 𝑝𝑇/√𝐻𝑧 and can
work up to 4 kHz; second, a micro-fluxgate designed by CEA LETI in 2005 with
dimensions 1mm × 400µm × few µm etched on Silicon has white noise about 1 𝑛𝑇/
√𝐻𝑧 [30, 31]. By simple comparison between the thermal noise of these two
examples, we can conclude that the reducing of fluxgate dimensions undergoes a loss
in fluxgate performances manifested by increasing of noise. The main problem of
miniaturized fluxgates stems from the volume of their coils. The fluxgates show
important restrictions due to their weight, power consumption and high temperature
fluctuations (no thermal stability).
1.7 Giant magneto-impedances
Magneto Impedance (MI) has been discovered in 1930 by Harisson et al in a nickel
wire [32]. Later, in the early 1990s, Panina et al. studied the phenomenon in
amorphous wires and films, and discovered Giant Magneto-Impedance (GMI) [33].
Giant magneto-impedance (GMI) is due to a significant modification of the impedance
of soft ferromagnetic materials (amorphous or nanocrystalline wires or ribbons) when
they are subjected to external magnetic field. The excitation frequency of GMI is in
the MHz range.
Figure 1.8 shows GMI sensor configuration formed by a simple ferromagnetic wire
crossed by an alternating current:
Figure 1.8- GMI sensor made of ferromagnetic wire crossed by
alternating current and subjected to an external field 𝐻𝑒𝑥
The impedance of conducting wire depends on skin effect 𝛿 (equation (2-7)), to
geometrical dimensions of the wire 𝑎, and to the wire resistance at low frequency 𝑅. 𝑗
is the complex number according to the relation:
𝑎
𝑍 = (1 + 𝑗) . 𝑅. (1-9)
𝛿
The impedance given here is only defined if there is linear relation between current
and voltage. This relation is valid if δ<<a. However, ferromagnetic material is non-
linear. The impedance Z is therefore valid only around the operating point [34, 35]. δ
depends on permeability μ, so the GMI impedance depends on permeability μ.
However, in ferromagnetic materials, the permeability μ depends on external magnetic
field 𝐻𝑒𝑥𝑡 . Thus, when an external field 𝐻𝑒𝑥 is applied, the permeability is changed
and this change produces an impedance variation.
GMI has relative impedance variations greater than 600% and relative impedance
variation of 1440 % / mT [36]. GMI sensors fabricated by AICHI Micro Intelligent
company [37] are unitary assemblies that are composed of amorphous wire of 20 μm
at diameter. These sensors are marketed with amplification electronics with total
dimensions of 31 × 11 𝑚𝑚2 (Figure 1.9), and have linearity range of 0.6 mT. Tannous
et al. summarized the different physical parameters that maximize the GMI impedance
variation [38].
Figure 1.9- GMI sensor marketed by AICHI Company [37]
Boukhenoufa and Dolabdjian have developed a GMI with a length of 10 mm and
white noise of 100 𝑝𝑇/𝑠𝑞𝑟𝑡(𝐻𝑧) at 10Hz [39]. This noise range allows GMI sensor to
compete with fluxgates. Traore has fabricated a GMI sensor that has 400 turns of
external coil, 2.5 mm of length, and 20 μm of diameter [40]. This GMI sensor has a
white noise about 1.8 𝑝𝑇/√𝐻𝑧 at 1 Hz.
Large impedance variations allow to GMI sensors to detect very weak magnetic fields.
GMI sensors have been evaluated by Vacher [41] and by Peng [42] for detection of
small magnetic fields for non-destructive testing application. These sensors show good
performance in detection of buried defects [43]. However, excitation circuit of GMI
sensors should operate at high frequencies (more than 20MHz) that makes difficult
their integration into probes for non-destructive testing. Because of their high
sensitivity at low frequencies, GMI sensors are used for development of magnetic
biosensors. Magnetic biosensors based on GMI sensors are used for detection of
magnetic field variation caused by magnetic microparticles [44]. Chen and al. use
GMI sensors to identify gastric cancer cells [45].
The weak point of GMI sensor compared to fluxgate is the remanent field effect after
subjecting to strong magnetic fields, because the ferromagnetic core is usually not
demagnetized during sensor operation [26]. Despite the high sensitivity of GMI
sensors, it is shown that the performance of these sensors is limited by the noise level
of electronics, especially in the white noise zone by comparing with GMR and TMR
sensors.
1.8 Hall Effect Sensor
The Hall effect is part of a set of phenomena called galvanometric effect. The
galvanometric effects are observed in solid which undergoes simultaneously the action
of electric field and magnetic induction. Hall effect occurs when a material crossed by
electric current I, is subjected to magnetic induction field B perpendicular to the
current direction (Figure 1.10).
Figure 1.10- Principle of Hall effect
In the presence of magnetic field, the charge carriers undergo the Lorentz force and are
deflected during the transient regime. The appearance of surface charges at the top and
at the base of the sample creates an electric field and generates a force, opposite to
Lorentz force. The potential difference, called Hall voltage V appears between the
faces of the sample. It is proportional to the injected current and to the magnetic field.
The sensitivity of Hall effect sensor can be increased [46] by ferromagnetic
concentrators. It increases from 10 mV / T to 1.4 V / T. The highest reported
detectivity limit of a combination of Hall sensor with magnetic concentrators is as low
as 10 pT [47]. Hall-effect sensors offer an advantage over a coil for depth penetration
structure that requires working at low frequencies and for spatial resolution. They are
cheap and easy to integrate. They are particularly interesting in terms of linearity and
they offer a wide range of magnetic field measurement (since there is no saturation).
Kejik describes in [48] an integrated micro-Hall plate sensor with an active area of
2.4 ×2.4 μ𝑚2 supplied by the spinning current and has a white noise of 300 𝑛𝑇/√𝐻𝑧.
The Hall effect sensor has been employed for applications of non-contact switching,
memory readout and current measurements. Ejsing has used this sensor for the
detection of magnetic induction generated by magnetic nanoparticles [49]. Also, Oota
has used a scanning Hall-sensor microscope with an active area 25×25 μ𝑚2 for non-
destructive testing of structural materials [50]. Lee has used these sensors with
0.52 mm spatial resolution for NDT and crack evaluation of ferromagnetic materials
[51] .
Figure 1.11- Photograph of the linearly integrated Hall
sensors (LIHaS): 64 InSb Hall sensors [51] .
1.9 Magnetoresistive sensors
Magnetoresistance effect (MR) has been discovered by William Thomson in 1856
[52]. Under the effect of external magnetic field (𝐻𝑒𝑥 ), the resistance of MR sensors
will be changed to a new resistance value (𝑅). This magnetoresistance effect (MR
ratio) is a performance index that can be defined. The MR sensors will have a
minimum resistance (𝑅𝑚𝑖𝑛 ) and a maximum resistance (𝑅𝑚𝑎𝑥 ). The magnitude of the
magnetoresistance effect (MR) can defined as follows with (𝑅0 = 𝑅𝑚𝑒𝑎𝑛 in the case of
AMR and GMR, and 𝑅0 = 𝑅𝑚𝑖𝑛 in the case of TMR ):
∆𝑅 𝑅𝑚𝑎𝑥 − 𝑅0
𝑀𝑅 = = (1-10)
𝑅0 𝑅0
There are different types of magnetic sensors like Anisotropic Magneto-Resistive
(AMR), Giant Magneto-Resistive (GMR) and Tunnel Magneto-Resistive (TMR)
sensors. At the beginning, the AMR sensors has been developed to replace coils in
hard disks read heads, with the aim of reducing the bit size on the writing media. After
the development of GMR, AMR sensors have been replaced by GMR sensors, and
now GMR sensors are partly replaced by TMRs. Magnetoresistive sensors have many
other applications, such as current measurement, velocity measurement, non-
destructive testing and magnetic imaging.
1.9.1 Anisotropic Magnetoresistance (AMR)
Anisotropic magnetoresistance (AMR) is a generic magnetotransport property of
ferromagnetic metals materials (and their alloys). The anisotropic magnetoresistance
effect, occurs when the electrical resistivity depends on the relative angle 𝜃 between
the direction of electrical current (direction of easy axis) and orientation of the
magnetic field [53] (Figure 1.12). When the external magnetic field is parallel to AMR
easy axis, the resistance 𝑅 of AMR does not vary, and is still equal to (𝑅0 ). The AMR
effect reaches its maximum when 𝜃 equal to 90° angle.
Figure 1.12-Principe diagram of AMR sensor
The resistance R varies as cos2 (𝜃) [54].
𝑅 = 𝑅0 + ∆𝑅. cos2 (𝜃) (1-11)
AMR sensors are more sensitive than Hall sensors. The AMR ratio ranging between
1% and 5 % for permalloy at room temperature [55].This effect can be used to
measure the components of magnetic field vector in the range of 1 nT to 1 mT.
Sensitec Company developed an AMR Fix Pitch sensors for angle and length
measurement with accuracy of 3 to 25 µm [56]. It is difficult to achieve pT noise
values, Zimmermann has presented AMR sensors that have a white noise of 200 𝑝𝑇/
√𝐻𝑧 [57].
Unlike coils, fluxgates, Hall effect and GMI sensors, AMR sensors can be easily
miniaturized and have dimensions smaller than 100μm. For example, micro-sensor
arrays based on anisotropic magnetoresistances with size of 16 x 5 𝜇𝑚2 are designed
for medical applications [58]. AMR sensors of 50 x 20 𝜇𝑚2 at size are used for NDT
application [59]. Also AMR has been used in pulsed eddy current non-destructive
testing (PEC NDT) [60]. We can also note that AMR sensors are used in the
development of magnetometers that have recently become a built-in feature in most
tablets and smartphones around the globe [61], and for vehicle speed and length
estimation [62].
The response of AMR sensors has the disadvantage of not being linear but a specific
configuration called barber pole where the angle is fixed around 45° is currently used
to linearize AMR sensors. They present a saturation point in mT range and are not
absolute sensors. Also, a very strong magnetic field (~200 mT) [26] can cause a
permanent change in magnetization direction of AMR material, which makes the
AMR sensor unusable.
1.9.2 Giant magnetoresistances (GMR)
Giant magnetoresistance sensors (GMR) have replaced the AMR sensors, as they
allow high storage density due to their high sensitivity and their smaller size. In terms
of resistance variation, the GMR is four to five times higher than the AMR. A giant
magnetoresistance is a multilayer sensor formed by a series of thin ferromagnetic and
non-magnetic layers. The thickness of individual ultra-thin layer can be few
nanometers. The resistance of the GMR reacts with the application of external
magnetic field. The phenomenon of GMR has been discovered by two European
Nobel Prize winners [63, 64] in 1988: Albert Fert and Peter Grünberg (Figure 1.13).
Figure 1.13- The two discoverers of GMR
The magnetoresistance ratio of GMR may reach up to 50 % [65], typically the
sensitivity of GMR sensors are between 15 and 40 V/V/T when they are biased with
1 V. The white noise of GMR sensors is less than 0.5 nT/√Hz at room temperature
[66]. Specific technologies integrate GMR sensors with micron-size and are able to
achieve 𝑓𝑇/√𝐻𝑧 detectivities in the white noise leading the possibility to detect weak
biomagnetic signals [67].
For example, GMR sensors have been used for detection of "Influenza A Virus" [68].
GMR sensors of 75 ×40 𝜇𝑚2 size with white noise level at 𝑛𝑇/√𝐻𝑧 and sensitivity of
8.5 V/V/T, are used to estimate the magnetic properties of diluted ferrofluid for
biomedicine application [69].They are widely used in hard disk read heads where they
detect the magnetic coding of the bits. GMR sensors based spin torque oscillators
(STO) with lateral size of 70nm are integrated to CMOS circuit [70]. Figure 1.14
shows an example of static field mapping realized in CEA SPEC laboratory for Earth
Science application, in particular micrometeorite magnetization mapping. The
discovery of gradient-field GMR sensors makes them interesting for pulsed eddy
current testing method, as the gradient-field measurements give better sensitivity and
accuracy in detection and sizing of hidden corrosion in layered conductive structures
meteorite collected in Antarctica, Right: Magnetic mapping of the meteorite magnetic dipole.
Coll. Cerege – Jérôme Gattacceca [66].
GMR sensors are not absolute sensors and have also a saturation that may limit their
operating range.
We will describe in the following part GMR technology in more details.
1.9.2.1 GMR with spin valve structure
Spin valves are a particular case of multilayered structure [72] composed of a hard
layer, a spacer and a free layer. The nanometric layers are stacked according to the
Figure 1.15.
Figure 1.15- Example of spin valve GMR [73]
The hard layer has high coercivity. This layer is usually composed of a pinned
ferromagnetic layer and an antiferromagnetic (AF) layer (pinning layer). The small
external magnetic field to detect has no impact on the AF layer. This AF layer is used
to fix the magnetization orientation of the pinned ferromagnetic layer due to exchange
bias direct pinning. Thus, magnetization orientation of hard layer is fixed. The
magnetization of free layer is easy to orient with external magnetic field. The free
layer is separated from the hard layer by a conductive layer (spacer) that is sufficiently
thick (2-5nm), so that the magnetic coupling between free and hard layers is weak and
their magnetizations are independent. Paragraph 4.5 gives a detailed example of GMR
of spin valve used in our work.
1.9.2.2 GMR effect
To illustrate the principle of the GMR sensors, we consider a configuration composed
by a stack of ferromagnetic layers with parallel magnetization and anti-parallel
magnetization (Figure 1.16). A nonmagnetic conductive layer is placed between the
two ferromagnetic layers in the two cases.
Figure 1.16- GMR structure: (a) Free layer, spacer and pinned layer. The spacer is non-magnetic (NM); (b)
The equivalent electrical circuit with two conducting channels is shown for anti-parallel magnetizations
(red) and for parallel magnetizations (blue); (c) The figures at the top illustrate the trajectory of the
electrons in antiparallel configuration and illustrate at the bottom the parallel configuration .
Electrons have only two possible directions: upwards or downwards. In anti-parallel
configuration (Figure 1.16), electrons are diffused and hence the resistivity of the
structure is high (red case). The spin up and spin down currents pass through each
ferromagnetic magnetization layer opposite to their spin. The equivalent resistance
𝑅┴ is:
𝑅↑↓ +𝑟↑↑
𝑅┴ = 2
≅ 𝑅/2 (1-12)
In the configuration with parallel magnetization of layers (Figure 1.16), the spin up
electrons pass through structure nearly without diffusion, while the spin down
electrons are strongly scattered (blue case). The spin electron current anti-parallel to
the magnetization vector encounters two resistances 𝑅↑↓ while the inverse spin current
encounters two resistances 𝑟↑↑ (Figure 1.16.c). The equivalent resistance 𝑅|| of the
structure is:
𝑅↑↓ 𝑟↑↑
𝑅|| = 2 ≅ 2𝑟, 𝑖𝑓 𝑅↑↓ ≫ 𝑟↑↑
𝑅↑↓ + 𝑟↑↑ (1-13)
Thus the resistance in the antiparallel state is higher than the resistance in the parallel
1.9.2.3 GMR sensitivity
The GMR sensitivity can be calculated using the sensitivity curve obtained by
measurement of sensor response in homogenous magnetic field. To obtain sensitivity
curve, we put the GMR in the center of the Helmholtz coil (Figure 1.17). Typical
GMR response of a linearized sensor is shown in Figure 1.18.
Figure 1.17- Measurement scheme of GMR sensitivity.
The Helmholtz coil is excited by a sinusoidal signal from the wave generator. The
relative variation of magnetoresistance (MR) is calculated according to equation
𝑅|| +𝑅┴
(1-10) with 𝑅0 = .Thus, the MR ratio could be defined as follow :
𝑅┴ − 𝑅||
𝑀𝑅 = (1-14)
2𝑅0
The sensitivity 𝑆𝐺𝑀𝑅 (V/T) is the variation of output voltage on the applied field given
by the following equations.
∆𝑉 ∆𝑅. 𝑖
𝑆𝐺𝑀𝑅 (𝑉/𝑇) = = (1-15)
∆𝐻 ∆𝐻
The sensitivity in (%/mT) is:
∆𝑅 1
𝑆𝐺𝑀𝑅 (%/𝑚𝑇) = × 100 (1-16)
∆𝐻 𝑅0
An example of MR calculation is presented: a DC current of 3 mA is applied to the
GMR sensor. This GMR sensor of spin valve type has a resistance of 79.6 Ohms. The
resistance variation of GMR as a function of magnetic field (H) is linear in the range [-
2 mT ; +2 mT] (Figure 1.18). In this case, 𝑅┴ = 81.7 Ohms and 𝑅|| = 77.5 Ohms.
These sensors have MR ratio of 5.2 %. The 𝑆𝐺𝑀𝑅 is 6.2 V/V/T.
Figure 1.18- Example of GMR response as function of magnetic field
applied along its sensitivity axis
1.9.3 Tunnel magnetoresistance (TMR)
Another type of multilayer system with resistivity variation as a function of relative
orientation of layers magnetizations is the Tunnel Magnetoresistance (TMR) sensor.
Tunnel Magnetoresistance effect has been proposed in 1975 by Michel Jullière [74],
and has been observed in room temperature in 1995 [75, 76]. Contrary to GMR
systems, the spacer is composed of an insulator layer. TMR is a quantum mechanical
effect, which occurs when two ferromagnetic layers are separated by a few atomic
layers of insulator. The transfer of electrons from one ferromagnetic layer to another is
done by tunnel effect. Transmission of electron is possible only if there is a free state
with the same energy and same spin on the other side of the insulator. Yuan [77] and
Lee [78] describe a method of tunneling magnetoresistance (TMR) fabrication. Figure
1.19 shows a typical stack of tunnel junction.
Figure 1.19-Typical stack of TMR sensor on thickness of Fe diffusion barrier for double
MgO-based p-MTJ spin-valves with top 𝐶𝑜2 𝐹𝑒6 𝐵2 free layer [79].
The high magnetoresistance ratio of TMR sensors with MgO barrier (spacer) makes
them very attractive. Tantalum (Ta) layer is an interface layer. The quality of the
interfaces is essential for performance of TMR sensor. MR exhibits a higher
sensitivity and higher signal-to-noise ratio (SNR) compared to Hall sensors and other
magnetoresistive sensors, including GMR [80]. TMR sensor has greater
magnetoresistance ratio than GMR, especially TMR with MgO junction that can reach
MR ratio up to 1000% [81]. Janeiro describes in [82] a TMR of size 5x20 μm2 that has
white noise of 13.8 𝑛𝑇/√𝐻𝑧. Besides high spatial resolution and high field sensitivity,
the TMR sensors have low energy consumption.
TMR sensors are used in non-destructive testing and biomedical applications. Pelkner
describes in [83] the benefits of GMR and TMR sensors in terms of high spatial
resolution testing for different NDT applications. We find the TMR in MRAM
(magnetoresistive random access memory) memory and in read head [79]. Lei presents
in [84] a contactless scanning prototype device based on TMR sensors used in
biomedical application for quantification of Magnetic nanoparticles (MNP) in lateral
flow strips (LFS). Also, TMR sensors are used for nanosatellites space application,
with white noise of 150 𝑝𝑇/√𝐻𝑧 and volume about (25 × 25 × 10 𝑚𝑚3 ) [85].
However, the injected voltage in TMR sensors is limited and the magnetoresistance
ratio of TMR decreases if the injected voltage increases. Despite its higher sensitivity
compared with AMR and GMR sensors, the 1/f noise limits its detectivity and thus its
performance. Also the high coercivity and low linearity of TMR sensors remain a
serious problem [86].
1.10 Conclusion
This chapter presented an introduction of magnetic sensors principles. Table 1 presents
a summary of the main properties of these magnetic sensors.
Measuring Detectivity (𝑝𝑇/√𝐻𝑧)* Dimensions Continuously Frequency (Hz) Measured Material type
range operating quantity
Coil 0.1fT<B<100T 1 at 1kHz for 1cm² > 30x18x100 𝜇𝑚3 No 10−3 − 109 Derivative of Massive
the flux
Fluxgate 0.1nT<B<0.01T 3 at 1 kHz >1×1×1 𝑚𝑚3 yes 𝐷𝐶 − 103 flux Massive
Hall Effect 1μT<B<1T 105 white noise <4×3×1.5 𝑚𝑚3 Yes 𝐷𝐶 − 105 field Thin film
GMI 0.1nT<B<1mT 100 at 1 kHz <5mm×30 μm Yes 𝐷𝐶 − 104 Flux Massive
AMR 1μT<B<5mT 1000 white noise µm size Yes 𝐷𝐶 − 107 Field Thin film
GMR 1nT<B<5mT 50 white noise µm size Yes 𝐷𝐶 − 108 Field Thin film
TMR 1nT<B<5mT 10 white noise µm size Yes 𝐷𝐶 − 108 Field Thin film
Table 1: Comparisons of characteristics of different magnetic sensors [34, 54, 87]
We can compare the magnetic sensors based on two parameters: size and detectivity of
these sensors. The limits related to the fabrication of very small coils, GMI and
fluxgate sensors limit their use in application that need high spatial resolution. For
example, we need high spatial resolution to detect very small defects in NDT and to
distinguish between small magnetic objects for magnetic imaging.
Figure 1.20 shows experimental comparison of detectivity between different magnetic
sensors [34, 88, 89].
Figure 1.20- Comparison of the detectivity of magnetic sensors : coil, fluxgate,
Hall, GMI, AMR, GMR and TMR sensors
We can observe in Figure 1.20 that the fluxgate and the GMI sensors have a lower
noise level, but have higher size than MR sensors. The micro-Hall sensor is smaller in
size and has linearity response, but is characterized by high noise level. The
magnetoresistive sensors have micron size, and high detectivity independent of
frequency in the white noise, that make them more effective at small scale.
For applications in NDT and micromagnetic imaging, we need a micron-size spatial
resolution with high detectivity, so the best tradeoffs between magnetic sensors are
MR sensors. With simple comparison between AMR and GMR sensors, we can
conclude that GMR sensors are the best for our application in terms of thermal noise
and operating frequency range. The high 1/f noise of TMR sensors at low frequencies
limits their interest compared to GMR and they are more difficult to process. The
tradeoff between spatial resolution and the noise level justifies the choice of GMR
technology for the development of probe using in NDT and magnetic imaging
(micromagnetic scanning).
Chapter 2. Non-destructive testing
We will describe in this chapter some NDT methods and their application limits. We
will consider the performances of eddy current method. One of the objectives of this
work is to develop eddy current probe based on magnetoresistive sensors for detection
of surface and subsurface small defects in conductive materials. Finally, we will
present in the conclusion of this chapter a study, that show more specifically the
choice of the GMR sensors between the MR sensors to replace coils for eddy current
non-destructive testing (EC NDT).
2.2 Non-destructive testing
NDT (non-destructive testing) methods are based on different technologies enabling to
guarantee structure and product reliability. They are essential in many industrial
sectors, including transportation, automotive, petrochemical, nuclear and aircraft
industries [90, 91]. NDT is used in industry from the beginning of the 20th century.
Figure 2.1- Non-destructive testing in the aerospace industry [92]
In general, NDT methods have the same principle which can be described down into
four steps:
- An excitation energy emits signal in the controlled part: electromagnetic or
acoustic waves;
- An interaction is created between the controlled part and emitted signals
- Receiver that receive the signals resulting from these interactions.
- Acquisition system in order to interpret the received signals and to deduce the
properties and the state of the part under test.
In the following, we present a general review of some techniques used in NDT, and
we will detail eddy current method.
2.3 Optical methods
Visual inspection is widely used in NDT. To control surfaces that are inaccessible to
direct vision of operator, many optical instruments like endoscope and stroboscope are
developed to increase the performance of the eye or more generally to give the
possibility of surface control. However, the non-automatic optical methods have limits
related in particular to the tiredness of operator. Thus, fully automatic optical control
equipment has been developed.
Another optical method shown in Figure 2.2 is Optical Holographic technique
developed for testing of materials (HNDT). The method could used to detect small
displacement of material surface. A surface or near-subsurface defects could be
detected due to surface displacement changes. Holography creates a three-dimensional
image of an object having arbitrary shape. HNDT allows to detect and to measure
deviation and dimension of the target [93, 94].
Figure 2.2- Optical instrument for surface deformations [94]
2.4 Penetrant Testing
Non-destructive testing by Penetrant Testing (PT) is the oldest NDT method as its first
application would date to the end of the 19th century. PT is a low-cost inspection
method and it is widely used to localize surface-breaking defects in all non-porous
materials (metals, plastics or ceramics). The piece is subjected to the action of a
penetrating liquid (or gaz). The physico-chemical characteristics of this liquid allow it
to penetrate inside the cracks of specimen. Its application is done either by immersion
or by spraying. PT method allows the detection of the discontinuities on the surface of
controlled piece in the form of colored or fluorescent indications, observed on a white
background or on a black background. The control of PT method is performed under
artificial white light or daylight (colored bleeding) or under ultraviolet radiation (UV-
A). The drying time, "dwell time", is the time to soak into cracks (generally 5 to 30
minutes). The dwell time mainly depends on used penetrant, material under test and
the size of cracks sought [95]. Figure 2.3 presents an example of penetrating testing
steps. PT is widely used for inspection ensuring surface flaws detection in large areas
and structures with complex shapes. However, this method is highly polluting and is
highly dependent on human interpretation of results.
Figure 2.3- The four steps of PT technique
2.5 Magnetic particle inspection (MPI)
Magnetic particle inspection is method of detection of surface defects in ferromagnetic
materials. Magnetic flux is created in the material and in the presence of defect the
flux leakage will be created. To detect this flux leakage ferrous particles are applied to
the part under test (like in penetrant testing). If there is no flux leakage thus there is no
defects in tested part, particles distribution is homogeneous. In the presence of flux
leakage, ferrous particles are attracted to this zone and form an indication of the
defect. MPI is very attractive for surface defects detection however it has the same
limitations as penetrating testing in terms of detection and pollution factors.
2.6 Ultrasonic testing
Ultrasonic testing (UT) exploits the propagation properties of ultrasonic waves
(transmission, reflection and absorption). It consists of emission of acoustic waves in
controlled piece and analyzing the echo returning from this piece. The sensors are
composed of one or more transducers acting as transmitter or receiver (Figure 2.4).
Figure 2.4- Shema of Ultrasound Principe [35]
Three types of transmitters are used to generate ultrasound signal: piezoelectric
generators, magnetostrictive transmitters and electrostrictive emitters. Receiver
transducer is located in generally on the same side as transmitter. The emitted acoustic
wave encounters various elements on its path like background of the piece and defects.
This reflected wave is received by the receiver transducer with some delay due to the
propagation of wave across the piece. If the velocity 𝑣 of wave propagation in the
material is known, it allows to distinguish between the echo from the bottom of the
piece and the echo from a defect located inside the piece. The propagation time 𝛿𝑡 of
the reflected wave from the defect depends on the distance between emitter and target
'𝑑1 ' and on the distance between receiver and the target '𝑑2 '. 𝛿𝑡 is expressed by the
𝑑1 + 𝑑2
𝛿𝑡 = (2-1)
𝑣
UT technique allows to locate or to characterize the defect. Also thickness
measurement could be realized. There are other techniques in ultrasound to improve
the detection and characterization of objects like for example phased array [96] or
laser ultrasound. Binder layer (water, oil) is needed to be applied between transducers
and controlled piece [97].
2.7 Radiography
Radiography method could be applied to all solid materials, which thicknesses vary
from few micrometers to several hundred millimeters, taking into account the nature of
controlled medium and nature of radiation. Radiography could be defined as a cross-
sectional 2D imaging technique that aims to create the image of a solid object from the
set of its projections [98]. The interest of this method is the identification of internal
defects. Radiography method consists of emitting electromagnetic radiation of short
wave length like X-ray or γ-rays in the piece. Photons energy will be partially or
completely absorbed inside the piece. The receiver on the other side of controlled
piece will detect the photons. Sensitive films, fluorescent screens or CCD detectors
could be used as receiver. Obtained images can be directly interpretable, have good
spatial resolution. For X-ray imaging (Figure 2.5), we can get the projections by
directing an X-ray beam to the object from different orientations and measuring its
intensity attenuation during the traversing of X-ray in the object. Images with good
resolution could be obtained, however important safety requirements should be
Figure 2.5-Example of uses of Radiography method [99]
2.8 Magnetic flux leakage
Magnetic flux leakage method is a technique used for corrosion and pitting detection
in steel structures. A magnet is used to magnetize steel. Magnetic flux is created in
material under test. If there is no defect, flux lines will pass through material. In the
presence of a defect or corrosion, magnetic leakage field will be created in the area of
a defect. This magnetic leakage field is detected by magnetic sensitive sensors (Hall
sensors, GMR sensors) placed between the poles of magnet.
2.9 Eddy current testing
Eddy current technique has been discovered by the French scientist Jean Bernard Leon
Foucault in 1855. This method is used since 1926. Eddy current testing (ECT) is a
magnetic method used to detect surface and sub-surface flaws in conductive materials
[100]. ECT is based on the interaction between magnetic field source (emitter coil),
the test material (piece) and receiver (magnetic sensor) [101]. The induced current
density depends on applied current, on geometry and dimensions of excitation coil
[102]. Eddy currents are created in conductive material by emitter. The emitter is in
general a coil traversed by excitation current 𝐼𝑒𝑥 , which produces a magnetic field
variable in time determined by Maxwell equations:
⃗
𝜕𝐷
𝑟𝑜𝑡 ⃗ = 𝐽+
⃗⃗⃗⃗⃗⃗ 𝐻 (2-2)
𝜕𝑡
⃗ =0
𝑑𝑖𝑣𝐵 (2-3)
𝜕𝐵
⃗⃗⃗⃗⃗⃗ 𝐸⃗ = −
𝑟𝑜𝑡 (2-4)
⃗ =𝜌
𝑑𝑖𝑣𝐷 (2-5)
𝐸⃗ and 𝐻
⃗ are respectively the electric and the magnetic fields, 𝐷
⃗ and 𝐵
⃗ - the electric
and magnetic inductions, 𝐽 - the surface density of electric current and ρ - the density
of electrical charges.
The variable magnetic field will create eddy currents in conductive material.
According to equation (2-4), these eddy currents have same frequency as the excitation
current. The induced currents circulate locally in the material (Figure 2.6) and have
spatial distribution that depends on excitation magnetic field, electrical conductivity 𝜎,
magnetic permeability 𝜇 of the examined piece. In the presence of inhomogeneity like
defect or inclusion, physical properties (conductivity and permeability) are changed,
eddy current lines are deviated and reflected magnetic field will be changed.
Figure 2.6- Principle of eddy current generation [35]
Receiver detects this reaction magnetic field resulting from the interaction between the
induced currents and the excited material (piece).
Eddy current distribution
The distribution of eddy current is not homogeneous at different depth and is governed
by the Maxwell equations. The intensity of eddy current in conductive material
characterized by electrical conductivity 𝜎 (𝑆/𝑚) and magnetic permeability 𝜇(𝐻/𝑚 ),
decreases with the depth 𝑧 (𝑚). In the case of infinitely thick piece with a flat surface,
the current density along z-axis 𝐽𝑧 (𝐴/𝑚²) excited at frequency 𝑓(𝐻𝑧)is expressed by
[103]:
Jz (z, t) = J0 𝑒 −𝑧√𝜋𝑓𝜇𝜎 cos(𝑤𝑡 − 𝑧√𝜋𝑓𝜇𝜎), (2-6)
where 𝐽0 𝑎𝑛𝑑 𝑤 are respectively the maximum of current density at piece surface, and
the pulsation of induced currents. According to equation (2-6), if the depth z increases,
the current density decreases. This decreasing of current density as function of depth is
shown in Figure 2.7.
Figure 2.7- Evolution of current density as function of piece depth
(z) in the case in the case of plane wave excitation [104].
The penetration depth 𝛿 (𝑚), also called skin depth is the depth at which eddy current
density decreases to the level of 37% of its surface value 𝐽0 [105]. This skin depth is
defined as:
𝛿=√ (2-7)
𝜋𝑓𝜎μ
The magnitude δ allows the estimation of appropriate excitation frequency according
to material electromagnetic parameters.
Figure 2.8 shows the evaluation of skin depth against frequency for different materials.
At low frequencies the penetration depth is more important, thus for buried flaws
detection low frequencies should be used, whereas for surface cracks with low
penetration depth high frequencies should be used. Working frequencies of EC
technique are from 10 Hz to 10 MHz. At low frequencies, the amplitudes of eddy
currents are low, at high frequencies capacitive effects of inductive part will influence
the measurement.
Figure 2.8- Evolution of skin depth against frequency for different materials
Receiver in eddy current
EC probe is composed of an emitter consisted of a coil and a receiver consisted of
magnetic sensors. Inductive sensor (coils) could operate in emission-reception mode
and in emission-reception separated mode. Other magnetic sensors could be used as
receivers to increase the performances of EC probes.
Magnetic sensors and their performances for ECT have been reviewed in the Chapter
1. Figure 2.9 presents an example of MR array sensor based probe developed during
IMAGIC project [106]. This project has been focused on the development of the
probes with MR sensors as receivers with miniaturized electronics for detection of
buried defects and small surface defects. Double coil has been chosen as emitter for
buried flaws detection, receiver array is formed of 32 GMR sensors. For surface cracks
detection GMR and TMR array sensors of 32 and more elements have been fabricated.
One wire emitter has been used for this application. Higher performances of GMR and
TMR sensors in comparison with inductive sensors for detection of small surface
defects have been shown.
Figure 2.9- Photo of the IMAGIC MR probe for buried flaws detection [107]
This chapter introduces different NDT methods. In particular, we present in details the
eddy current method. Table 2 presents a comparison of the main NDT techniques:
Physics Type of controlled material Strong point Weakness
principle detected defect
Optical Forming an surface all rapid Fine defects
method image detection
Penetrant Capillarity surface defects all except porous global method, Productivity, Little
Testing effect simplicity quantitative
Ultrasound Disturbance Surface/buried all defect sizing Coupling
of ultrasound flaws
Radiography Flow surface/buried all defect sizing safety requirement
Eddy currents Current Surface and Conductive material defect sizing Interpretation,
perturbation subsurface sensitive to defect
defects orientation
Table 2: Comparison of the main NDT techniques [96, 99, 100, 101, 108]
EC technique is a method that is widely used for defects detection in conductive
materials. It also could be used in order to partially replace the polluting methods like
penetrating testing and X-ray inspection. Nowadays EC testing is undergoing the
change in order to answer to industrial specifications in terms of product quality. The
aim is to detect very small (<100µm) surface cracks and deep buried flaws (under
several mm). In order to overcome the limits of conventional winding coils classically
used in ECT in terms of detectivity at low frequencies and micro-coils fabrication
limits, magnetic sensors have been investigated since several years as receivers for EC
probes. Moreover, several components of magnetic field are needed to be measured
for defects characterization. Oka et al. introduced EC probe using 3-axis measurement
coil to characterize the shape of detected defect [109]. This probe has limitation due to
its large size. During this PhD thesis 3D GMR based EC probe has been developed for
detection and sizing of small surface cracks. This probe will be detailed in Chapter 4.
Chapter 3. Magnetic Imaging
The second application of our probe is in micro-imaging application. This chapter will
explore state of the art of local and non-local microscopes for magnetic imaging as
well as weighing up their advantages and limitations. We will present more in detail
the optical magnetic microscopy, because its spatial resolution is comparable to the 3D
probe.
3.2 Microscope for magnetic imaging
By definition a microscope is an instrument allowing the visualization of small objects
that are difficult to see by the naked eye. Microscopy probes allow surface mapping
and the extraction of properties like topography, electronic density, interaction force,
tunnel current etc. The major types of microscopy (optical, electronic, near-field ...)
are similar in their basic principle, it is a result obtained by an interaction between an
object and an energetic radiation. On the other hand, these different techniques are
distinguished by their spatial resolution level (some Angstroms for near-field
microscopy, 250 nm for optical microscopy) and by their magnification capacity
(about 1000 for optical microscopy, about 100,000 for electron microscopy).
Microscopes can be classified as follow: microscope that takes directly an image of
sample (wide field optical microscopes and transmission electron microscopes) or not
directly by scanning the sample to be analyzed via a scanning point (confocal optical
microscopes, scanning electron microscopes and scanning probe microscopes).
Scanning magnetic microscopy [110], scanning Hall probe microscopy [111, 112],
scanning superconducting quantum interference device microscopy [113], Lorentz
transmission electron microscopy [114], Kerr microscopy [115], and others are used to
study magnetic structure and small magnetic elements in many domains [116, 117,
118] Scanning techniques offer a good spatial resolution (nanometric) and good
magnetic resolution (for example up to femtoTesla for SQUID [119] and about
microTesla resolutions for Hall probes [120]). But, these techniques don't allow the
rapid acquisition of entire sample surface images. Microscope that takes directly an
image (global microscope) has fairly poor spatial resolution compared to a scanning
microscope. This resolution of microscope depends on the size of sensors used and
also of the distance sensor-sample. For example, Magneto-optical (MO) imaging have
spatial resolution at micrometer range, and magnetic resolution in the order of 0.1mT.
Table 3 shows a comparison of microscopy:
Measured Lateral resolution Physical Quantity
technique (nm)
Lorentz microscope Electron 10nm stray field
Spin-polarized scanning Transport atomic resolution Spin polarization
tunneling microscopy of sample
(SP-STM)
MFM (Magnetic Force Magnetic Force 10 stray field
Microscopy)
SMRM (Scanning Transport micrometers stray field
Garnets films, Kerr Optical Sub-micrometers Stray field /
Nitrogen Vacancy (NV) Optical ~50 stray field
Table 3- Comparison between different types of Microscopy [121, 122]
Finally, the efficiency of different microscopes techniques depends on application
domain and their limits in terms of spatial resolution and field sensitivity. In the
following, we detailed some microscopy cited in Table 3.
3.3 Transmission electron microscopes (TEM)
The transmission electron microscopes (TEM) has series of electromagnetic and
electrostatic lenses to focus a high energy beam of electrons on a sample. In the TEM
microscope, to produce an image, the electrons pass through the sample, analogous to
basic optical microscopy [123]. This microscope can attend a resolution of 0.1 nm. It
is very efficient to help researcher in cancer research, virology, materials science as
well as pollution, nanotechnology and semiconductor research. For example, it allows
to obtain three-dimensional views of the virus (20-300 nm) and the DNA (2 nm in
width) samples [124]. The Lorentz microscopy based on TEM is a useful tool to get
information of both physical structure and micromagnetic structure of the thin samples
at nanoscale. It allows exploring the magnetic properties of the specimen. There are
two modes in Lorentz microscopy: the Fresnel mode, and the Foucault mode [125].
The specimen should be less than 100 nm thick, so the electrons can pass through it.
This thickness limitation is the main drawback of TEM microscope.
Figure 3.1- In situ TEM study of magnetization reversal in Co80Ir20 film: Fresnel images in a hysteresis
sequence on the hard axis in plane of the film [126].
3.4 Local probe microscopy
Local probe microscopy has become an essential research tool in the last few decades,
particularly in the development of nanotechnology and semiconductor. Local probe
microscopy has started with the invention of tunneling microscopy (Binnig, 1982 -
Nobel Prize in 1986), and the atomic force microscopy (Binnig, 1986). The principle
function of this microscopic technique consists of scanning the surface using a probe
(tip) placed to the proximity of the surface and measuring the interaction between the
probe and the surface of the specimen. It exists currently a variety of scanning probe
microscope able to detect diverse properties as the stray field emitted by a magnetic
sample (MFM), ferroelectric properties (PFM)…
The atomic size of the tip (the tip is ideally a cone ending by single atom) allows to
obtain atomic size resolution images, which not possible with traditional microscopy.
One difference between atomic force microscopy and competing technologies such as
optical microscopy and electron microscopy is that AFM does not use lenses or beam
irradiation. Therefore, it does not suffer from a limitation in spatial resolution due to
diffraction and aberration. The local microscope allows obtaining indirectly the image
thanks to the interaction between the tip and the surface. Image obtained by the local
microscopy is put together by slowly scanning sample surface. Scan areas range from
a few to 200 micrometers. In local microscopy, piezoelectric scanning moves the
sample in an x, y and z directions, wich allow to precise sensor-to-sample distance.
3.4.1 Scanning Tunneling Microscopy (STM)
Scanning Tunneling Microscopy (STM) discovered by Binning measures surface
electronic states using a tunneling current between the probe tip and a sample surface.
The tip is formed by one single atom at its extremity. The distance between point and
sample is few Angstroms (Figure 3.2). The electrons are forced to pass from the tip to
the conductive sample through the insulating layer composed by air or vacuum and are
measured by an "ammeter". The electric current is therefore a tunnel current that
decreases exponentially with the distance between the tip and the specimen. STM
allows determining the morphology and the density of electronic states of conductive
or semi-conductive surfaces with atomic spatial resolution. Among STM microscopy,
the spin-polarized scanning tunneling microscopy (SP-STM) can provide information
on magnetic domain and electronic structure with extremely high spatial resolution.
SP-STM is sensitive to magnetic local density of states of the sample surface, the spin
polarization of the magnetic surface.
Figure 3.2- Schematic view of an STM (left part) Schematic illustration of the
SP-STM method [127] (right part)
We show in Figure 3.2 (right part) that the tunneling current depends on the relative
magnetization orientations between tip and sample spins.
The major limitation of STM is that the technique is only efficient for conductive
samples, in particular for applications in semiconductor physics and microelectronics
[128, 129]. Historically, this technique served as the groundwork for the subsequent
advancement to Magnetic Force Microscopy (MFM).
3.4.2 Magnetic force microscopy (MFM)
Binnig has continued innovations of local probe instruments in 1986 by demonstrating
the ability to scan the surface of the sample, whether it is conductive or not. A new
microscope named Atomic Force Microscopy (AFM) allows the measurement of
forces between the surface of the sample and the tip [130, 131, 132]. AFM allows the
analysis of all types of surface of materials in air, in liquid medium or in controlled
atmosphere. Depending on the forces in presence, the AFM can take images of
different physical or chemical surface information such as: mechanical, magnetic,
electrostatic properties, etc. Atomic force microscopy provides the topography of a
surface of about: 50 × 50 μ𝑚2, with vertical resolution less than one nanometer and
lateral resolution of the order of nm [131].
The tip is placed in the end of a flexible cantilever, which acts as a spring (Figure
3.3.a). The deflection and torsion measurement by optical or tuned fork technics of the
cantilever allows the measurement of forces between the tip and the sample (Figure
3.3.c).
forces dependent of various distance (c) [131, 133]
The choice between atomic microscopy types depend on the nature of controlled
samples and the predominant forces between the tip and the sample.
The Magnetic Force Microscope (MFM) is a variation of the AFM. MFM has the
same form of AFM microscopy, but the tip is coated with ferromagnetic deposit
(FeNdB, CoPtCr, CoZrNb, Co or Ni). Magnetic force microscopy (MFM) has been
widely discussed in literature [134, 135, 136] as it is one of the most used techniques
for magnetic field mapping to measure weak forces. MFM leads to magnetic
interactions between the tip and the stray filed emitted by the analyzed sample. MFM
is used to study magnetic domains smaller than 100nm [137].
and I+ indicating the direction of current flow [138]
One of MFM's disadvantage is the difficulty of extracting direct quantitative
information from images as the signal is proportional to emitted field gradients and is
strongly dependent on the magnetic tip properties. In addition, forces of the sample
topology can interfere with magnetic force signals. In addition, the response frequency
in MFM microscopy is limited by the mechanical resonant frequency of the force
sensor [136, 139, 140].
3.4.3 Scanning magnetoresistance microscopy (SMRM)
MFM does not deduce the magnetic field value because it is difficult to analyze
quantitatively the field detected by traditional MFM. The integration of self-sensing
cantilevers based on Hall effect sensor, SQUID or magnetoresistive sensor (MR) in
atomic force microscopes propose a solution to overcome these issues [141, 142, 143,
144]. Scanning magnetoresistance microscopy (SMRM) [145, 146] provides
quantitative measurement with high spatial resolution of the magnetic fields emanating
from sample surfaces. SMRM have been achieved micrometer and sub-micrometer
resolutions on magnetic nanomaterials by attaching a commercial read and write head
of conventional hard disk drives on a raster scanning setup [147].
Relatively, the local microscopy has short lateral scanning range (order of hundreds
micrometers). The mechanical motors used in scanning magnetoresistance probe
microscopy allows users to perform a fast, high resolution mapping of magnetic field.
The magnetoresistance probe microscopy allows to overcome the limitations of short
lateral scanning range of local microscopy. The scanning volume of this type of
microscopy is more than ten centimeters to the detriment of sensor-to-sample distance
controlled, so it is not able to precise exactly the sensor-to-sample distance [148].
3.4.4 Nitrogen Vacancy (NV) centers
Scanning probe microscopy using an artificial atom Nitrogen Vacancy (NV) situated
in a diamond (Figure 3.5) is suitable for physics, chemistry, life sciences, earth and
planetary sciences. The NV center has proved that it can be integrated in a local probe
microscope for magnetic field imaging [149, 150]. Some of its key advantages are its
high spatial resolution and high sensitivity to magnetic field. NV is a nitrogen-gap pair
in a monocrystalline carbon. The photoluminescence emitted by the NV exhibits a
strong variation when it is excited by a resonant radio-frequency field. In the presence
of magnetic field along the NV axis, the resonant frequency varies about 100 MHz per
10 Gauss (10 Oe). This variation is linear for fields up to 100 Gauss. These properties
are known and are already used in several ways, but a true magnetic microscope using
them did not exist yet [151]. This type of microscope is based on AFM microscope. A
single diamond with a size of 20 nm is glued to AFM tips. The spatial resolution by
this microscope is about 100 nm [151]. It allows quantitative and non-invasive
measurements of a magnetic field at ambient conditions, by the calculation of shifted
resonant frequency. The disadvantage of this microscope is that the calculation is non-
direct. Furthermore, it is less efficient than the microscope based on magnetic sensors
when dynamic pressure is applied.
Figure 3.5- Example of NV-center Nanodiamond used for thermos dynamic
application [152]
3.5 Optical Microscope
The first and most common microscope invented is the optical microscope, which uses
light to pass through a sample to produce an image. The optical microscope is an
optical instrument containing a system of lenses producing an enlarged image of a
sample placed in the focal plane. Optical microscopes have refractive glass
(occasionally plastic or quartz), to focus light on the eye or onto another light detector.
The spatial resolution limit of optical microscope is 250 nm [123]. Recently, CMOS or
charge-coupled device (CCD) technology sensors similar to those used in a digital
camera, are integrated to optical microscope to view directly the object through the
eyepiece to obtain digital images. We present some types of optical microscopes for
magnetic imaging like Magneto-optical devices based on Garnets films.
3.5.1 Magneto-optical devices based on garnets films
Past research has been devoted to exploiting magneto-optical properties of
ferromagnetic material. Magneto Optical Imaging film (MO) determines locally and in
real time the distribution of magnetic field. Magneto-optical (MO) effects result from
the interaction of light with magnetic material. This interaction is undertaken to
modify the polarization state of light. Magneto-optical devices have been conceived
from garnet films due to their excellent MO properties (large Faraday effect), and used
in a range of applications in nanophotonics, integrated optics, communications and
imaging [153].
Figure 3.6- Schematic drawing of the Faraday effect. The sample is illuminated by linearly polarized light.
Polarizer and analyzer are set in crossed position. The light enters the magneto-optically active layer (MOL)
and is reflected at the mirror layer. In areas where no flux is present in the sample, no Faraday rotation takes
place hence these regions stay dark in the image. In all areas where flux is present, the Faraday rotation
changes the polarization plane so that this light is able to pass through the analyzer thus leading to bright
areas. In the drawing, a reflection angle is shown for clarity whereas in the experiment we have perpendicular
incident light. [154]
The Kerr magneto-optic effect has been used to extract information of materials
magnetization [155, 156]. The magneto- optic Kerr effect has been used to develop
several scanning Kerr microscopies that have submicron spatial resolution [157].
Garnets are magnetic materials which present a strong Kerr effect more than scanning
Kerr microscopies , and this why they are used (Figure 3.6). When crossing an
optically active medium, the plane of polarization of light rotates under the action of
external magnetic field (Figure 3.6). During a double crossing of the optical active
medium (garnets) by the light, the magneto-optical effects are added. This property is
exploited in MO imaging, which proceeds by the analyses of the light reflected by the
garnet put in contact with the magnetic layer to be analyzed [158, 159]. If UV or blue
range visible light sources are used, sub-micron spatial resolutions can be attained
[160]. Figure 3.7 shows two examples of MO images.
Figure 3.7- Magneto-optical images used to calculate distribution of electric field at 4.2 K in the superconductor.
The image shows such distributions for a YBCO film with a grain boundary [161]
There are two main types of Faraday effect garnets used for MO imaging:
 Europium, EuS and EuSe garnets are used at low temperature (T ≤ 20K) [162],
and have a measuring range of several Teslas.
 YIG (Yttrium Iron Garnet) garnets have a strong Faraday rotation
(3.8°/cm/kOe) and can be used at temperatures from 4 K to 300 K. On the
other hand, the measuring range is only a few thousands Gauss.
Typical detectivity of a Garnet film is few µT.
Magnetic microscopies like MFM (Magnetic Force Microscopy), EFM (Electric Force
Microscopy), and CFM (Chemical Force Microscopy) are local microscopies [163].
They have high spatial resolution (into nanometer range) and field sensitivity. In
general, MFM does not deduce the magnetic field value because it is difficult to
analyze quantitatively the field detected by traditional MFM. The integration of self-
sensing cantilevers based on Hall effect sensor, SQUID or magnetoresistive sensor
(MR) in atomic force microscopes propose a solution to overcome these issues.
The local microscopes mentioned above are used to scan small surface. The ability to
detect and map the magnetic field at nanometer scale is essential in many areas of
science. The disadvantage of MFM is the unsuitability of measuring samples for high
magnetic fields with micrometric and millimeter surfaces. Many probes are developed
to evaluate magnetic properties in large area of samples [164]. For example, the
magnetic field microscopy of rock samples using giant magnetoresistance [73]. The
3D probes developed during the PhD (will be described in Chapter 4) aim to evaluate
magnetic properties in large area of samples. The use of high sensitive sensors such as
GMR sensors can improve the magnetic field sensitivity with high spatial resolution
(into the micrometric range). The GMR sensor has a high field sensitivity for DC field
detection without the need of any external bias magnetic fields, and ensures
quantitative magnetic field measurements. GMR sensors can also be used in detecting
of AC signals in wide frequency ranges up to a few GHz. The limitation is given by
the ferromagnetic resonance of the free layer. 3D probes developed in this thesis has a
spatial resolution compared to that of optical microscopy. In addition, this probe has
more important measured magnetic range than the MO microscopes described above.
PART 2: 3D PROBE AND APPLICATIONS FIELDS
Chapter 4. 3D PROBE
Many studies have already used the characteristics of magnetic sensors to improve the
performance of scanning magnetic probes [34, 41, 165]. The majority of oldest probes
are sensitive to two components of magnetic field at maximum. Oka et al. has
introduced a 3D EC probe using 3-axis measurement with coils allowing to detect
simultaneously three field components, however detection cannot be realized at the
same measurement point [109]. The probes developed during this thesis allow to
complete and to improve the results obtained through these previous works. In this
chapter, we will demonstrate the viability of using four orientations of GMR sensors to
develop a 3D probe allowing simultaneously reconstruction of three components of
magnetic field at the same measurement point. Different types of GMR sensors have
been used for the 3D probe developing. The first GMR sensor has sensitivity of
19 V/V/T and has a maximal working field up to ±15 Oe. The second type of GMR
sensor has a larger magnetic range than the first one to the detriment of its sensitivity.
This sensor has sensitivity of 3.5 V/V/T and it has a maximal working field of
±200 Oe (± 20 mT). These 3D probes are interesting for magnetic imaging of
magnetic objects with arbitrary shapes, for evaluating of magnetic properties with
micrometric spatial resolution and they are interesting to detect small variations of
magnetic field caused by imperfections in the wires or magnetic materials. Also, eddy
current excitation combined with our 3D receiver probe offers new advantages to
characterize and detect the defects.
In this chapter, we will give the description of 3D probes. The reconstruction method
of three components of magnetic field at the same measurement point will be also
described in this chapter. We report in detail the characteristics of the GMR sensors
used in developing of the 3D probes: stack, sensitivity, noise measurement and
detectivity. We will start by the description of electronic circuit associated to the 3D
probes for signals amplification.
4.2 Electronic Circuit
We have developed an electronic circuit composed of three parts described in Figure
4.1. This electronic circuit is based on a very low noise instrumentation preamplifier
adapted to the resistance range of our GMR sensors, typically about 80 and 200 Ohms.
The electronic circuit is used for each GMR sensor, thus we use four electronic circuits
for each probe.
Figure 4.1-Electronic circuit
The first part is used to feed the circuit (Figure 4.2). To avoid electrostatic discharge,
the power supply is not connected directly to the GMR. With 'V+' is DC voltage, the
Regulator "1" fix the input voltage " 𝑉𝑖𝑛 " to 9V. The Regulator "2" fix the input
voltage to -9V.
Figure 4.2 Alimentation part of electronic circuit. Regulators 1 and 2 +9V are used for preamplifier INA103, regulator 3 is
used for Wheatstone bridge
The output voltage V0 at "Regulator 3" in Figure 4.2 is equal to:
V0 = Vref (1 + 𝑅4⁄8) + 𝑅4. 𝐼𝐴𝐷𝐽 (4-1)
The Wheatstone bridge configuration in which the GMR sensor is inserted (Figure
4.3) is used to remove the DC component. The diodes are used for circuit protection,
by diverting the discharge current into circuit supply circuit [166]. We relied two
diodes in parallel to GMR (each one has inversed orientation of the other). Thus, the
lower resistance of diode allows better power dissipation. We choose the value of
R2 = R3 ≥ 10 × 𝑅𝐺𝑀𝑅 , to ensure that the current is the same in both branches of the
bridge. The voltage at GMR sensor 𝑉𝐺𝑀𝑅 is equal to:
𝑉 .𝑅𝐺𝑀𝑅
𝑉𝐺𝑀𝑅 = 𝑅 0 (4-2)
𝐺𝑀𝑅 +𝑅3
Figure 4.3- Full Wheatstone bridge circuit relied to INA103
The full bridge is associated with low noise preamplifier "INA 103" to increase the
gain of useful signal. The INA 103 is fed by the out of regulators "1 and 2" (± 9 V).
The noise of "INA 103" is of 1.2 𝑛𝑇/√𝐻𝑧 for a gain of 500. The output voltage signal
𝑉𝑜𝑢𝑡 is equal to :
𝑉𝐺𝑀𝑅 = 𝐺𝐼𝑁𝐴103 . (𝑉 + − 𝑉 − ) (4-3)
With 𝐺𝐼𝑁𝐴103 is the INA103 gain.
Figure 4.4- 3D probe with electronic circuits
4.3 Experimental set-up
4.3.1 Experimental set-up for DC signal
Figure 4.5- Experimental set-up for DC signal.
Experimental set-up consists of test desk, where the sample can be moved horizontally
along the X and Y directions. The distance between sensor and sample is set by
moving the sensor vertically down until contact. The output voltage of the
preamplifier is connected to a commercial voltage amplifier. This low noise amplifier
has 4 nV/√Hz input noise, 1 MHz bandwidth and variable gain from 1 to 50,000 with
filters used to reduce the noise intensity. For DC signal measurement, we use low-pass
filter (LPF) at 30 Hz to avoid 50 Hz electrical network perturbations. We use a typical
additional gain of 20, resulting in a total chain gain of 10000. The results are displayed
on a PC using an acquisition system (Figure 4.5). Noise measurements have been
realized with full experimental set-up, no parasitic noise has been identified.
4.3.2 Experimental set-up for AC signal
The experimental set-up used for AC signal measurement (Figure 4.6), is comparable
to the experimental chain used for DC signal measurement. We add a frequency
generator to excite the emitter and the output signal is demodulated by a SR830 DSP
Lock-In Amplifier that simultaneously displays the magnitude and phase of a signal. A
Mux (multiplexer) is used to select the signals received by the sensors. SR830 uses
digital signal processing (DSP) to replace the demodulators, output filters and
amplifiers found in conventional lock-in and has a noise about 6 nV/√Hz.
Figure 4.6- Experimental set-up for AC signal.
4.4 Description of 3D probe formed by four GMR
Figure 4.7 shows the details of three axis probe based on GMR sensors. Four GMR
sensors are fixed on mechanical support in order to reconstruct the three components
of magnetic field. Each of these sensors is oriented at 45° one in front of another. The
support has been fabricated by 3D printer by Gerald Le Goff.
Figure 4.7- Description of the probe (left part), Right: GMR sensors (right part)
Exploiting the signals received from each sensor of the probe and the sensitivity
matrix of these sensors, we can reconstruct the three components of the field.
GMR sensors C1 and C2 are mainly sensitive in 'x-z' plan, and the sensors C3 and C4
are mainly sensitive in 'y-z' plan (Figure 4.8). This sensors conception will be used to
reconstruct the three components of magnetic field.
Figure 4.8- GMR sensors position in three axis
First step is to calculate the sensitivity of each sensor in three axis in order to create
the sensitivity matrix (𝑆𝑖𝑗 is the absolute value of sensitivity of 'i'sensor according to
direction 'j'). For applied voltage V=1V to each GMR sensor, sensitivity matrix S
(V/V/T) is:
𝑆1𝑥 𝑆1𝑦 𝑆1𝑧
𝑆= (4-4)
Each sensor has a sensitivity in two axis, with sensitivity theoretically equal to zero in
third axis (Figure 4.8). So S becomes
𝑆1𝑥 0 𝑆1𝑧
𝑆= 𝑆3𝑦 (4-5)
0 𝑆3𝑧
( 0 𝑆4𝑦 𝑆4𝑧 )
Figure 4.9 shows the distribution design of GMR sensors according to x-z plane and to
y-z plane.
(right part)
𝑉𝑖 represents the output signal amplitude acquired by the sensor 'i'. 𝐻𝑗 is the magnetic
field component according to the direction 'j' and 𝛼 = 45°.
As 𝑉𝑖 = 𝑆𝑖𝑗 . 𝐻, we conclude the following numerical formulas:
𝑉1 = 𝑆1𝑥 . 𝐻𝑋 . 𝑠𝑖𝑛(𝛼) + 𝑆1𝑧 . 𝐻𝑍 . cos(𝛼) (4-6)
𝑉2 = −𝑆2𝑥 . 𝐻𝑋 . 𝑠𝑖𝑛(𝛼) + 𝑆2𝑧 . 𝐻𝑍 . cos(𝛼) (4-7)
𝑉3 = −𝑆3𝑦 . 𝐻𝑦 . 𝑠𝑖𝑛(𝛼) + 𝑆3𝑧 . 𝐻𝑍 . cos(𝛼) (4-8)
𝑉4 = 𝑆4𝑦 . 𝐻𝑦 . 𝑠𝑖𝑛(𝛼) + 𝑆4𝑧 . 𝐻𝑍 . cos(𝛼) (4-9)
With α = 45°, cos(45°) = 𝑠𝑖𝑛(45°) = , the equations presented above allow to
obtain the following matrix representation:
𝑉1 𝑆1𝑥 0 𝑆1𝑧
1 −𝑆2𝑥 0 𝑆2𝑧 𝐻𝑋
( 2) = −𝑆3𝑦 ( 𝐻𝑌 ) (4-10)
𝑉3 √2 0 𝑆3𝑧
𝐻𝑍
𝑉4 ( 0 𝑆4𝑦 𝑆4𝑧 )
𝑉 = 𝑆𝑒 . 𝐻 (4-11)
So the matrix '𝐻' that contains the three components of magnetic field is equal to:
𝐻 = 𝑆𝑒 −1 . 𝑉 (4-12)
4.5 GMR sensors used for 3D probe development
Developed probe is based on giant magnetoresistance (GMR) sensor operating at room
temperature. The GMR sensors used in this work are spin valves with length of 30 µm
and width of 3 µm. The GMR of spin valve type is formed of three layers: free layer,
spacer, pinned layer (hard layer). The three layers are deposited according to the
scheme of Figure 4.12 and Figure 4.19. It has high sensitivity and is effective for small
object detection due to its micron size.
GMR sensors used in PhD thesis have yoke shape (Figure 4.10). This form is
interesting for magnetic noise suppression and domain walls elimination [6]. Magnetic
domains are generally responsible of 1/f noise increasing and of the presence of RTN;
also, they affect high-frequency noise near the resonance frequency.
Figure 4.10- GMR sensor in a yoke-shape before cutting
Magnetic instabilities are moved to the corners of yoke shape, thus, the magnetization
field is aligned without instability at the long part of free layer. Simulation of magnetic
moment distributions is shown in Figure 4.11 [13].
Figure 4.11- Distribution of magnetic moments in free layer with a yoke shape structure.
The GMR has been fabricated by standard UV lithography technics, means ion bean
etching, deposition by sputtering of Ta/Cu/Ta contacts and lift off. The full structure is
then covered by 200 nm Al2O3 passivation/protective layer. Elodie Paul has
performed the process.
We will present below the types of GMR sensors used in 3D probe.
4.5.1 Type 1 GMR sensors
One of the 3D probes developed in the PhD uses four type 1 GMR sensors. It is
patterned in a stack of the following composition:
Ru(3)/Ta(3)/PtMn(18)/CoFe(2)/Ru(0.85)/CoFe(2.1)/Cu(2.3))/CoFe(1.5)/Ta(0.1)/NiFe(
3.5)/Ta(3). The thicknesses are in nanometers. The role of the Ta layer is to decouple
the crystallinity of the CoFe layer and the NiFe layer.
Figure 4.12- Spin valve stack of type1 GMR sensors. The thicknesses (X) are given in nm
GMR's hard layer (reference layer) is composed of PtMn/CoFe/Ru/CoFe, where PtMn
is anti-ferromagnetic coupled by exchange bias to the CoFe - ferromagnetic layer
[167]. Ruthenium (Ru) is chosen with optimized thickness to create an anti-
ferromagnetic coupling between the two CoFe ferromagnetic layers, based on theory
of RKKY interaction [168, 169]. This structure is call a synthetic antiferromagnetic.
The main advantage of CoFe/Ru/CoFe coupling is the cancelation of static field
created by the hard layer on the free layer in microscopic structures. The free layer is
an association of CoFe with a soft ferromagnet NiFe having a magnetization which
rotates easily with an external applied field. The spacer (Cu) is a nonmagnetic layer of
copper. The spacer makes the free and the hard layers magnetically independent.
Tantalum constitutes the cap layer and the seed layer when it is respectively on the top
and the bottom of the GMR.
The resistance of type 1 GMR sensors used in 3D probe is about 180 Ohms. We put
the GMR sensors in the Helmholtz coils to obtain the sensitivity curve according to the
three axis. The applied voltage to each GMR sensor is 0.35 V.
The response of GMR sensors to magnetic field collinear (according to z-axis) with
the pinned layer magnetized orthogonally to the reference layer orientation is shown in
Figure 4.13 – The response of four type 1 GMR sensors as function of magnetic
field applied along their z-axis
Figure 4.13 shows that the four GMR sensors are sensitive to magnetic field according
to z-axis. Also, we show that the slope (that equal to the sensitivity) of the four GMR
responses according to z-axis is positive and has the same sign (in agreement with
theoretical results presented in paragraph 4.4).
The response of GMR sensors to magnetic field collinear (according to y-axis) with
the pinned layer is shown in Figure 4.14.
Figure 4.14- The response of four type 1 GMR sensors as function of magnetic
field applied along their y-axis.
Figure 4.14 shows that the GMR sensors C3 and C4 are sensitive to magnetic field
according to y-axis and that the GMR sensors C1 and C2 are less sensitive to y
magnetic field component. However, the response of C1 and C2 is not negligible,
typically 1/6 of the response along the normal axis and hence, the sensitivity matrix
has to integrate that response. Also, we show that the slope of C3 response according
to y-axis is negative, and the slope of C4 response according to y-axis is positive (in
agreement with theoretical results presented in paragraph 4.4).
The response of GMR sensors to magnetic field collinear (according to x-axis) with
field applied along its x-axis.
according to x-axis and that the GMR sensors C3 and C4 are less sensitive to
x magnetic field component. Also we show that the slope of C2 response according to
x-axis is negative, and the slope of C1 response according to x-axis is positive (in
agreement with theoretically results presented in paragraph 4.4).
Figure 4.13, Figure 4.14 and Figure 4.15 show that type 1 GMR sensors can measure
field intensities between ± 15 Oe. We should be able to measure a larger range with a
well centered response of the curve. The small offset of the field is due to a residual
ferromagnetic coupling between the free layer and the hard layer. Beyond this
maximum field, the sensor response becomes nonlinear. These experimental
sensitivity results of the 3D probe based on type 1 GMR sensors constitute a validation
of theoretical results presented in paragraph 4.4.
Finally, the sensitivity matrix Se (V/V/T) of the 3D probe based on type 1 GMR
sensors is:
𝑆𝑒 = ( −17.1 2.3 18.2 )
2.1 −16.8 18.1
The sensitivity matrix Se in (%/mT) is:
𝑆𝑒 = ( −1.71 0.23 1.82 )
0.21 −1.68 1.81
Low frequency noise
Despite that the yoke structure stabilizes magnetically the free layer of GMR, there
may stay magnetic domains in the free layer, probably caused by the quality of the
lithography. Figure 4.16 shows the voltage noise (voltage fluctuations) of four type 1
sensors in a time interval. This figure shows the proportional relation between the
level of the noise and the 𝑉𝐺𝑀𝑅 .
Figure 4.16- Voltage noise of four type 1 sensors with 𝑉𝐺𝑀𝑅 = 0 mV, 100 mV, 300 mV and 1V
In time domain, we clearly see to states of the sensors. At low voltage typically
300mV, sensors are exhibiting a 1/f noise whereas at large voltage, they exhibit an
additional random telegraph noise. This is confirmed by the spectral analysis of the
noise of each sensor.
Noise spectral density and detectivity
The detectivity is the magnetic field level detected by the sensor with SNR equal to
one and for frequency band of 1 Hz. As C1 and C2 are sensitive in x-z plane, the
detectivity of these sensors according to y-axis is approximately equal to zero. It is
equal to zero according to x-axis in the case of C3 and C4 sensors. We will show the
noise level of four type 1 sensors.
Figure 4.17-Noise spectral density in V ⁄ √Hz of four type 1 sensors with 𝑉𝐺𝑀𝑅 = 0 mV, 100 mV, 300 mV
and 1V. Due to the appearance of RTN noise, the maximum voltage to be used for these sensors is below
1V.
Spectral density measurements confirm first analysis. Up to 300 mV, we have a 1/f
noise with a cutoff at several kHz, which is usual for the size of GMR. At 1 V, C2 and
C4 are exhibiting also a classical 1/f noise up to 10kHz. The drop at 3 kHz is due to
the low pass filtering of the noise measurement setup. C1 and C3 are exhibiting a
bump followed by a 1/f² decrease which is typical of a RTN noise with a characteristic
frequency of few kHz. This RTN has a magnetic origin and it disappears when the
GMR is saturated. Figure 4.19 shows the detectivity or field equivalent noise 𝑇/√𝐻𝑧
𝑉 𝑉
of four type 1 sensors calculate as 𝑁𝑜𝑖𝑠𝑒 ( )/𝑆 (𝑇 ).
√𝐻𝑧
Figure 4.18-Field equivalent noise in T⁄√Hz of fourtype 1 sensors according to z-axis with
𝑉𝐺𝑀𝑅 = 100 mV
Table 4 resumes the detectivity (𝑛𝑇/√𝐻𝑧) value of type 1 sensors according to its
sensitive axis at different frequencies with 𝐷𝑖𝑗 - detectivity of 'i' sensor according to
direction 'j'.
f = 10 Hz f = 100 Hz f = 1 kHz
𝐷1𝑥 17 4.5 1.6
𝐷1𝑧 16 4 1.5
𝐷3𝑦 10 4.3 1.7
𝐷3𝑧 9 3.9 1.5
𝐷4𝑦 8 3.4 1.2
𝐷4𝑧 7.5 3.1 1.1
Table 4- Detectivity given in (𝑛𝑇/√𝐻𝑧) of type 1sensors according their sensitive axis with 𝑉𝐺𝑀𝑅 =
For magnetic imaging, we are working at low frequency. For NDT, we work typically
between 1 kHz and several MHz depending on the application and hence it is
interesting to increase strongly the voltage on the GMRs.
The other 3D probes developed in the PhD thesis are based on four type 2 GMR
sensors. They are patterned in a stack of the following composition:
Si/SiO2(500)/Ru(1)/PtMn(15)/CoFe(2)/Ru(2)/NiFe(4.5)/CoFe(1)/Cu(2.3)/CoFe(2)/Ru
(0.85)/CoFe(2.1)/PtMn(18)/CoFe(2)/Ru(0.85)/CoFe(2.1)/Cu(2.3)/CoFe(1)/NiFe(4.5)/
Ru(1.8)/CoFe(2)/PtMn(15)/Ta(5). The thicknesses are in nanometers.
Figure 4.19- Spin valve stack of type 2 GMR sensors. The thicknesses (X) are given
in nm
GMR's hard layer (reference layer) is again composed of antiferromagnetic- synthetic
antiferromagnetic CoFe/Ru/CoFe/PtMn/CoFe/Ru/CoFe structure. The spacer (Cu) is a
nonmagnetic layer of copper. The free layer is an association of CoFe with a soft
ferromagnet NiFe. This stack is more sophisticated. First additional exchange biased
CoFe/PtMn orientated at 90° from the reference layer are added. These pinned layers
are weakly coupled to the free layer by a RKKY coupling. The effect is to force the
free layer to be oriented in a specific direction at zero field to destroy the hysteresis
and to insure a linear response on a controlled field region. The second change is to
create a dual GMR in parallel with shared reference layer. The first free layer (top) is
ferromagnetically coupled to the pinning layer whereas the second free layer (bottom)
is antiferromagnetically coupled to its pinned layer. This stack insures a wide linear
range and a symmetric response. In order to achieve the proper orientation, the stack is
annealed at 300°C with 3 different fields orientation, one at 1T and the second about
50mT.
The resistance of type 2 sensors used in 3D probe is about 80 Ohms. The applied
voltage to each GMR sensor is 0.17 V.
field applied along its z-axis
Figure 4.20 attests that the four GMR sensors are sensitive to magnetic field according
to z-axis. Also we show that the slope of the four GMR responses according to z-axis
is positive (in agreement with theoretical results presented in paragraph 4.4). For an
unexplained reason, C3 and C4 are presenting a magnetic offset, which is not present
in the full sheet measurements. It is probably due to lithography process issue.
the pinned layer is shown in Figure 4.21
Figure 4.21- The response of four type 2 GMR sensors as function of magnetic field
applied along its y-axis
according to y-axis and that the GMR sensors C1 and C2 are not sensitive to y
magnetic field component. Also we show that the slope of C3 response according to y-
axis is negative, and the slope of C4 response according to y-axis is positive (in
agreement with theoretically results presented in paragraph 4.4). In the case of the
response of C1 and C2 sensors, the two peaks showed are due to the presence of two
pinned layers with opposite couplings. We can see that the response to a field in a
direction perpendicular to the normal response is non-negligible and has to be included
in the sensitivity matrix. Another information can be obtained from these curves. The
coupling between the pinned layer and free layers is 7 mT for the ferromagnetic
coupling and 11 mT for the AF coupling. A fine tuning of the Ru layer is hence
necessary to obtain a better symmetry.
applied along its x-axis
according to x-axis and that the GMR sensors C3 and C4 are not sensitive to x
magnetic field component. Also, we show that the slope of C2 response according to
response of C3 and C4 sensors, the two peaks are also due to the presence of two
pinned layers.
Figure 4.20, Figure 4.21 and Figure 4.22 show that the type 2 GMR sensor can
measure field intensities up to ±20 mT. Beyond this maximum field, the sensor
response becomes nonlinear.
𝑆𝑒 = ( −2.72 −0.38 2.80 )
The sensitivity matrix Se in (%/mT) at 0 external field is:
0.27 −0.042 0.28
𝑆𝑒 = ( −0.27 −0.038 0.28 )
0.044 −0.31 0.31
0.037 0.34 0.32
Figure 4.23 shows the low frequency noise (voltage fluctuations) of four type 2
sensors in a time interval of one second. We see that the noise level of the type 2
sensors is lower than the type 1 sensors. In particular, we do not see RTN noise which
can be explained by the strength of the pinning layer and thus to magnetic domain
fluctuation stabilizations.
Figure 4.23- Low frequency noise in µV of four type 2 sensors with 𝑉𝐺𝑀𝑅 = 0 mV, 100 mV, 300 mV
and 1V
Detectivity
We will show the noise level of four type 2 sensors.
Figure 4.24- Noise spectral density in V⁄√Hz of four type 2 sensors with 𝑉𝐺𝑀𝑅 = 0 mV, 100 mV, 300
mV and 1V
Figure 4.25 shows the field equivalent noise in 𝑇/√𝐻𝑧 of four type 2 sensors.
Figure 4.25- Noise spectral density in T⁄√Hz of four type 2 sensors according to z-axis with
Table 5 resumes the detectivity (𝑛𝑇/√𝐻𝑧) value of type 2 sensors according to their
sensitive axis at different frequency with 𝐷𝑖𝑗 is the detectivity of 'i' sensor according
to direction 'j'.
𝐷1𝑥 40 14 7
𝐷1𝑧 36 13 6
𝐷2𝑥 29 12 5.5
𝐷2𝑧 25 10 4.7
𝐷3𝑦 29 10 6.7
𝐷3𝑧 25 9 6
𝐷4𝑧 22 7.8 5
Table 5- Detectivity given in (n𝑇/√𝐻𝑧) of type 2 sensors according to its sensitive axis
with 𝑉𝐺𝑀𝑅 = 0.1 V
In terms of detectivity type 2 probe is only a factor of 2 worse at low frequency than
type 1 probes but allows a factor 10 in terms of field range. At high frequencies, in the
thermal noise the type 1 probe becomes better than type 2 and ultimately we find a
ratio of detectivity related to the ratio of sensitivity as expected. Hence, we will prefer
to use type 1 for NDT of small defects in the MHz range whereas type 2 can be
interesting for low frequency measurements.
More generally, we can optimize the coupling of the pinned layers of type 2
configuration to increase the sensitivity and decrease the linear range for each specific
By using a single GMR sensor, if we want to measure all three components of the
field, we have to take a measurement three times for three different probe-sample
relative orientations. This method increases the error of results to be analyzed. In this
chapter we have presented 3D probes allowing to extract the three spatial components
of the magnetic field at the same point of measurement and simultaneously. We have
exploited the small size and the high sensitivity of GMR sensors in the development of
3D probe. In the following chapters, we will see the advantages of 3D probes based on
different types of GMR sensors for magnetic imaging and NDT.
Chapter 5. 3D PROBE IN MAGNETIC IMAGING
This chapter reports on the results obtained by the 3D probe described in Chapter 4.
We have demonstrated the viability of using four orientations of GMR sensors
allowing the reconstruction of three components of magnetic field for 3D magnetic
imaging. In particular, the probe is useful for magnetic imaging of magnetic objects
with arbitrary shapes. This is due to the detection of small variations of magnetic
fields due to the imperfections in the wires or magnetic materials as well as evaluating
magnetic properties with micrometric spatial resolution. We will see in this chapter
that the 3D probes allow the reconstruction of three components of DC magnetic field
emitted by current line, dollar bin and soft steel objet. Also we will show the
performances of 3D probe for susceptibility measurement (AC magnetic signals). For
the detection of magnetic footprint of different shape objects in soft steel mock-up, we
have used a GMR sensor able to detect strong magnetic fields. For that we will use the
3D probes based on type 2 sensors. Other measurements have been performed with
type 1 probes. We will start by the description of measurement set-up.
5.2 Set-ups
Two different setups have been used for magnetic imaging. The first one installed at
LIST. This setup has been used in particular for current line imaging (Figure 5.1). This
experimental set-up consists of test desk controlled by a motor, where the probe can be
moved horizontally along the X and Y directions. The motor has spatial resolution of
0.2 µm in z-plane and 5 µm in x-y plane. The mapping system is controlled by an
extremely easy-to-use-software built on MS Windows platform and LabVIEW. The
3D probe associated to a mechanical support is connected to a voltage preamplifier
and acquisition system to display the results.
Figure 5.1- Experimental set-up at CEA-LIST
A second setup at SPEC has been used for dollar bin, soft steel material and
susceptibility measurements ( Figure 5.2). This experimental set-up consists of test
desk controlled by a motor, where the sample can be moved horizontally along the X
and Y directions. The motor has spatial resolution of 0.2 µm in z-plane and 2 µm in x-
y plane. The 3D probe is connected to a voltage preamplifier and acquisition system to
display the results. For susceptibility measurement (AC signal), we have adjusted the
coil to emit a magnetic field by using a frequency generator to feed this coil, "SR830
DSP Lock-In Amplifier", and a Mux.
Figure 5.2- Experimental set-up at CEA-SPEC
5.3 Study of current line
The current line has been studied with the probe based on type 2 GMR sensors. The
sensor-to-sample distance (lift-off) during scanning operation is 0.7 mm. The scanning
step is 0.1 mm for the x-axis and 0.6 mm for the y-axis. Current line is excited by a
DC current of 20 mA (Figure 5.3). The straight wire consists of 1 mm thick line with a
length of 15 mm. The current flow is parallel to the Y-axis and the scan direction is
perpendicular to the current flow (X direction). A constant voltage V= 1V is applied to
the GMR sensors.
Figure 5.3- Measurement scheme of current line
The estimated magnetic field signal is accomplished by the fundamental physical
relation between magnetic field and current, described by Biot-Savart law:
⃗⃗⃗ × 𝑟
μ0 𝐼𝑑𝑙
⃗⃗⃗⃗⃗
𝑑𝐵 = (5-1)
4𝜋 𝑟 2
Where B is magnetic induction, Idℓ is an element of the current I, the constant 𝜇0 is
the permeability of free space, and r is the distance between the current and the sensor.
Thus, the current can be directly calculated from the magnetic field if we know the
distance that separates the current line and magnetic field sensor [170, 171].
Magnetic field emitted by the current line has a two-dimensional distribution in the x-z
plane. This magnetic field emitted by the current line has a zero value on the y-axis.
Figure 5.4, Figure 5.5, and Figure 5.6 compare the experimental results and the
simulation results obtained on current line.
As the probe moves over the wire, the Hx component of the magnetic field will be
negative, as seen in Figure 5.4. Blue color corresponds to the negative value of the Hx
component. In addition, the measured field amplitude decreases slightly when the
probe moves away from the current line.
The maximum amplitude of magnetic signal calculated with the sensor according to
the x-axis (Hx) by Biot-Savart law is equal to 1.25 µT. The maximum amplitude of the
magnetic signal obtained by the 3D probe in the case of Hx component is equal to
1.2 µT. The Hx component reconstructed by the 3D probe shows a good correlation
with simulated results.
current according to x-axis; simulation results of Hx. component (left part) reconstruction of Hx
component measured by 3D probe. The maximum amplitude is equal to 1.2 μT with SNR of 8.2 dB
The asymmetry in the experimental results may be due to a small gradient of the
laboratory static field of about 50 nT/cm.
The value of magnetic field projection to the y-axis is zero, as shown in Figure 5.5.
The magnetic signal calculated on y-axis (Hy) by Biot-Savart law is equal to zero. The
Hy component is perpendicular to the sensitive axis of the GMR sensor, which
confirms that the magnetic field reconstructed in the case of Hy is almost equal to zero.
Observed residual component of field is about 30 nT. Thus, the error bar of the
accuracy of the reconstruction is about 3%.
current according to x-axis; simulation results of Hy component (left part) reconstruction of Hy
component measured by 3D probe.. The amplitude of Hy is almost equal to zero (right part)
The magnetic field projection to z-axis is in two opposite directions (Figure 5.6). As
the probe moves over the wire, the Hz component of magnetic field will be first
positive, then zero, then negative, as seen in Figure 5.6. Red and blue colors
correspond to the positive and negative values of the Hz component. The maximum
amplitude of magnetic signal calculated with on z-axis (Hz) by Biot-Savart law is
equal to ±1.35 µT for a sample to sensor distance of 700 µm. The Hz component
reconstructed from the measurement with 3D probe is equal to ±1.3 µT, which allows
to estimate the experimental lift off. We still observe a small field gradient due to the
setup.
Figure 5.6- One dimensional (top) and two dimensional scans (bottom) of the magnetic field of a line current
according to x-axis; simulation results of Hz component (left part) reconstruction of Hz component measured
by 3D probe.. The maximum amplitude is equal to 1.3 μT with SNR of 8.9 dB (right part)
5.4 Magnetic imaging of dollar bin
The performance in spatial resolution within a developed probe can be demonstrated
with a magnetic image of a one-dollar bill. US Federal Reserve uses ink that contains
particles of iron oxide that contains natural remnants of magnetization with a
minimum grain size equal to 10 µm. Magnetic ink is used to reduce counterfeiting that
gives the user a sense of security by capturing basic "amateur" counterfeit money bills
[172].
A one-dollar bill has been placed on the test desk. The test desk is moved relative to
the stationary 3D probe using a three-axis (x; y; z) mechanical stage. The distance
between the probe and bill is 400 µm±100 µm. The scan step is 100 µm in the x and
y axis. The magnetic image of George Washington's face on a dollar bill is saturated
at 1 T along the vertical direction (z-axis) before measurements. A constant voltage
V= 1 V is applied to the GMR sensors.Figure 5.7-right shows the result of the Hz
component reconstructed by our 3D probe, which includes a major part of
Washington's face compared with the result obtained by Kletetschka (Figure 5.7-left)
[165]. Figure 5.7-center shows a photo of George Washington's face taken with a
Nikon D7100 camera equipped with a macro lens with a focal length of 90 mm. In this
image, we reach the limit of resolution of the printing process used on the bill.
Kletetschka scanned the dollar bill using a Hall sensor. This sensor is sensitive to the
vertical component of the magnetic field. The z distance between the Hall sensor
probe and dollar specimen is 0.125 mm.
Figure 5.7- Measurement results of George Washington's face in the case of Hz component detection obtained
by Kletetschka [173] (left part). Picture of part of Washington's face in one dollar bill (center part) and
measurement results obtained by developed 3D probe (right part)
The obtained image is a mirror of the real dollar bill. It is compared to results obtained
in [73, 173]. Our results show a good performance to detect the small quantities of
magnetic ferrite material, giving more details of Washington's eye, nose, hair and
neck. We can conclude that the distribution of magnetic ink in the dollar bill is not
homogenous as expected.
In Figure 5.8, a bigger image of George Washington's face obtained by developed 3D
probe is shown. Three magnetic field components are presented.
Figure 5.8- Picture of Washington's face in one dollar bill (a) Magnetic image of a portion of a $1.00 bill scanned
at a working distance of height of 400 µm, Hz component. The gray scale varies from about −300 µT(downward,
black) to +310 µT(upward, white) (b) Magnetic image of Hx component. The gray scale varies from about −80 µT
to +85 µT (c) Magnetic image of Hy component. The gray scale varies from about −65 µT to +60 µT in the case of
Hy (d)
SNR is 38 dB for a normal component of magnetic field Hz, 13 dB for the Hy
component and 11 dB for the Hx. As magnetic field within the dollar bill is oriented to
the z-axis, the amplitude of normal component Hz is higher than that of Hx and Hy
(Figure 5.8). The amplitudes of Hx and Hy components are different to zero due to the
deviation of the induced field in the dollar bill. One interesting aspect is also the large
contrast observed for example in Hx component at the edges, which shows that a part
of the magnetization in the large dark areas (not so dark in Hz) remains with a planar
magnetization.
5.5 Imaging of soft steel objects
The performance of 3D probe in detection of magnetic footprint of different shape
objects in soft steel mock-up has been evaluated (Figure 5.9 and Figure 5.10). The aim
of that study is to evaluate the performances of the probe for large static permanent
fields as the mean field radiated by the object was several milliTesla. The distance
between the objects is 0.5 mm and 1.5 mm. Steel is a metal alloy consisting mainly of
iron and carbon and is processed to withstand mechanical and chemical stresses. The
self-magnetic field in this mockup is very weak. For this reason, two magnets are put
under the mock-up to induce a magnetic field. The steel mock-up shown in Figure 5.9
and Figure 5.10 has an electric conductivity of 5.9 S.m-1.
Figure 5.9- Steel mock-up with different shape objects
This specimen contains objects of different shapes: four circles, snake, square, and
near bars. (Figure 5.10).
Figure 5.10- Steel objet that contains many shapes
To facilitate the study of these objects and to facilitate the analysis of the obtained
results, we have divided the specimen in six parts presented in the table below:
Shape Dimensions
P.1 4 circles 1: dex=5 mm dint=2.4 mm
2: dex=5 mm dint=2 mm
4: dex=5.2 mm dint=2.9 mm
P.2 Snake Length L=36mm
Thickness w=2mm
P.3 Square 9.2 x x6.8 mm²
P.4 5 bars 1.4 x 3.6 mm²,
distance between bars 0.5mm
distance between bars 0.5 mm
During the acquisition, the only movement is within the x–y plane, and the distance
between the 3D probe and the mock-up is fixed at 400 µm. The scan step is 200 µm in
the x and y axis. The magnet emits a non-homogenous field in steel mock-up of
several milliTesla. Magnetic field in the specimen is oriented according to the
z component. The normal component of magnetic field (Hz) obtained with the 3D
probe is shown in Figure 5.11.
Figure 5.11- Hz component, the grey scale varies from −100 µT
(downward, black) to +80 µT (upward, white)
It is observed that the objects are detectable with SNR = 27 dB. The black regions of
the image correspond to an antiparallel magnetic field (the magnetic field emitted by
the mockup is antiparallel to the easy axis of GMR sensor), whereas white color ones
correspond to a parallel magnetic field.
 P.1: Circular shapes are well detected and their diameter could be estimated.
It is observed that a circle number 1 (Figure 5.9) doesn't have the same
magnetic signature (downward amplitude value = -86 µT) as other circles
(upward value: 1= +54 µT, 2= +57 µT and 4= +80 µT). This difference is due
to a non-homogenous magnetic field in the specimen.
 P.2: Snake shape is well detected. The magnetic signature is upwards (white
color) and has a signal amplitude between +40 µT and +60 µT.
 P.3: Square object is well detected with amplitude value equal to +45 µT.
 P.4: Five bars separated with distance of 0.5 mm are well detected,
confirming good spatial resolution of the 3D probe. The magnetic amplitude
values of five bars are between +30 µT and +60 µT.
 P.5: Five bars separated with distance of 0.5 mm are detected. The magnetic
amplitude values of five bars are between -70 µT and 70 µT. The magnetic
signature of bars is in the opposite value (upward and downward value) due
to a non-homogenous magnetic field in the mock-up.
 P.6: Five bars separated with distance of 1.5 mm are well detected. The
magnetic amplitude values of five bars are between - 80 µT and -100 µT.
Magnetic field emitted by the magnets is in the z-axis direction, however the magnetic
field of studied objects undergoes a deviation in the x-y plane of the specimen. Figure
5.12 shows the Hx. and Hy. magnetic field components detected by the 3D probe.
(downward, black) to +40 µT (upward, white) in the case of Hx (left part) the gray scale varies from
about −35 µT to +40 µT in the case of Hy (right part)
We can see that the magnetization of the objects is dominated by internal domains,
which can be imaged by the 3D probes. Each domain orientation creates a positive or
negative stray field, which is highlighted by the patterns done on the surface. The
origin of this large scale inhomogeneity is unclear but may be related to an
inhomogeneity of composition.
5.6 Susceptibility measurement
This part has been removed for confidentiality
In this chapter, we have described the use of 3D GMR probe in magnetic imaging. The
developed probe shows a good detection of magnetic objects with different shapes.
The combination of the responses of the four GMR with the sensitivity calibration
allows us to achieve accuracy along the three directions of fields better than 5%. The
two types of probes have been evaluated. We have demonstrated that by using type 2
probes, we are able to image patterns in strongly magnetized objects.
The main limitation of the present set-up is related to the accurate control of the
distance between the sample and the probes. Ultimately, an automatic system
measuring the local distance should be incorporated in order to use efficiently the
spatial resolution of GMR sensors.
Finally, we have tested the probe to perform susceptibility images of a sample. That
technique should be further developed by incorporating the possibility of scanning the
frequency on a wide range in parallel to the physical scanning. Interest will be to
determine the local susceptibility of materials as function of frequency in order to
characterize the local inhomogeneities.
Chapter 6. 3D Probe in Non-Destructive Testing
Eddy current excitation combined with giant magnetoresistive sensor is interesting in
defect detection that requires working at low frequencies, i.e. detection of buried
flaws, and with high spatial resolution at high frequencies for very small surface
cracks (under 100 µm) detection. In this chapter, eddy current excitation combined
with 3D probe will be presented. This emitter part has been optimized in CIVA
software [174]. We will show theoretical example of 3D probe performances in EC-
NDT with reconstruction of three components of magnetic field. We will see in this
chapter that the 3D probes enable to detect the defects at both surface and sub-surface
levels. We will use the 3D probes based on type 1 sensors to exploit their high
sensitivity (about 20 V/V/T) for defect detection. For subsurface defect detection, the
experimental results obtained in Aluminum and Inconel mock-ups are analyzed and
compared with simulation results. Titanium and Aluminum samples containing small
surface defect (under 600 µm) have been used for probe evaluation for surface cracks
detection. The performance of 3D probe for small surface defects detection in
Aluminum and Titanium mock-ups are compared with simulation results.
6.2 Optimization of EC probe
The design of the probe (the emitter part and the 3D probe) for NDT application has
been optimized with CIVA software.
Figure 6.1 shows the details of the three axis probe with GMR sensors as receiver. For
NDT application, 3D probe described in Chapter 4 is used as receiver and two current
foils are used as emitters (Figure 6.2). 3D probe and the current foils are fixed on
mechanical support. Emitters are placed in horizontal. The GMR sensors detect a low
magnetic field in the absence of defect.
Figure 6.1-3D Probe based on a GMR array receiver and two current foils
emitters
The two current foils used as emitter have the following dimensions: internal length
L = 20 mm, the internal width W = 12 mm and the total width E = 2 mm. A
rectangular hole is placed in the center between two current foils in order to place
receiver. Each current foil has eight layers and each layer has four turns. The current
foils parameters are detailed in Figure 6.2. These parameters have been optimized in
CIVA software.
Figure 6.2- The parameters of two current foils
In order to obtain the field almost equal to zero (according to z component) in the
absence of defect, the emitters are excited in opposite direction (in opposition phase).
Figure 6.3 illustrates the form of current into the two current foils.
Figure 6.3- Schematic of opposite excitation in two current foils emitters
In the case of working with normal (z) component of magnetic field, this method of
excitation avoids the saturation of GMR receiver. This excitation allows to obtain
homogenous and parallel current in the mock-up. Also, it allows to use GMR receiver
with high sensitivity, operating in weak field.
On the contrary, when the foils emitters are excited at the same direction, the GMR
sensors receivers evolve in strong field even in the absence of mock-up, the value of
magnetic field measured by receiver in z-plane is very high.
Figure 6.4 illustrates the magnetic induction in aluminum mock-up for the two types
of excitation current cited above. These simulation results are made using CIVA
software. The applied current is 100 mA. It is shown that the coupling noise is much
lower in the case of the two current foils excited in opposition phase than the case of
excitation in phase (the same direction).
Figure 6.4- Image of 1D z component distribution of magnetic induction reflected by aluminum
mock-up for the two forms of excitations: phase and opposition phase
The optimal distance between the coils has been defined by simulations. Figure 6.5
illustrates the magnetic induction reflected by aluminum mock-up for different
distances between the two current foils.
Figure 6.5- Distribution of z-component of magnetic induction reflected by aluminum mock-up at different distance
between the two current foils with the applied current of 100 mA (left part), 1D z-zone distribution at fixed x and y
positions (GMR position) (right part)
We have chosen the distance between the two current foils equal to 4 mm,
compromise between the coupling noise value and the magnetic induction intensity.
6.3 3D EC probe
6.3.1 Simulation results with 3D EC probe
To validate the operation of 3D probe in the reconstruction of the three magnetic fields
components, we made a configuration in CIVA software similar to 3D EC probe
(Figure 6.6).
Figure 6.6- Configuration realized in CIVA for 3D probe.
Aluminum mock-up (𝜎 = 19.5 𝑀𝑆. 𝑚−1 ) has been used for simulations of 3D probe.
The defect with following dimensions has been studied:
length × width × depth = 5 mm × 0.15 mm × 2 mm. The ligament (the distance
between the flaws and the mock-ups surface) is 0.5 mm. The excitation frequency f =
50 kHz and excitation current of I=20 mA have been chosen. The applied voltage to
GMR receivers is 0.2 V.
The results of reconstruction of Hx component (real part of the signal) are compared
with results of simulations of the case when receiver is placed along x-axis (Figure
Figure 6.7- CSCAN of the defect in aluminum mock-up obtained with receiver placed according to
x- axis (left part), reconstruction of Hx component obtained with 3D EC probe (right part).
The lobe signature in Figure 6.7 corresponds to the flaw position. Hx component has
the advantage of having a monopole signature that is simple to interpret.
The results of reconstruction of Hy component (real part of the signal) are compared
with Hy obtained by simulations of the case when receiver is placed according to y-
axis (Figure 6.8)
Figure 6.8- CSCAN of the defect in aluminum mock-up obtained with receiver placed according to y-
axis (left part), reconstruction of Hy component of magnetic field with 3D EC probe (right part).
The four lobes signature in Figure 6.8 corresponds to the four edges of the flaw. This
configuration has the advantage of having zero direct coupling.
The result of reconstruction of Hz component (real part of the signal) are compared
with Hz obtained by simulations of the probe with receiver placed according to z-axis
(Figure 6.9)
Figure 6.9- CSCAN of the defect in aluminum mock-up obtained with receiver placed according to z-axis (left
part), reconstruction of Hz component of magnetic field with 3D EC probe (right part)
The two lobes signature in Figure 6.9 corresponds to two extremities of the flaw.
Moreover, this configuration has the advantage of having zero direct coupling in the
case of receiver placed along z axis.
The three magnetic field components reconstructed theoretically by the 3D Probe are
in good agreement with the results obtained by depositing sensors according to the
three axis.
6.3.2 Experimental Results
6.3.2.1 Subsurface defects
Aluminum mock-up
Aluminum mock-up with flaws has been used for probe performance evaluation.
Applied current for emitters is 20 mA at frequency at 50 kHz. The applied voltage to
the GMR receivers is 1 V. The distance between the 3D probe and the mock-up is
fixed at 700 µm. The scan step is 500 µm in the x-axis and 100 µm in the y-axis The
CSCAN image highlights detection and reconstruction of the three field components
of a defect of size 5×0.2×2 mm3, located at ligament of 0.5 mm (Figure 6.11, Figure
6.12, Figure 6.13).
Experimental set-up is shown in Figure 6.10. It is based on the acquisition card
National Instruments PCI6251, signal generator with 4 channels to generate three
synchronized frequencies, and lock-in. Signal generator is used for alimentation of
emitters and reference signal to the lock-in. The signal from receiver element is sent to
the lock-in where it is demodulated and is sent to acquisition card. Acquisition is then
analyzed using CIVA software.
Figure 6.10- Experimental set-up for EC measurements (left part) 3D probe installed (right
part).
Figure 6.11, Figure 6.12, and Figure 6.13 show comparison between simulated results
(real part) with receivers position according to x-, y- and z-axis and reconstruction of
three components of magnetic field (Hx, Hy, and Hz) from experimental results
obtained with 3D probe at the defect of size 5×0.2×2 mm3, located at ligament of
0.5 mm.
component measurement
The one lobe signature of CSCAN in Figure 6.11 corresponds to the flaw detected
with 6 µT as maximal amplitude for experimental results, with a signal to noise ratio
(SNR) is equal to 21 dB.
The four lobes signature in Figure 6.12 corresponds to the 4 edges of the flaw. The
maximal amplitude in EC measurements is 1.5 µT, SNR=19 dB.
component measurement.
The two lobes signature in Figure 6.13 corresponds to two extremities of the flaw. The
maximal detected amplitude is 12 µT. The defect is detected with signal to noise ratio
of SNR=27 dB.
The experimental results of magnetic response show a strong correlation with
simulation results in the case of three analyzed components of magnetic field.
Inconel mock-up
Inconel mock-up (𝜎 = 0.97 𝑀𝑆. 𝑚−1) has been also used for 3D EC probe
performances evaluation for subsurface defects detection. The excitation frequency of
f = 70 kHz and current of I=50 mA applied to emitters have been chosen. The applied
voltage to the GMR receivers is 1 V. The distance between the 3D probe and the
mock-up is fixed at 700 µm. The scan step is 500 µm in the x and 100 µm in the
y axis. An example of detection of defect with following dimensions:
length × width × depth = 4 × 0.1 ×0.93 𝑚𝑚3 with ligament of 1.3 mm is analyzed
Figure 6.14, Figure 6.15 and Figure 6.16 show comparison between the simulated
results with receiver position to x-, y- and z-axis and reconstruction of three
components of magnetic field (Hx, Hy, and Hz) from experimental results obtained
with 3D EC probe. Real part of the signal is studied
The lobe signature in Figure 6.14 corresponds to the flaw position. The maximal
amplitude of detected signal is 2.5 µT with SNR=23 dB.
The four lobes signature in Figure 6.15 correspond to the four edges of the flaw.
Maximal detected amplitude is 1.6 µT. The defect is detected with SNR= 17 dB.
The two lobes signature in Figure 6.16 correspond to two extremities of the flaw.
Maximal detected amplitude is 10 µT. The defect is detected with SNR= 28 dB.
The experimental results of defect magnetic response show a good correlation with
simulation results in the case of three components of magnetic field.
6.3.2.2 Surface defects
Experimental testing in Aluminum mock-ups (𝜎 = 18 𝑀𝑆. 𝑚−1 ) containing four
defects (Figure 6.17) has been performed for evaluation of 3D probe for small defects
detection. The defects have the same width and height: w= 0.04× h=0.05 𝑚𝑚2 and
have the following lengths : 0.7 mm, 0.5 mm, 3 mm and 2 mm. Applied current for
emitters is 20 mA. All defects are well detected with excitation frequency f = 100 kHz.
The applied voltage to the GMR receivers is 1 V.
Figure 6.17- Aluminum mock-up.
The three following figures show comparison between simulated results with receivers
position according to x-, y- and z-axis and reconstruction of three components of
magnetic field (Hx, Hy, and Hz) from experimental results obtained with 3D EC probe.
Real part of the signal is studied. The scan step is 100 µm in the x-axis and 25 µm in
the y-axis. The distance between the 3D probe and the mock-up is 400 µm.
Figure 6.18 shows that each defect is detected with one lobe signature. Experimentally
the defects are detected with SNR =13.8 dB for defect 1 (l=0.7 mm), SNR=13 dB for
defect 2 (l=0.5 mm), SNR=16 dB for defect 3 (l=3 mm) and SNR=15 dB for defect 4
(l=2 mm). Maximal detected amplitude is 1.5 µT, 1.3 µT, 2.5 µT and 2.3 µT
We show in Figure 6.19 (right part) that the 3D probe doesn't detect the Hy
component of magnetic field. This inability of detection is due to the small width of
four defects (40 µm). Estimated weak magnetic amplitude of Hy component of order
of 0.2 µT (Figure 6.19-left part) and the noise level is equal 2 µT. Thus, Hy
component is hidden into the intrinsic noise.
Figure 6.20 shows that each defect is detected. The signature of detected defects
corresponds to two extremities of the defects. Experimentally, the defects are detected
with SNR =16.7 dB for defect 1 (l=0.7 mm), SNR=16 dB for defect 2 (l=0.5 mm),
SNR=17.6 dB for defect 3 (l=3 mm) and SNR=17 dB for defect 4 (l=2 mm). Maximal
detected amplitude is 2.8 µT, 2.6 µT, 3.5 µT and 3 µT respectively.
Titanium mock-up
Titanium mock-ups (𝜎 = 0.6 𝑀𝑆. 𝑚−1 ) have also been used for 3D Ec probe
evaluation for surface defects detection. This material is rather difficult due to it low
conductivity compared with Aluminum. This makes the detection of small defects harder
as EC signals are small. Applied current for emitters is 50 mA. The chosen excitation
frequency is 1 MHz. The applied voltage to the GMR is 1 V. The scan step is 100 µm
in the x-axis and 25 µm in the y-axis. The distance between the 3D probe and the
mock-up is fixed at 400 µm. The measurements have been performed on the mock-up
with the defect with following dimensions: length × width × depth =
0.6 × 0.05 ×0.03 𝑚𝑚3 .
The three following figures show comparison between the simulated results with
receivers position according to x-, y- and z-axis and reconstruction of three
with 3D EC probe. Real part of the signal is studied. We show only 1-D results, as
detection of defects has been performed in one-line scan.
The results shown in Figure 6.21 correspond to the defect response in tangential
component Hx. The defect is detected with maximal amplitude of 1.5 µT and with
SNR = 12.3 dB. Experimental results are in good agreement with the simulation
the defect (50 µm), to the weak magnetic amplitude of Hy component (estimated to
0.4 µT) (Figure 6.22 -left part), and to the noise level in experimental results (Figure
6.22 –left part) that is equal to 0.8 µT
The two lobes signature in Figure 6.23 corresponds the extremities of the defect. It is
is detected with maximal amplitude of 1.9 µT and SNR= 13 dB. Experimental results
are in good agreement with the simulation results.
In this chapter, performances of 3D EC probe have been evaluated. The 3D receiver
combined with emitter part described in this chapter has achieved a detection of μm-
sized defects. For this purpose, we measured the magnetic field response caused by
defects in Aluminum, Inconel and Titanium mock-ups. For subsurface and surface
defects detection, the developed 3D EC probe has demonstrated the capability of
detection of subsurface defect at ligament of 0.5 mm in Aluminum mock-up and at
ligament of 1.3 mm in Inconel mock-up. Small surface defect of 500 μm in Aluminum
mock-up has been detected with magnetic field detection limit of 2 μT. Also small
surface defect of 600 μm in Titanium mock-up has been detected with magnetic field
detection limit of 0.8 μT.
Conclusion and Future works
The main goal of this thesis work was to investigate probe allowing simultaneously reconstruction
of three components of magnetic field at the same measurement point of measurement. 3D probes
have been used for non-destructive testing and magnetic imaging applications. We have
succeeded to overcome the limits of detection of three components of magnetic field
simultaneously at the same point of measurement by using four orientations of GMR sensors.
The design of probe which comprises of various aspects including mechanical, electronic and
software elements has been developed. Different types of micron-sized GMR sensors have been
developed for different applications with a compromise between detectivity and magnetic field
range. Probe layouts and coil dimensions have been optimized with CIVA software for ECT
application. In micro-magnetic imaging, the 3D probe has detected the three components of
magnetic field emitted by current line in good agreement with simulations. Magnetic imaging of
one-dollar bill, and structured metallic object has been realized. Also, susceptibility measurements
have been realized with the 3D probe.
Performances of these probes have been evaluated in cases of sub-surface flaws and surface
cracks detection for EC-NDT. For sub-surface flaws detection, the 3D probe has demonstrated
their capacity of defect detection at size of 600 µm in titanium and of 500 µm in aluminum mock-
ups. Results in terms of signal to noise is comparable to single component probe detection.
 The next-stage work will be the development of GMR–array based 3D probes to reduce testing
time with the possibility of greater area inspection at one time. This work has begun with the
fabrication of 2D lines of sensors, which can be mounted at 45°. But during the time of my
PhD I did not have enough time to test these new probes.
 For 3D detection in NDT applications, it would be interesting to combine different
technologies. For example, micro-coils can be used to detect the perpendicular field whereas
GMR sensors are used for detection of planar components of the field. This would permit the
fabrication of 2D arrays of sensors.
 Investigate a correlation relationship between the defect shape and output signals of the 3D
probe by developing a hierarchical structure for defect classification and quantification: The
relationship between the magnetic field signal reflected by the defects and defects dimensions
can be used to characterize defects. For this reason, develop a statistical approach for the
influence of each defect parameters on the three components of field (frequency, ligament, lift-
off, length, width and depth) is very interesting. Indeed, further investigation will be
undertaken by using wide range of defect samples to create a big database.
 We have tried to use the frequency information to have a more accurate determination of a
defect in NDT. This is particularly suitable with GMR sensors as they are wideband detectors.
I have made a number of simulations but this work would require more in depth work.
 Optimization of the sensing elements and the downstream electronics, i.e. integrated the ASIC
(application-specified integrated circuit) system to the 3D probe for processing the sensor
signals [175].: The ASIC is a specific CMOS circuit that should provide the biasing for the
sensor array and do the reading of the sensors output for further processing. More
specifically the ASIC drives the magnetic sensors; multiplexes/sample the signals;
amplify/filter the signals.
 One key point of this work is an accurate control of sensor-surface distance. We can have a
calibration with a current line like we have done but a continuous measurement of the distance
by, for example, laser interferometry will be particularly important for magnetic imaging and
would allow a better image acquisition.
 Replace the GMR sensors by TMR sensors in application that need to work at high
frequencies. For example, for surface cracks detection, TMR are very promising in terms of the
possibility of detection and spatial resolution.
 Insert the GMR sensors in half bridge configuration with two active resistances and two GMR
sensors or full bridge configuration with four GMR sensors to eliminate the thermal effects.
With this configuration the noise level decreases, so the detection limit of magnetic field
reduces. We have not developed such probes in my PhD but it is possible to locally reverse the
magnetization of GMR sensors and then suppress thermal drifts. This is important for DC
magnetic imaging where thermal fluctuations are the main cause of image deformation.
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Titre : Sonde 3D à base des capteurs magnétoresistives pour l'imagerie magnétique et le contrôle
non-destructive.
Mots clés : Imagerie magnétique, Capteur magnétorésistif géante (GMR), Microscopie magnétique à
balayage, Détection des trois composantes des champs, Contrôle non-destructif
Résumé : La thèse est dédiée au développement lecture, la conception et la réalisation de la
des sondes à base de capteurs magnétorésistifs partie mécanique et d'émission. Pour cela un
capable de détecter les trois composantes du travail important de simulation a été nécessaire.
champ simultanément pour le contrôle non L'application de ces sondes sur des cas modèle
destructif par courants de Foucault et pour pour l'imagerie magnétique avec une résolution
l'imagerie magnétique. Une première partie submillimétrique est ensuite décrite. La sonde
donne un aperçu de l'état de l'art des capteurs et proposée dans cette thèse a été aussi utilisée
des méthodes d'imagerie et du contrôle. Dans avec succès pour détecter des défauts dans des
une seconde partie, la réalisation des sondes échantillons d'aluminium et de titane avec un
trois axes est donnée. Cela a inclus la micro bon rapport signal sur bruit.
fabrication, la réalisation de l'électronique de
Title : 3D Probes based on magnetoresistive Sensors for magnetic micro-imaging and NDT
Keywords : Magnetic imaging, spin valve Giant magnetoresistive sensor (GMR), scanning
magnetoresistance microscopy, 3D magnetic field, Non-destructive testing
Abstract : The thesis is dedicated to the realization of the reading electronics, the
development of probes based on design and realization of the mechanical part
magnetoresistive sensors capable of detecting and emission. For this, an important simulation
the three components of the field work was necessary. The application of these
simultaneously for eddy current non- probes to model cases for magnetic imaging
destructive testing and for magnetic imaging. A with submillimeter resolution is then described.
first part provides an overview of the state of The probe proposed in this thesis has also been
the art of sensors, imaging and control used successfully to detect defects in aluminum
methods. In a second part, the realization of the and titanium samples with a good signal-to-
three-axis probes is given. This included the noise ratio.
micro-fabrication, the.
Espace Technologique / Immeuble Discovery
Route de l'Orme aux Merisiers RD 128 / 91190 Saint-Aubin, France
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