Microstructure and magnetic properties of core-shell Nd-La-Fe-B sintered magnets

To balance the utilization of rare earth (RE) resource, abundant La has been incorporated into the Nd-Fe-B sintered magnets via the binary main phase (BMP) approach. Microstructure and magnetic properties of the Nd-La-Fe-B BMP magnets with increasing La substitution from 0 to 45 wt% were investigated. The results show that RE-rich intergranular phase with high oxygen content forms easily in the triple junction region for the La-substituted magnets. Those hcp-RE2O3 precipitates exhibit poor wettability with the matrix phase, leading to the direct exchange coupling of adjacent grains. As a result, the coercivity of Nd-La-Fe-B BMP magnets is dramatically reduced. Fortunately, the remanence can be maintained at a high level, ascribing to the inhomogeneous RE distributions among the grains (core-shell morphology), and the stable tetragonal phase after high-temperature sintering. With 27 wt% La substitution for Nd in the BMP magnet, the remanence decreases from 13.15 to 13.01 kGs by merely 1.06%. This work suggests that if one can optimize the microstructure by constructing magnetically isolated grains, low-cost Nd-La-Fe-B BMP magnets can be promising candidates for mass production.