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

Abstract

Temperature dependence of initial permeability $\mu _{i}$ for Co-contained Finemet-type alloy annealed at 500 °C through 600 °C was measured for observing the changes of $\mu _{i}$ at elevated temperature. It was found that the $\mu _{i}$ 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, $T_{c}^{A\ast }$ , 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 $T_{c}^{A\ast }$ did not enhance after nanocrystallization; however, $T_{c}^{A}$ can be elevated drastically up to the Curie temperature of crystalline phase $T_{c}^{{\textrm {cry}}}$ when the exchange field between adjacent nanograins penetrated the amorphous interphase thoroughly. In addition, the effective exchange penetration length of Co-contained Finemet alloy $L_{\rm {FeCo}}$ 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.