  We investigated a polycrystalline sample of the ferrimagnetic compound
$\mathrm{Tb_{0.15}Co_{0.85}}$ by magnetometry and small-angle neutron
scattering (SANS). The magnetization curve at 300 K is characteristic for soft
ferrimagnets but at 5 K the hysteresis indicates the existence of magnetic
domains. The magnetic SANS signal suggests that at 300 K the Tb and Co moments
are correlated over large volumes within the micrometer-sized grains with
correlation lengths > 100 nm. At 5 K, however, the magnetic SANS analysis
reveals a reduced correlation length of around 4.5 nm, which indicates the
formation of narrow magnetic domains within the ferrimagnet with one dimension
being in the nm range. We attribute the observed changes of the domain
structure to the temperature-dependence of the magnetic properties of the Tb
sublattice.
