  Hot-deformed anisotropic Nd$-$Fe$-$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 Nd$-$Fe$-$B nanocrystals
surrounded by a Pr$-$Cu-rich intergranular grain-boundary phase, has been
investigated using unpolarized small-angle neutron scattering (SANS) and very
small-angle neutron scattering (VSANS). 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.
