# Phonon Spectroscopy Near Phase Transition Temperatures in Multferroic BiFeO$_{3}$ Epitaxial Thin Films 

$^{1}$Department of Physics and Institute for Functional Nanomaterials, University of Puerto Rico, San Juan, PR 00931-3343, USA 

R. Palai $^{1}$, J.F. Scott$^{2}$, and R.S. Katiyar1 

$^{2}$Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, UK 

(Dated: March 5, 2018) 

We report a Raman scattering investigation of multiferroic bismuth ferrite (BiFeO 3 ) epitaxial ( c -axis oriented) thin films from -192 to 1000 ◦ C. Phonon anomalies have been observed in three temperature regions: in the γ -phase from 930 ◦ C to 950 ◦ C; at ∼ 370 ◦ C, N´ e el temperature ( T N ), and at ∼ -123 ◦ C, due to a phase transition of unknown type (magnetic or structural). An attempt has been made to understand the origin of the weak phonon-magnon coupling and the dynamics of the phase sequence. The disappearance of several Raman modes at ∼ 820 ◦ C ( T c ) is compatible with the known structural phase transition and the Pbnm orthoferrite space group assigned by Arnold et al. [1]. The spectra also revealed a non-cubic β -phase from 820-930 ◦ C and the same non-cubic phase extends through the γ -phase between 930-950 ◦ C, in agreement with Arnold et al. [2], and an evidence of a cubic δ -phase around 1000 ◦ C in thin films that is not stable in powder and bulk. Such a cubic phase has been theoretically predicted in [3]. Micro-Raman scattering and X-ray diffraction showed no structural decomposition in thin films during the thermal cycling from 22-1000 ◦ C. 

Multiferroics are the materials which display a coexis- tence of at least two of the switchable states: polariza- tion, magnetization or strain in the same phase [4]. In addition, they may also exhibit a magnetoelectric (ME) effect: magnetization induced by an electric field and electric polarization by means of magnetic field [5]. The current interest in multiferroics is largely based on engi- neered epitaxial and heterostructured thin films, because their physical properties are as good as bulk and per- mit technological applications in data storage, magnetic recording, spintronics, quantum electromagnets, and sen- sors [6–8]. Devices made up of multiferroic materials can perform more than one task and facilitate device miniaturization. A weak ME effect has been observed in most multiferroics, generally showing a small change in their spontaneous polarization under applied magnetic field [5, 9, 10]. However, the complete switching of fer- roelectric domains by applied magnetic fields has rarely been observed. Why and under what circumstances a large coupling should exist and how to control the cou- pling are still open questions. Understanding the physics of the different possible interactions between magnetic and electric order parameters i.e. giving rise to magne- toelectric (ME) coupling would be very useful. 

Magnetism and ferroelectricity are involved with lo- cal spin ordering and off-center structural distortions, re- spectively [11]. These are quite complementary phenom- ena that coexist in certain multiferroic materials. Cur- rently, BiFeO$_{3}$(BFO) is one of the most widely studied multiferroics because it is one of only two or three single- phase multiferroics at room temperature i.e an antifer- romagnetic (AFM) incommensurate phase with cycloidal modulation ( λ ≈ 60 nm) below ≈ 370 ◦ C [12, 13], fer- 

PACS numbers: 77.55.+f, 78.30.-j, 77.80.Bh, 78.66.Nk 

## I. INTRODUCTION 

roelectric up to ≈ 820 ◦ C [14], and ferroelastic between 820-930 ◦ C [15]. Bulk BFO crystallizes in a rhombohe- dral ( a = 5.58 ˚ A and α = 89.5 $^{0}$) structure at room temperature (RT) with space group R3c (C 6 3 $_{v}$) and an- tiferromagnetism of G -type [13, 16]. The structure and properties of bulk BFO have been studied extensively [13, 16–18] and although early values of polarization were low ( P$_{r}$ = 6.1 µ C/cm $^{2}$) due to sample quality, P$_{r}$ = 40- 100 µ C/cm 2 was recently found in bulk by several dif- ferent groups [19, 20]. The epitaxially grown thin films of BFO on SrTiO$_{3}$ (STO) substrates show very high values of P$_{r}$ ( ∼ 100 µ C/cm $^{2}$) [6] compared to the best known ferroelectrics such as PbZrTiO$_{3}$ ( ∼ 70 µ C/cm $^{2}$) and BaSrTiO$_{3}$ ( ∼ 30 µ C/cm $^{2}$). This makes BFO a po- tential material for novel device applications. 

The motivation for the present study is manifold. The first objective is to test the recent space group deter- mination of the γ -phase reported by Arnold et al. [1] as being orthorhombic. Their definitive neutron study showed that the γ -phase is indeed stable (which in itself had been controversial), and that it has the same or- thorhombic Pbnm orthoferrite symmetry as does the β - phase. A cubic Pm3m perovskite structure was definitely ruled out, although a body- centered orthorhombic space group was indistinguishable from the primitive Pbnm. A main aim of our Raman study is to test the orthorhom- bic crystal class for the γ -phase and see whether we can further distinguish between primitive and body-centered orthorhombic structures. 

We investigated 300 nm (001) BFO thin films on STO (100) substrates with ∼ 25 nm thick SrRuO$_{3}$ (SRO) 

## II. EXPERIMENTAL DETAILS 

