Magnetization reversal mechanism for core-shell grains in GBD Nd-Fe-B sintered magnets based on in-situ domain observation

Understanding the magnetization reversal mechanism of a core-shell structure grains formed in the grain boundary diffused Nd-Fe-B magnet can further improve the utilization efficiency of heavy rare earth resources to more effectively improve the coercivity of Nd-Fe-B sintered permanent magnet. In this paper, magnetic domain structure and its dynamic evolution of core-shell structural grains in grain-boundary-diffused Nd-Fe-B magnet were in-situ observed. Wider domains are found in shell regions due to higher anisotropy constant and lower saturation magnetization. The domain motion revealed that the demagnetization process of core-shell structure has a multi-step nucleation process. The nucleation of the core-shell structural grain firstly occurs at the core regions, while another reverse nucleation appears at the outmost portion in shell-region at a larger reverse field. The micromagnetic simulation results confirm the possibility of the nucleation and reversal process observed in the experiment. Our results can help understanding the demagnetization process and the coercivity enhancement mechanism of grain-boundary-diffused Nd-Fe-B sintered magnet.