Mechanism of Improved High-Temperature Magnetic Softness for Co-Contained Finemet Alloy

Temperature dependence of initial permeability μi for Co-contained Finemet-type alloy annealed at 500 °C through 600 °C was measured for observing the changes of μi at elevated temperature. It was found that the μi of 600 °C-annealed sample did not drop dramatically at the Curie temperature of the amorphous phase, which is a new magnetic phenomenon observed only 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 TcA. The results indicated that T_c^A* did not enhance after nanocrystallization; however, T_c^A can be elevated drastically up to the Curie temperature of crystalline phase T_c^cry when the exchange field between adjacent nanograins penetrated the amorphous interphase thoroughly. In addition, the effective exchange penetration length of Co-contained Finemet alloy LFeCo was also evaluated, which is larger than that of Finemet alloys. This should be the main mechanism of improved high-temperature magnetic softness for Co-contained Finemet alloys.