Encompassing Investigation of the Correlation between Secondary Phase Formation, Heat Treatment and Coercivity for Nd–Fe–B Sintered Magnets by <i>in situ</i> High-Temperature Synchrotron X-ray Diffraction

The magnetic properties of Nd–Fe–B sintered magnets depend on the secondary phase and its microstructure, which in turn depend on the post-sintering annealing conditions. Therefore, clarifying the correlation between magnetic properties, secondary phases, and annealing conditions is crucial. In this study, we examined the dependence of the formation/decomposition of crystalline phases during the cooling process of Nd–Fe–B sintered magnets containing Ga and C on the heat-treatment temperature using synchrotron radiation high-temperature in situ X-ray diffraction (HT in situ XRD). We also analyzed the correlation between the secondary phase formation/decomposition and magnetic properties. As a result, the existence of a secondary phase that correlated with the magnetic properties was clarified. An exhaustive search was conducted to determine the optimum heat-treatment conditions for the formation of secondary phase correlated with coercivity, thereby demonstrating the usefulness of the HT in situ XRD method in the development of high-performance permanent magnets.