Study on the difference in formation mechanism and demagnetization process between core–shell and reverse core–shell structures of sintered Nd-Fe-B magnets diffused with Tb

During the grain boundary diffusion (GBD) of Tb, the core–shell and reverse core–shell structures are the two main microstructures influencing the magnetic properties of the sintered Nd-Fe-B magnets. These two microstructures are all composed of the (Nd, Tb)2Fe14B phase, but the formation mechanisms are different. The difference in formation mechanism of the core–shell and reverse core–shell structures was studied by quenching the magnets at different temperatures and holding times. The (Nd, Tb)2Fe14B shell of the core–shell structure is the precondition for forming the reverse core–shell structure. The triple-junction phases (TJPs) area change proves that the Nd elements diffuse from the TJPs to the surface of the (Nd, Tb)2Fe14B shell to form the Tb-poor shell in the reverse core–shell structure and the Gaussian distribution of Tb in the shell of the core–shell structure. In addition, the difference in the Tb content distribution leads to different demagnetization processes, resulting in the opposite effect of these two microstructures on the coercivity. The GBD aims to increase the entire coercivity by enhancing the surface anisotropy field (HA), such as the Tb-rich shell for the grains and the Tb-rich surface (∼200 μm) for the magnets. Therefore, for the reverse core–shell structure forming in the surface of the magnets, the surface with a low HA decreases the coercivity of the grains, reducing the coercivity of the magnets.