Annealing effect on microstructure and asymmetric giant magnetoimpedance in Co-based amorphous ribbon

We investigated the modification of microstructure and the change in GMI profile in a weak field-annealed amorphous ribbon as a function of the annealing time. The FWHM of x-ray diffraction patterns decreased in 8 h annealed samples, suggesting the reduction of the amorphous phase in the surface layer. Auger electron spectroscopy represented the compositional depth profile, where the oxide layer was heavily enriched in B and Si contents, balanced by a depletion of these elements in the underlying materials. The reduction of B and Si content decreases the crystallization temperature of the depletion layer, resulting in the formation of crystal or crystallites during annealing in air. The sharp change of GMI profiles was observed for the annealed samples over 1 h, but there was hysteresis in the GMI profile for increasing and decreasing field for the 1 h annealed sample. As the annealing time increased, the peak in the antiparallel field region to annealing field disappeared. Eventually, the GMI-valve in parallel field region was revealed in a 8 h annealed sample.