The evolution of structure and magnetoimpedance characteristics of amorphous Co69Fe4Cr4Si12B11 microwires under heat treatment

Amorphous Co69Fe4Cr4Si12B11 glass-coated microwires after heat treatment in the temperature range of 250-600 degrees C during 30 min were investigated. Changes of microstructure, phase composition, fracture morphology, and giant magnetoimpedance (GMI) properties were shown. It is confirmed that a significant increase in the GMI effect was possible not only due to the structural relaxation, but also as a result of phase transformations at the temperature close to the onset of crystallization. It is shown that at the very initial stage of the nucleation of Co nanocrystals, a sharp increase in circumferential diagonal GMI component was observed, a further increase in the amount of the crystalline Co phase was accompanied by degradation of this effect. The most significant GMI ratio was obtained for microwires annealed at 430 degrees C. At 450 degrees C and above, an irreversible decrease of the GMI ratio took place. It was caused by the formation of Co crystals, decrease in amount and composition change of the amorphous phase during primary and secondary crystallization. At the final stage of the Co crystals segregation the secondary crystallization of the residual amorphous phase occurred with formation of a metastable tau-phase with a Me23B6 type structure. Thermal stability of the tau-phase was analyzed.