Asymmetric giant magnetoimpedance in field-annealed Co-based amorphous ribbon

The giant magnetoimpedance (GMI) has been measured in weak-field-annealed Co-based amorphous ribbon as functions of both the annealing field, H-a, and the measuring frequency. The GMI profile measured at 0.1 MHz shows a typical two-peak behavior for the sample with H-a less than 50 mOe. The GMI peak in the region where the applied field is antiparallel to the annealing field decreases with H-a, showing an asymmetry in the GMI profile. Eventually, the GMI peak in the antiparallel-field region disappears, and a drastic step-like change in the GMI peak is revealed for an applied field which is parallel to the annealing field of 500 mOe. The asymmetric GMI phenomenon for the 0.1 MHz measuring frequency, the so-called "GMI valve," is due to a peculiar domain structure, which occurs in the sample surface during the field annealing. The GMI peak in the parallel-field region appears again for measuring frequencies over 0.5 MHz due to the remarkable contribution of the magnetization rotation to the GMI. (C) 1999 American Institute of Physics. [S0021-8979(99)18108-8].