Quantitative Identification of Constituent Phases in a Nd-Fe-B Sintered Magnet and Temperature-Dependent Change in the Electron Density of Nd 2Fe 14B Studied by Synchrotron X-Ray Diffraction

We measure the temperature-dependent XRD profiles of an isotropic Nd-Fe-B-Cu sintered magnet during the annealing process. Through Rietveld refinement, we demonstrate the changes in the volume fractions of the Nd₂Fe₁₄B main phase and the other secondary phases as a function of increasing temperature up to 1047 °C. The secondary phases include dhcp-Nd, fcc-NdO_x, and hcp-Nd₂O₃ at room temperature and fcc-Nd at elevated temperatures. The main phase remains relatively stable up to 600 °C and starts melting above 900 °C, while diffraction from the dhcp-Nd phase completely disappears at around 600 °C. Taking advantage of the excellent quality of the XRD profiles in the powdered single-crystal sample, we also investigate the electron density distribution of the main phase by the maximum entropy method (MEM)/Rietveld analysis in order to elucidate the origin of the large magnetic anisotropy. From the subtraction of the electron density distributions recorded above (-123 °C) and below (-173 °C) the spin reorientation transition (-138 °C), we observe an anomalous change of the electron density distribution around the Nd f and g sites of Nd₂Fe₁₄B whose electron density distributions are intimately related to the magnetic anisotropy.