Role of Ga on the high coercivity of Nd-rich Ga-doped Nd-Fe-B sintered magnet

We have investigated the effect of Ga on the microstructure and coercivity in Nd-rich Ga-doped Nd-Fe-B sintered magnets with different amount of Ga additions using focused ionbeam scanning electron microscope (FIB/SEM), aberration corrected scanning transmissionelectron microscope (STEM), three dimensional atom probe (3DAP) and synchrotron X-raydiffraction. While a erromagnetic Fe-rich amorphous phase is a dominant grain boundary phasein the Ga-free magnet, the trace addition of Ga resulted in the formation of the Nd6Fe13Gaantiferromagnetic phase at grain boundaries as well as in triple junctions after post-sinterannealing above 480 °C. In addition, non-ferromagnetic Nd-rich phases with the Ia3 structureand the amorphous structure were formed along the grain boundaries. The structures andchemical compositions of these three types of grain boundary phases were identified. The highcoercivity is closely related with the formation of the three types of non-ferromagnetic grainboundary phase rather than the amount of the Nd6Fe13Ga triple junction phase; hence thecoercivity enhancement after the optimal heat-treatment is attributed to the magneticisolation of the Nd2Fe14B grains through the formation of the non-ferromagnetic grainboundary phases. The underlying mechanism for the formation of these grain boundary phasesis discussed based on the experimental results.