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 degrees 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 Nd-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 optimallyannealed 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 Nd2Fe14B 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 Nd2Fe14B grains from magnetization reversal by exchange-decoupling the Nd2Fe14B grains. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

In-situ elevated-temperature x-ray diffraction is used to identify crystalline phases in Nd-Fe-B sintered magnets, revealing a correlation between the presence of a Nd-6(Fe, Ga)(14) phase and large coercivity enhancements. Scanning soft x-ray absorption microscopy is employed to observe Fe and Nd distribution directly, showing similarities between surface and bulk microstructures. Fe L-3-edge x-ray magnetic circular dichroism measurements reveal insights into the demagnetization process and the role of grain boundary phases in magnetization reversal.