A Nd-Fe-O intergranular phase in Nd-Fe-B sintered magnets and its effect on coercivity

The intergranular microstructure and coercivity of Nd richer Nd/sub x/Fe/sub 93-x/B/sub 7/ (x=16-28) sintered magnets have been systematically studied by means of scanning electron microscopy, analytical transmission electron microscopy, scanning Auger multiprobe, thermomagnetic analysis, magnetic measurement and annealing experiments. A new stable Nd-Fe-O ferromagnetic phase with a composition NdFe/sub 2/O/sub x/ (x/spl ap/0.3) and a Curie temperature of 145/spl deg/C is found in the intergranular regions of the magnets, due to the introduction of a considerable amount of oxygen during magnet processing and its segregation in the intergranular regions. The phase forms at /spl sim/650/spl deg/C and volume fraction reaches a maximum of /spl sim/4% in the Nd/sub 22/Fe/sub 71/B/sub 7/ magnets. This new phase has a strong hindrance effect on the propagation of reversed domain walls between Nd/sub 2/Fe/sub 14/B grains, and thus leads to a considerable enhancement of coercivity. Raising the Curie temperature of this phase would be a new important approach to improve the high-temperature coercivity of Nd-Fe-B magnets.<
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