Cryogenic Joule annealing induced large magnetic field response of Co-based microwires for giant magneto-impedance sensor applications

We have presented herein the results of microstructure, surface magnetic domains (SMDs), and giant magneto-impedance (GMI) effect of melt-extracted Co_68.15Fe_4.35Si_12.25B_11.25Nb ₂Cu₂ amorphous wires for the first time employed by using a cryogenic Joule annealing (CJA) technique with large DC current amplitude. Compared with the conventional JA method, experimental results indicate that the maximum GMI ratio [ΔZ/Z₀]_max achieves up to 425% at 8.1MHz with monotonic increase of the axial magnetic field H_ex up to 6.5Oe for 300mA (equal to around 1.06×10⁶A/dm^-2) CJA-ed wire, which is about 75% larger than the [ΔZ/Z₀] _max for the 100mA (nearly 3.53×10⁵A/dm^-2) JA-ed microwires. The remarkable features of large and linearly sensitive response field (2.5∼6.5Oe) and the sensitivity of 99.4%/Oe with higher GMI ratio simultaneously make the CJA tailored melt-extracted microwires promising candidate materials for miniaturized GMI sensors. Another interesting result of GMI profiles of 200mA (appropriately equal to 7.07×10⁵A/dm ^-2) CJA-ed wire show a linear response to H_ex (ranging from 10 to 80Oe or more), this behavior of GMI curves can be explored to fabricate bi-sensor. Large response field proves to originate from the intensive coupling between the radial stress field and the circumferential magnetic field during CJA process. The effect of outer-shell microstructure and complex SMD for 300mA CJA-ed microwire is attributed to the fact that liquid nitrogen hinders the evolution of circumferential domain structure to some extent and protects the amorphous structure in the shell region.