The role of exchange coupling on the giant magnetoimpedance of annealed amorphous materials

The characteristics of giant magnetoimpedance (GMI) profiles were measured in annealed amorphous Co66Fe4B15Si15 ribbons in the open air as functions of annealing time, temperature and field. The GMI profile measured at 0.1 MHz exhibits a drastic step-like change, the so-called 'GMI-valve' in samples produced with the optimum annealing parameters: time ta≥8 h, temperature Ta∼380 deg. C and in the weak-field range, 0.5 Oe≤Ha≤3 Oe. The GMI-valve is related to an exchange coupling of a bias-field with the magnetization of the soft amorphous phase, where the bias-field is caused by HCP-Co, FCC-Co and/or Co2Si crystalline phases in a B and Si depleted layer