Effect of annealing on the surface magnetic and magnetoimpedance properties of Co-based amorphous microwires

Magnetic domains of negative magnetostrictive amorphous microwires (AWs) form a core-shell type structure with the core and shell domains preferred along axis and circumference, respectively. In this work, we have studied the effect of a combined current modulation annealing on the bulk and surface magnetic properties of melt-extracted Co$_{68.2}$Fe$_{4.3}$B$_{15}$Si$_{12.5}$ and Co$_{68.2}$Fe$_{4.3}$B$_{14}$Si$_{12.5}$Nb$_{1}$ AWs and their correlations with the magnetoimpedance (MI) effect. The AWs were annealed by a combination of ac (90 mA, 50 Hz) and dc (60, 63, and 65 mA) currents for 480 seconds each, and magnetic hysteresis loops were measured by VSM and a magneto-optic Kerr effect (MOKE) magnetometer. Compared to VSM loops, MOKE loops for the annealed AWs were observed to have higher coercive and anisotropy fields, indicating that the near-surface region was magnetically harder. The anisotropy fields of the AWs defined as the peaks of the MI spectra at 1 MHz and 500 MHz were found to have a correlation with those probed by VSM and MOKE, respectively. These findings are of practical importance in designing MI-based sensors.