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.
Highlights
Nd-Fe-B sintered magnets have been extensively investigated to improve their magnetic properties, especially coercivity.1–3 It has been reported that coercivity is closely related to the microstructure of Nd-Fe-B sintered magnets, which are composed of Nd2Fe14B crystal grains and Nd-rich secondary phases, such as hcp-Nd2O3, fcc-NdOx, dhcp-Nd, and fcc-Nd, on the grain boundaries and at the grain boundary triple junctions.4–13 For this reason, clarification of the constituent phases and their structural properties has been seen as critical to the further understanding of Nd-Fe-B sintered magnet magnetic properties
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
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
Summary
Nd-Fe-B sintered magnets have been extensively investigated to improve their magnetic properties, especially coercivity. It has been reported that coercivity is closely related to the microstructure of Nd-Fe-B sintered magnets, which are composed of Nd2Fe14B crystal grains and Nd-rich secondary phases, such as hcp-Nd2O3, fcc-NdOx, dhcp-Nd, and fcc-Nd, on the grain boundaries and at the grain boundary triple junctions. For this reason, clarification of the constituent phases and their structural properties has been seen as critical to the further understanding of Nd-Fe-B sintered magnet magnetic properties. We clarified that, for rod-shaped isotropic (randomly oriented) and anisotropic (c-axis oriented) bulk-sintered magnets, their powdered samples, and fine single crystals, the lattice constants of the Nd2Fe14B phase were different below the Curie temperature (TC: approximately 580 K), depending on sample texture.. We clarified that, for rod-shaped isotropic (randomly oriented) and anisotropic (c-axis oriented) bulk-sintered magnets, their powdered samples, and fine single crystals, the lattice constants of the Nd2Fe14B phase were different below the Curie temperature (TC: approximately 580 K), depending on sample texture.20 This difference probably originates from the internal stress generated in bulk-sintered magnets, with such internal stresses likely to affect secondary phase crystallographic properties Nd2Fe14B magnetostriction. Some reports have mentioned that the structural mismatch between Nd2Fe14B and its secondary phases can cause local strain near the interface. Recently, we clarified that, for rod-shaped isotropic (randomly oriented) and anisotropic (c-axis oriented) bulk-sintered magnets, their powdered samples, and fine single crystals, the lattice constants of the Nd2Fe14B phase were different below the Curie temperature (TC: approximately 580 K), depending on sample texture. This difference probably originates from the internal stress generated in bulk-sintered magnets, with such internal stresses likely to affect secondary phase crystallographic properties
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