Vortex-like magnetic domain evolution in soft magnetic dual-phase Fe-Co-Ti-Ge compositionally complex alloy

Soft magnetic compositionally complex alloys (CCAs) have attracted great attention in recent years. However, the magnetic domain structures and magnetic properties have rarely been studied in a correlative way for dual-phase CCAs. Here we investigate the microstructure and magnetic domain structure of a recently proposed dual-phase Fe30Co40Ti10Ge20 CCA using scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS), Lorentz TEM, differential phase contrast (DPC)-STEM and 4D-STEM. Three types of Fe-Co-rich phases with different shapes were discerned in the Heusler phase matrix. The magnetic domain type of the Fe-Co-rich phase is determined by its shape. A new vortex-like structure with subdomains in the Fe-Co-rich phase was observed. The evolution of magnetic domain structure was investigated by in situ Lorentz TEM experiments and correlated to the microstructure. These results provide insights into understanding the magnetic domain structure in Fe-Co-Ni-based magnetic CCAs to design CCAs with better magnetic performance by manipulating the microstructure.