Relationship between the microstructure, local magnetism and coercivity in Ga-containing Nd-Fe-B sintered magnets

The crystalline phases that comprise the bulk microstructure of an isotropic Ga-containing Nd-Fe-B sintered magnet are identified by in situ elevated-temperature x-ray diffraction for temperatures between 25 and ${800}^{\circ }$C. Comparing the temperature-dependent volume fractions of the crystalline phases with the coercivity as a function of annealing temperature reveals that the large coercivity enhancements are remarkably well-correlated with the existence of a crystalline $N{d}_6$${(Fe,Ga)}_{14}$ phase. Scanning soft x-ray absorption microscopy measurements are used to directly observe the distribution of Fe and Nd in anisotropic as-sintered and optimally-annealed samples and demonstrate that the surface microstructure is similar to the bulk. Fe $L_3$-edge x-ray magnetic circular dichroism measurements are used to directly map the demagnetization process in the fractured surfaces of anisotropic as-sintered and optimally-annealed samples. In the as-sintered sample, the demagnetization curves of compositionally different regions all have the same shape but with a maximum magnetic signal that only depends on the Fe content within the probing depth of the x-ray absorption measurements. In the optimally-annealed sample, the demagnetization curves from compositionally different regions have rather different shapes and the exposed $N{d}_2$$F{e}_{14}B$ grains without any grain boundary coverage have much lower coercivity than those with. This provides evidence that the grain boundary phase that forms upon annealing is able to protect the $N{d}_2$$F{e}_{14}B$ grains from magnetization reversal by exchange-decoupling the $N{d}_2$$F{e}_{14}B$ grains.