Studies on heat treatment of NdxFe93-xB7 (x = 16-28) sintered magnets

The effect of heat treatment on Nd richer NdxFe93-xB7 (x = 16-28) sintered magnets has been systematically studied. The coercivity could be greatly enhanced by heat treatment below 650 C. Microstructure studies reveal that a NdFe2Ox phase appears in the intergranular regions of the magnets after the heat treatment. With the increase in the Nd content of the magnets, its volume fraction increases, reaching a maximum of ∝ 4% at the Nd content of 22 at%, and then decreases. The enhancement of coercivity upon heat treatment is found to result not only from the increase in the nucleation field for the reversed domains, but also from the increase in the pinning field on the reversed domain walls, which is proportional to the volume fraction of the NdFe2Ox phase. Thus the former plays a major role in magnets with Nd contents lower than 19 at% or higher than 26 at%, and the latter plays a major role in magnets with Nd contents between 19 and 26 at%. (orig.)