Stress resistance improvement of soft magnetic properties for Fe-based nanocrystalline cores via setting a multi-stage annealing prior to transverse magnetic field annealing

Development of Fe-based nanocrystalline cores with enhanced stress resistance in soft magnetic properties is a prerequisite for ensuring the performance stability of power electronic devices under complex service conditions. A multi-stage annealing conducted prior to transverse magnetic field annealing (TFA2) was proposed and performed to the as-quenched FINEMET ribbon. The corresponding magnetic properties, microstructure, domain structures of the nanocrystalline samples before and after passive strain were systematically compared with normal isothermal annealing (NA) and standard transverse magnetic field annealing (TFA1). Results indicate that the TFA2 core exhibits superior performance, including a higher permeability (μ) of 27,290 at 100 kHz, a lower core loss (P) of 9.08 W/kg at 20 kHz, 0.5 T, as well as better stress resistance with only 2.6% permeability degradation and minimal 1.6% loss increment after undergoing 20% radial compressive strain, in comparison to those of the NA and TFA1 cores. The excellent soft magnetic properties and good stress resistance for the TFA2 core can be attributed to a combination of factors, including fine grain size, low magnetostriction coefficient, relatively high magnetic field-induced K_u, and the effective competition between magnetic field-induced K_u and external stress-induced K_u-σ. This methodology provides offers both theoretical insights and experimental validation for designing next-generation stress-immune soft magnetic materials in high-frequency power conversion systems.