  We report the results of an unpolarized small-angle neutron scattering (SANS)
study on Mn-Bi-based rare-earth-free permanent magnets. The magnetic SANS cross
section is dominated by long-wavelength transversal magnetization fluctuations
and has been analyzed in terms of the Guinier-Porod model and the distance
distribution function. This provides the radius of gyration which, in the
remanent state, ranges between about $220-240 \, \mathrm{nm}$ for the three
different alloy compositions investigated. Moreover, computation of the
distance distribution function in conjunction with results for the so-called
$s$-parameter obtained from the Guinier-Porod model indicate that the magnetic
scattering of a Mn$_{45}$Bi$_{55}$ sample has its origin in slightly
shape-anisotropic structures.
