Effect of Ni Substitution for Si Element on Thermal and Soft Magnetic Properties of Fe_73.5Ni_xSi_15.5-xB₇Nb₃Cu₁ Nanocrystalline Alloys

Fe_73.5Ni_xSi_15.5-xB₇Nb₃Cu₁ nanocrystalline alloys were synthesized for improving the high-frequency performance without decreasing the saturation magnetic induction (B_s). Thermal and microstructural analysis reveals that Ni substitution favors the precipitation of initial α-Fe(Ni)(Si) phase. After proper annealing, the alloy ribbons with minor Ni addition obviously possess better comprehensive soft magnetic properties, including low coercivity of 0.34–0.51 A/m and high effective permeability of 23,000–56,000 in a wide annealing temperature range from 510 °C to 590 °C. The B_s is obviously improved from 1.21 T to 1.43 T, due to the increasing content of ferromagnetic Ni and density. Thermal expansion measurement demonstrates that all investigated alloys have obvious Invar effect and low thermal expansibility, which presents a moderate decrease with increasing Ni addition after crystallization, implying a smaller saturation magnetostriction. The Ni-doping nanocrystalline magnetic cores also exhibit better high-frequency characterization, including high relative permeability and low core loss at high-frequency region.