  We have performed unpolarized and polarized neutron diffraction experiments
on monodisperse 8 nm and 13 nm antiferromagnetic MnO nanoparticles. For the 8
nm sample, the antiferromagnetic transition temperature $T_N$ (114 K) is
suppressed compared to the bulk material (119 K) while for the 13 nm sample
$T_N$ (120 K) is comparable to the bulk. The neutron diffraction data of the
nanoparticles is well described using the bulk MnO magnetic structure but with
a substantially reduced average magnetic moment of 4.2$\pm$0.3 $\mu_B$/Mn for
the 8 nm sample and 3.9$\pm$0.2 $\mu_B$/Mn for the 13 nm sample. An analysis of
the polarized neutron data on both samples shows that in an individual MnO
nanoparticle about 80$%$ of Mn ions order. These results can be explained by a
structure in which the monodisperse nanoparticles studied here have a core that
behaves similar to the bulk with a surface layer which does not contribute
significantly to the magnetic order.
