Optimization of the high-frequency magnetoimpedance response in melt-extracted Co-rich microwires through novel multiple-step Joule heating

The optimization of the high frequency giant magnetoimpedance (GMI) effect and its magnetic field sensitivity in melt-extracted Co_69.25Fe_4.25Si₁₃B_12.5Nb₁ amorphous microwires was systematically studied through a multi-step Joule annealing (MSA) technique. The surface morphology, microstructure, surface magnetic property, and radio frequency GMI response of the Co-rich microwires were explored using techniques like microscopy, magneto-optical Kerr effect (MOKE) magnetometry, and the magnetic field dependence of the wire's radio frequency impedance (GMI). The multi-step Joule annealing protocol begins with an initial dc current amplitude (i_dc) of 20 mA that was stepped up by 20 mA every 10 min to a maximum amplitude of 300 mA. Radio frequency GMI measurements at 20 MHz demonstrated a remarkable improvement of the GMI ratio and field sensitivity to 760% (1.75 times of that of the as-prepared wire) and 925%/Oe (more than 17.92 times of that of the as-prepared wire), respectively, after the MSA protocol with a maximum current amplitude of 100 mA. Microscopy and MOKE suggest that the MSA technique can enhance the microstructure and surface magnetic domain structure of the Co-rich magnetic microwire, which would give rise to an improved GMI ratio. The high GMI sensitivity at small magnetic fields renders these MSA-treated Co-rich microwires highly promising materials for biomedical devices that sense and monitor small, biological magnetic fields.