  The optimization of high frequency giant magnetoimpedance (GMI) effect and
its magnetic field sensitivity in melt-extracted Co69.25Fe4.25Si13B12.5Nb1
amorphous microwires, through a multi-step Joule annealing (MSA) technique, was
systematically studied. The surface morphology, microstructure, surface
magnetic property, and high frequency GMI response of the Co-rich microwires
were explored using scanning electron microscopy (SEM), magneto-optical Kerr
effect (MOKE) magnetometry, transmission electron microscopy (TEM), and
impedance analyzer, respectively. An initial dc current (idc) of 20 mA, which
was then increased by 20 mA at every time-step (10 min) up to 300 mA, was
applied to the microwires. The MSA of 20 mA to 100 mA remarkably improved the
GMI ratio and its field sensitivity up to 760% (1.75 time of that of the
as-prepared), and 925%/Oe (more than 17.92 times of that of the as-prepared) at
an operating frequency of 20 MHz, respectively. Our study indicates that the
MSA technique can enhance the microstructures and the surface magnetic domain
structures of the Co-rich magnetic microwires, giving rise to the GMI
enhancement. This technique is suitable for improving the GMI sensitivity at
small magnetic fields, which is highly promising for biomedical sensing and
healthcare monitoring.

Instrumentation and Detectors