Quantitative identification of constituent phases in a Nd-Fe-B-Cu sintered magnet and temperature dependent change of electron density of Nd₂Fe₁₄B studied by synchrotron X-ray diffraction

We have measured 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 a Nd₂Fe₁₄B main-phase and the other secondary phases as a function of increasing temperature up to 1047 °C. The secondary phases mainly include dhcp-Nd, fcc-NdO_x, and hcp-Nd₂O₃ at room temperature and fcc-Nd at elevated temperatures. The main phase starts melting above 900 °C but remains relatively stable up to 600 °C, while the dhcp-Nd phase completely disappears at around 600 °C. Taking an advantage of the excellent quality of the XRD profiles in the powdered single crystal sample, we have also investigated the electron density distribution of main-phase by MEM/Rietveld analysis in order to elucidate the origin of the large magnetic anisotropy. The change in asymmetric part of electron density is derived by the subtraction of the electron density distributions between those recorded at -123 and -173 °C, where the spin reorientation transition at -138 °C changes the magnetic anisotropy. This change is attributed to be a difference of electron density distribution between Nd f and g sites of the Nd₂Fe₁₄B, relating to the magnetic anisotropy.