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

Abstract

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.