Influence of magnetostriction on the lattice constants of the secondary phases in Nd-Fe-B sintered magnets studied by synchrotron X-ray diffraction

We investigated the structural influence of Nd2Fe14B phase magnetostriction on the Nd-rich secondary phases of hcp-Nd2O3, fcc-NdOx, dhcp-Nd, and fcc-Nd, in Nd-Fe-B bulk-sintered magnets. The temperature dependence of the secondary phase lattice constants was evaluated by synchrotron X-ray diffraction using rod-shaped isotropic (randomly oriented) and anisotropic (c-axis oriented) Nd-Fe-B-Cu sintered magnets. The lattice constants of each secondary phase in the rod-shaped isotropic and anisotropic samples exhibited an invar-type expansion below the Nd2Fe14B phase Curie temperature (TC), despite the phases not showing ferromagnetic ordering at TC—and peak broadening was seen for the secondary phases below TC. Since such volume expansion has not been seen in powdered sintered magnets, we viewed it as a phenomenon particular to rod-shaped sintered magnets. These results indicated that the Nd2Fe14B phase spontaneous volume expansion induced the volume expansion observed in all the constituent phases of the bulk sintered magnets.