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 degrees C. The secondary phases mainly include dhcp-Nd, fcc-NdOx, and hcp-Nd2O3 at room temperature and fcc-Nd at elevated temperatures. The main phase starts melting above 900 degrees C but remains relatively stable up to 600 degrees C, while the dhcp-Nd phase completely disappears at around 600 degrees 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 degrees C, where the spin reorientation transition at -138 degrees 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. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.