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 _xFe_93-xB₇ (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 NdFe₂O_x (x≈0.3) and a Curie temperature of 145°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 ~650°C and volume fraction reaches a maximum of ~4% in the Nd₂₂Fe₇₁B₇ magnets. This new phase has a strong hindrance effect on the propagation of reversed domain walls between Nd₂Fe₁₄B 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