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Changes in magnetic properties after applying grain boundary diffusion process to Nd-Fe-B sintered magnets with a thickness of several millimeters were investigated, and their microstructures were also observed. Coercivities of the magnets coated with heavy rare earths (HRE=Dy and Tb) oxide or fluoride powder and then heat-treated, were enhanced significantly without a considerable reduction in a remanence. During the heat treatment, HRE element diffused into the magnet through the grain boundary, and formed HRE-rich shells around the grain boundary of the Nd 2 Fe 14 B phase by substituting Nd. The coercivity increased by more than 400 kA/m with a thickness of up to 3 mm for the magnet processed with TbF 3
Hirota et al. (Fri,) studied this question.
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