Improved thermal stability of FeNiCo-based nanocrystalline soft magnetic alloys enabled by Ni segregation in the amorphous matrix

Recently, the growing need to minimize energy losses and enhance the performance of electronic devices has underscored the importance of developing soft magnetic materials. Specifically, high-frequency alternating current (AC) applications demand soft magnetic materials with excellent thermal stability and high electrical resistivity. In this study, we designed FeCoNi-based amorphous alloys with the composition (Fe, Ni)_{82.5 −x}Co_xSi₈B₄P₄C_1.5 (x = 0, 5, 10, 15 at%). Among the alloys, the (Fe, Ni)_67.5Co₁₅Si₈B₄P₄C_1.5 alloy displayed crystallization behavior with a single body-centered cubic phase after annealing at temperatures ranging from 350–500 ℃ for 288 h. Following crystallization, the microstructure of the alloy comprised spherical crystallites embedded within a residual amorphous matrix, stabilized by Ni species expelled during crystallization. Annealing at 500 ℃ produced a finer grain structure compared to that at 350 ℃, owing to the higher nucleation rate observed at elevated temperatures. This unique microstructure characterized by nanocrystallites embedded within a stable amorphous matrix enabled the (Fe, Ni)_67.5Co₁₅Si₈B₄P₄C_1.5 alloy, annealed at 500 ℃, to exhibit excellent thermal stability at high temperatures, along with favorable soft magnetic properties. Furthermore, the electrical resistivity of the nanocrystalline alloy exceeded 138 μΩ·cm, surpassing the resistivities of commercial polycrystalline, amorphous, and nanocrystalline soft magnetic alloys. These attributes highlight its potential as a promising material for high-frequency AC applications.