  Recent studies revealed that the electric multipole moments of insulators
result in fractional electric charges localized to the hinges and corners of
the sample. We here explore the magnetic analog of this relation. We show that
a collinear antiferromagnet with spin $S$ defined on a $d$-dimensional cubic
lattice features fractionally quantized magnetization $M_{\text{c}}^z=S/2^d$ at
the corners. We find that the quantization is robust even in the presence of
gapless excitations originating from the spontaneous formation of the N\'eel
order, although the localization length diverges, suggesting a power-law
localization of the corner magnetization. When the spin rotational symmetry
about the $z$ axis is explicitly broken, the corner magnetization is no longer
sharply quantized. Even in this case, we numerically find that the deviation
from the quantized value is negligibly small based on quantum Monte Carlo
simulations.
