The effect of oxygen on the surface coercivity of Nd-coated Nd-Fe-B sintered magnets

摘要

We have investigated the effect of oxygen on the surface coercivity of Nd-coated Nd-Fe-B sintered magnets as a model experiment to understand the coercivity mechanism. The coercivity values of the surface Nd_2Fe_(14)B grains of Ta-coated and uncoated Nd(2 μm)/Nd-Fe-B magnets were compared after annealing at 400 to 600 ℃ in vacuum. The coercivity of the Ta coated sample did not recover after the heat treatment, whereas that of uncoated sample recovered to a bulk value. Transmission electron microscopy indicated that most of the phases near the Nd/Nd-Fe-B interface were double hcp Nd (metallic Nd) in the coated sample, while an fcc phase was dominant in the uncoated sample, suggesting that the cubic NdO_x phase is effective to suppress the nucleation of reverse magnetic domains on the surface of an Nd_2Fe_(14)B grain. This suggests that a certain amount of oxygen is an indispensable ingredient to achieve a high coercivity in Nd-Fe-B sintered magnets.