Oxidation and coercivity of near single domain size Nd-Fe-B-type alloy particles

摘要

Oxidation and coercivity of the near single domain size Nd_2Fe_(14)B-type particles prepared by ball milling of HDDR-treated Nd_(12.5)Fe_(80.6)B_(6.4)Ga_(0.3)Nb_(0.2) alloy were investigated. Feasibilty of surface nitrogenation of the fine Nd_2Fe_(14)B-type particles for improving the stability of coercivity was also studied. The fine Nd_2Fe_(14)B-type particles in near single domain size had high coercivity over 9 kOe. However, the coercivity was radically deteriorated as the temperature increased in air (< 2 kOe at 200 °C). This radical coercivity reduction was attributed to the soft magnetic phases, α-Fe and Fe_3B, which were formed on the surface of the fine particles due to the oxidation. Surface nitrogenation of the fine particles significantly improved the stability of coercivity in air. Thin nitrogenated outer shell of the fine particles suppressed the dissociation of the Nd_2Fe_(14)B to a mixture of Nd-oxides, Fe_3B and α-Fe at the surface of the fine particles.