Coercivity of Nd-Fe-B-Type Fine Particles Prepared by Mechanical Milling of HDDR Material

For bonded micro-magnet application, fine particles with high permanent magnetic performance are required. As the HDDR-treated Nd-Fe-B-type material consists of very fine grains (~0.3 μm), the fine particles prepared from the HDDR-treated material are expected to have high magnetic performance even in the form of fine particles. Fine Nd-Fe-B-type particles were prepared by mechanical milling of the HDDR-treated Nd_12.5Fe_80.6B_6.4Ga_0.3Nb_0.2 material, and coercivity of the fine particle was investigated. Coercivity of the HDDR-treated Nd-Fe-B-type material was radically reduced by mechanical milling. Surface oxidation was the major contributory factor for the radical coercivity reduction. Chemical etching of the oxidized surface led to significant recovery of the coercivity. Unlike the mechanically milled fine particles of sintered Nd-Fe-B-type magnet, which lose coercivity almost completely, the moderately milled and etched fine particles (20-80 μm) of HDDR-treated Nd-Fe-B-type material exhibited high coercivity over 12 kOe.