Mechanism of improved high-temperature magnetic softness for Co-contained Finemet alloy

Herein, the temperature dependence of initial permeability was investigated for nanocrystalline (Fe_0.75Co_0.25)_73.5Si_13.5B₉Nb₃Cu alloy annealed at 530 and 660. It has been observed that the initial permeability of 530-annealed sample gradually droped to zero at 425 (the Curie temperature of as-quenched amorphous alloy), however the initial permeability of 660-annealed sample did not drop to zero at 425, which is a new magnetic phenomenon in two-phase nanocrystalline alloys. The origin of the phenomenon has been analyzed by estimating the Curie temperature of as-quenched amorphous alloy,T_c^A*, which has the same composition with the residual amorphous phase in annealed nanocrystalline alloy as well as the Curie temperature of the intergranular amorphous region T_c^A. The results indicated that the T_c^A* did not enhanced after nanocrystallization, however the Curie temperature of intergranular amorphous region T_c^A can be elevated drastically up to the Curie temperature of crystalline phase (T_c^A =T_c^cry) when the exchange-field between adjacent nanograins penetrated the amorphous interphase thoroughly. Furthermore the effective exchange penetration length of Co-contained Finemet alloy (LFeCo) was evaluated, which is about 0.61nm, larger than that of Finemet alloys.