Anisotropic diffusion mechanism in grain boundary diffusion processed Nd-Fe-B sintered magnet

We investigated the anisotropic diffusion mechanism of Dy in a DyH₂ dip-coated magnet in terms of both the crystal orientation of the Nd₂Fe₁₄B phase and the aligned direction of the magnet. A Dy-rich shell was formed preferentially at the interface, which is parallel to the [001] axis (c-axis) of the Nd₂Fe₁₄B crystal, during the grain boundary diffusion process (GBDP) because lattice diffusion of Dy perpendicular to the c-axis of the Nd₂Fe₁₄B crystal is much easier than that parallel to the c-axis. In contrast, the grain boundary diffusion depth in the direction perpendicular to the aligned direction (c-axis) of the magnet was much shorter (∼100 μm) than that in the direction parallel to the aligned direction (∼250 μm). Anisotropic grain boundary diffusion depending on the aligned direction of the magnet was not the result of the anisotropic Nd-rich grain boundary phase distribution but of anisotropic lattice diffusion depending on the Nd₂Fe₁₄B crystal orientation. Increasing the ratio of the surface areas perpendicular and parallel to the aligned direction (c-axis) of the magnet is expected to further improve the coercivity of the GBDP magnet.