Effect of grain-boundary diffusion process on the geometry of the grain microstructure of Nd -Fe -B nanocrystalline magnets

Hot-deformed anisotropic $\mathrm{Nd}\ensuremath{-}\mathrm{Fe}\ensuremath{-}\mathrm{B}$ nanocrystalline magnets have been subjected to the grain-boundary diffusion process (GBDP) using a ${\mathrm{Pr}}_{70}{\mathrm{Cu}}_{30}$ eutectic alloy. The resulting grain microstructure, consisting of shape-anisotropic $\mathrm{Nd}\ensuremath{-}\mathrm{Fe}\ensuremath{-}\mathrm{B}$ nanocrystals surrounded by a $\mathrm{Pr}\ensuremath{-}\mathrm{Cu}$-rich intergranular grain-boundary phase, has been investigated using unpolarized small-angle neutron scattering and very small-angle neutron scattering. The neutron data have been analyzed using the generalized Guinier-Porod model and by computing, model independently, the distance distribution function. We find that the GBDP results in a change of the geometry of the scattering particles: In the small-$q$ regime, the scattering from the as-prepared sample exhibits a slope of about 2, which is characteristic for the scattering from two-dimensional platelet-shaped objects, while the GBDP sample manifests a slope of about 1, which is the scattering signature of one-dimensional elongated objects. The evolution of the Porod exponent indicates the smoothing of the grain surfaces due to the GBDP, which is accompanied by an increase of the coercivity.