Multiscale structural origins of stress annealing tailored magnetic properties in Fe-based nanocrystalline cores

Fe-based nanocrystalline alloys (FNAs) are critical for high-frequency electronics, yet precise magnetic property control remains challenging. Here, a continuous stress annealing (CSA) technique is developed to regulate AC/DC soft magnetic properties of Fe-based nanocrystalline cores (FNCs) via thermomechanical field coupling (TFC). A non-monotonic stress-property relationship is revealed: coercivity (H_c), magnetic anisotropy constant (K), and AC loss (P_s) decrease first and then increase with stress, while permeability (μ_e) exhibits the opposite trend. There exists an optimal state at σ≈ 28.6 MPa, where H_c= 5.4 A/m, μ_e= 6532.5, P_s= 401.2 W/kg (at 300 mT, 200 kHz). To unveil the structural origins, a multiscale characterization approach is employed, integrating atomic force microscopy (AFM), magnetic force microscopy (MFM), in-situ synchrotron radiation X-ray diffraction (SRXRD), and high-resolution transmission electron microscopy (HRTEM). The findings show that low-stress annealing (LSA, σ<28.6 MPa) yields isotropic nanocrystalline distribution and irregular strip-like domains, whereas high-stress annealing (HSA, σ > 28.6 MPa) induces lamellar domains, anisotropic nanocrystalline distribution and lattice distortion, and reduced short-range order (SRO) clusters (from 18.15 vol% to 15.62 vol%). We propose a competitive coupling model where magnetic anisotropy (MA) arises from mesoscopic nanocrystalline distribution (MND), microscopic lattice distortion (MLD), and atomic-scale SRO clusters, and optimal properties at σ≈ 28.6 MPa stem from the synergy of MND and MLD in suppressing atomic-scale SRO clusters. This work establishes a quantitative multiscale framework for designing high-performance soft magnetic materials.