Effects of texture on lattice constants of Nd2Fe14B and their relationship with internal stress in Nd-Fe-B permanent magnets

Although crystallographic properties are considered to be intrinsic, several studies have reported significant different values for the lattice constants of the Nd2Fe14B phase. In this study, by analyzing rod-shaped isotropic (randomly oriented) and anisotropic (c-axis oriented) Nd-Fe-B bulk-sintered magnets, their powdered counterparts, and powder made from grinding a single crystal, we experimentally demonstrate that the lattice constants of the Nd2Fe14B phase in the ferromagnetic ordering state depend on the sample texture. The temperature dependence of the lattice constants was investigated by focusing on their expansion associated with ferromagnetic ordering, that is, the spontaneous linear expansion along the a-axis (lambda(a)) and c-axis (lambda(c)). The lambda(a) value of the isotropic (anisotropic) bulk-sintered magnets at 105 K is 10% (2%) smaller while the lambda(c) value is 130% (40%) greater than that of the powdered single crystal. These results indicate the presence of an internal stress perpendicular to the c-axis in the isotropic and anisotropic bulk-sintered magnets. In addition, we observed that the ratios of the length along the a-axis to that along the c-axis (c/a) for the various Nd-Fe-B magnets as well as those reported previously are correlated to the sample texture. In other words, the c/a values of the sintered magnets are greater than those of the powdered samples. This result suggests that the sample texture and hence the internal stress has a determining effect on the lattice constants.