Abstract: Fe-based nanocrystalline alloys offer high saturation magnetic flux density (B_s) for miniaturized, energy-efficient electronics, yet are plagued by an inherent magnetization-softness trade-off. In this work, the exchange-coupling interaction of Fe-Co atoms was incorporated to enhance the B_s and high-temperature magnetic stability of FeCoNiBCuSi amorphous alloy. Based on this mechanism, a novel magnetic field-assisted two-step annealing strategy was proposed to decouple the magnetization-softness trade-off by designing and regulating the microstructure and magnetic structure of the present alloy. Field-assisted pre-annealing accelerated atomic reconfiguration and yielded a more uniform microstructure at the sub-nanometer scale. Conversely, the magnetic field applied during post-crystallization annealing reduced the activation energy barrier for nucleation, thus promoting the formation of high-density crystal-like ordered nuclei instead of big-sized nanograins. This is well corroborated by wide-angle X-ray scattering analysis. Unlike the synergy of induced anisotropy (K_u) and random anisotropy in single-step annealing, the total magnetic anisotropy in the magnetic field-assisted two-step annealing process was predominantly governed by K_u, which aligned well with the magnetic domain patterns revealed by the magneto-optical Kerr microscope. Consequently, the optimized (Fe_0.8Co_0.2)₈₄Ni₂B_12.5Cu₁Si_0.5 alloy subjected to the two-step annealing demonstrated a large B_s of 1.86 T, a low coercivity of 3.7 A/m, and a good effective permeability of 10,700 at 1 kHz. Our findings offer a possible pathway for modulating the B_s–H_c trade-off in Co-rich Fe-based nanocrystalline alloys.

Key words: Co-rich Fe-based nanocrystalline alloy, Magnetic field-assisted two-step annealing, Magnetic softness, Microstructure