Taming crystallization to achieve superior soft magnetic performance via ultrafine nanocrystallines

The drive for high-efficiency, high-power-density power electronics in emerging technologies such as 5th generation mobile networks (5G) and artificial intelligence (AI) is impeded by a fundamental challenge in soft magnetic materials (SMMs): the intrinsic trade-off between high saturation magnetization ( B _s ) and low coercivity ( H _c ). Conventional approaches like thermal annealing rely on stochastic nucleation and growth, and thus struggle to precisely control nanocrystal size and distribution at the sub-10-nm scale, which is essential for simultaneously optimizing both properties. Here, we report a study on achieving performance breakthroughs in the Fe-(Co)-Si-B-Cu-Nb and Fe-Co-B-P alloy systems through a rotational magnetic field annealing process. This process significantly enhances the nucleation density by introducing dynamic structural fluctuations in the amorphous precursor, while simultaneously inhibiting grain growth. As a result, ultrafine nanocrystalline structures with an average grain size of only 1 ~ 5 nanometers and uniformly distributed in the amorphous matrix were successfully prepared. The obtained materials maintained an extremely high coercivity ( B _s ) of up to 1.94 T while achieving a coercivity ( H _c ) as low as approximately 0.8 A/m. This combined performance far exceeds that of existing soft magnetic materials. Computational simulations confirmed that the rotational magnetic field induced spatially non-uniform dynamic and structural fluctuations in the amorphous solid, thereby creating high-density and uniform heterogeneous nucleation sites. This work not only demonstrates a new paradigm for obtaining ultrafine nanocomposite structures through external field dynamics to control the crystallization path, but also provides a new design strategy for achieving high-performance soft magnetic materials beyond traditional performance limits, laying the material foundation for the next generation of efficient power electronic technologies.