Exceptionally High Saturation Magnetic Flux Density and Ultralow Coercivity via an Amorphous–Nanocrystalline Transitional Microstructure in an FeCo-Based Alloy

High saturation magnetic flux density (B_s) of soft magnetic materials is essential for increasing the power density of modern magnetic devices and motor machines. Yet, increasing B_s is always at the expense of high coercivity (H_c), presenting a general trade-off in the soft magnetic material family. Here, superior comprehensive soft magnetic properties, i.e., an exceptionally high B_s of up to 1.94 T and H_c as low as 4.3 A m⁻¹ are unprecedentedly combined in an FeCo-based alloy. This alloy is obtained through a composition design strategy to construct a transitional microstructure between amorphous and traditional nanocrystalline alloys, with nanocrystals (with < 5 nm-sized crystal-like regions around) sparsely dispersed in an amorphous matrix. Such transitional microstructure possesses extremely low magnetic anisotropy caused by the annihilation of quasi-dislocation dipoles, and a strong magnetic exchange interaction, which leads to excellent comprehensive magnetic properties. The results provide useful guidelines for the development of the next generation of soft magnetic materials, which are promising for applications of high-frequency, high-efficiency, and energy-saving devices.