The origin of coercivity decrease in fine grained Nd–Fe–B sintered magnets

Microstructures of fine grained Nd–Fe–B sintered magnets that were produced by the pressless process were investigated to understand the origin of the sudden coercivity decrease below a certain grain size. The intrinsic coercivity is inversely proportional to ln D2 with the highest coercivity of 17 kOe at D∼4.5 μm, below which the coercivity drops as the grain size decreases. We found that the degradation of the coercivity of the magnet with a grain size of 3 μm was mainly caused by the inhomogeneous distribution of fcc-Nd oxide whose volume fraction increased with respect to the dhcp Nd-rich phase.