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

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 Fe69.5Co3Nb2Mo1.5Si14B9Cu1 nanocrystalline alloys. Across all applied stresses (0–300 MPa), the alloys retain an ultrafine grain size (≤11 nm), yet the induced uniaxial anisotropy constant (Ku) rises sharply from 22.5 to 665 J/m3. This increase in Ku 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 Ku, domain structure, and effective permeability (μe) reveals that higher stress suppresses μe at low frequencies but yields exceptional frequency stability: μe ≈ 2330 is maintained up to 1 MHz at 50 MPa, and μe ≈ 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.