Stress-induced anisotropy for MHz-stable permeability in Fe-based nanocrystalline alloys

Abstract

Tensile stress annealing (TSA) is an effective strategy for tailoring magnetic anisotropy and high-frequency performance in nanocrystalline soft magnetic alloys. Here, we systematically investigate the influence of TSA on the microstructure, magnetic domain evolution, and permeability stability of Fe_69.5Co₃Nb₂Mo_1.5Si₁₄B₉Cu₁ nanocrystalline alloys. Across all applied stresses (0–300 MPa), the alloys retain an ultrafine grain size (⩽11 nm), yet the induced uniaxial anisotropy constant (K_u) rises sharply from 22.5 to 665 J/m³. This increase in K_u refines the magnetic domain structure, reducing average domain width from 110 to 36 µm, and shifts the magnetization mechanism from domain-wall displacement to rotation-dominated reversal. Quantitative correlation between K_u, domain structure, and effective permeability (μ_e) reveals that higher stress suppresses μ_e at low frequencies but yields exceptional frequency stability: μ_e ≈ 2330 is maintained up to 1 MHz at 50 MPa, and μ_e ≈ 585 remains constant from 1 kHz to 10 MHz at 300 MPa. These findings demonstrate that stress-induced anisotropy is a decisive factor in governing high-frequency magnetic response, offering both mechanistic insight and a practical framework for designing next-generation soft magnetic materials for precision current transformers, EMC filters, and MHz-class power electronics.