  Zero field cooled (ZFC)/Field Cooled (FC) magnetization curves of a bulk MnO
single crystal show a peculiar peak at low temperatures (~40K) similar to the
low temperature peak observed in MnO nanoparticles. In order to investigate the
origin of this peak, the spin structure of a MnO single crystal has been
studied and compared with a single phase powder sample using magnetometry and
polarized neutron scattering. Both magnetometry and polarized neutron
diffraction results confirm the antiferromagnetic (AF) phase transition at the
N\'eel temperature T_N of 118K, in both powder and single crystal form.
However, the low temperature peak in the ZFC/FC magnetization curves is not
observed in single phase MnO powder. To better understand the observed
behavior, ac susceptibility measurements have been employed. We conclude that
the clear peak in the magnetic signal from the single crystal originates from a
small amount of ferrimagnetic (FiM) Mn2O3 or Mn3O4 impurities, which is grown
at the interfaces between MnO crystal twins.
