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 ion beam scanning electron microscope (FIB/SEM), aberration corrected scanning transmission electron microscope (STEM), three dimensional atom probe (3DAP) and synchrotron X-ray diffraction. While a ferromagnetic Fe-rich amorphous phase is a dominant grain boundary phase in the Ga-free magnet, the trace addition of Ga resulted in the formation of the Nd₆Fe₁₃Ga antiferromagnetic phase at grain boundaries as well as in triple junctions after post-sinter annealing above 480 °C. In addition, non-ferromagnetic Nd-rich phases with the Ia3¯ structure and the amorphous structure were formed along the grain boundaries. The structures and chemical compositions of these three types of grain boundary phases were identified. The high coercivity is closely related with the formation of the three types of non-ferromagnetic grain boundary phase rather than the amount of the Nd₆Fe₁₃Ga triple junction phase; hence the coercivity enhancement after the optimal heat-treatment is attributed to the magnetic isolation of the Nd₂Fe₁₄B grains through the formation of the non-ferromagnetic grain boundary phases. The underlying mechanism for the formation of these grain boundary phases is discussed based on the experimental results.