Microstructure optimization to achieve high coercivity in anisotropic Nd-Fe-B thin films

A high coercivity of 2.35 MA m−1 (29.5 kOe) is demonstrated using perpendicular anisotropy Nd-Fe-B films. Single layer columnar grown Nd-Fe-B thin films with a coercivity comparable with that of sintered magnets (1.11 MA m−1, 14.0 kOe) were used as the starting film. The coercivity was doubled by annealing films capped with Nd, Nd/Cu, and Nd/Ag layers. Transmission electron microscopy indicated that the Nd2Fe14B columnar grains were isolated by a Nd-rich grain boundary phase formed by diffusion of the capped layers into the grain boundaries of the polycrystalline Nd-Fe-B films. The temperature dependence of coercivity was lower than that of commercial sintered magnets. The coercivity of the Nd/Cu diffusion processed sample at 473 K was 0.979 MA m−1 (12.3 kOe), which is higher than that of Nd10Dy4Fe80B6 sintered magnets.