The dependence of intrinsic coercivity on Tb concentration and core-shell structure of diffused Nd-Fe-B magnets

In this paper, the variations of composition, core-shell structure, and magnetic properties of diffused Nd-Fe-B magnet with the diffusion depth have been intensively investigated. It was confirmed that the Tb concentration, core-shell structure and intrinsic coercivity (Hci) along the diffusion depth of magnets indicated obvious gradient characteristics. The Hci of diffused magnet was increased by 824.4 kA/m and reached 1864.7 kA/m. The Hci of the residual sample gradually decreased with cutting and removal of surface. However, it was emphasized that the Hci of 1702.6 kA/m with the increment of 662.3 kA/m was obtained in the central sample, in which the concentration of Tb was 0.13 wt%. The electron probe microanalyzer (EPMA) analyses showed that Tb-rich shells were continuously visible in the depth of 100 mu m, and became discontinuous in the depth of 500 mu m, and were nearly invisible in the center of diffused magnet with the depth of 2500 mu m. In order to further characterize the atomic-scale morphology and elements, a high-angle annular dark field (HAADF) detector and an energy dispersive X-ray spectroscopy (EDS) on the double aberration-corrected scanning transmission electron microscopy (STEM) were employed. The results indicated that the thick Tb-rich shells were clearly visible near the surface of the diffused magnet, and the lattice constant c of 2:14:1 phase was reduced by 0.05 angstrom due to the partial substitution of Tb element for Pr/Nd elements. However, Tb-rich shells in the central part of diffused magnet was discontinuous and inhomogeneous, in which the Tb-rich and Tb-poor regions were occurred. It was mainly attributed to the low Tb concentration at grain boundaries in the center of the magnet, which was difficult to form complete Tb-rich shells.