Effect of core–shell structure on magnetic properties and subsequent grain boundary diffusion in the Ce-rich dual main phase magnets

Abstract

Dual-main-phase (DMP) magnets, a promising approach for the efficient utilization of high-abundance rare earth elements, exhibit enhanced coercivity compared to single-main-phase (SMP) magnets. This study demonstrates that a DMP magnet exhibits a 43 kA/m coercivity increase over an SMP magnet of equivalent composition. Microstructural characterization reveals two main-phase grains with distinct core–shell structures in the DMP magnet. Micromagnetic simulations indicate that the increased Nd content enhances the anisotropy field of the shells in Ce-rich grains, crucially contributing to the coercivity enhancement. Conversely, Nd2Fe14B grains do not significantly enhance coercivity. A micromagnetic model, constructed by substituting Nd2Fe14B grains with (Nd0.5Ce0.5)2Fe14B grains, demonstrates a slight coercivity increase compared to the DMP magnets. Moreover, retaining only the core–shell structure in grains near the end faces maintains higher coercivity than that of DMP magnets. Experimental results of DyCoCu grain boundary diffusion show a 406 kA/m coercivity increase in the DMP magnet, less than the 510 kA/m increase in the diffused SMP magnet. Although diffusion significantly increases the anisotropy field in the shell, the core region of the Ce-rich grains maintains a low anisotropy field, limiting magnetic property enhancement. These findings underscore the critical role of an optimized core–shell structure in enhancing coercivity for Ce-rich magnets, suggesting that the DMP method may not represent the most effective strategy.