  We report about the magnetic properties of antiferromagnetic (AF) MnO
nanoparticles (NPs) with different sizes (6-19nm). Using a combination of
polarized neutron scattering and magnetometry we were able to resolve
previously observed peculiarities. Magnetometry, on the one hand, reveals a
peak in the zero field cooled (ZFC) magnetization curves at low temperatures
(~25K) but no feature around the N\'eel temperature at 118K. On the other hand,
polarized neutron scattering shows the expected behavior of the AF order
parameter vanishing around 118K. Moreover, hysteresis curves measured at
various temperatures reveal an exchange bias effect indicating a coupling of an
AF core to a ferromagnetic (FM)-like shell. ZFC data measured at various fields
exclude a purely superparamagnetic (SPM) scenario. We conclude that the
magnetic behavior of MnO particles can be explained by a superposition of
SPM-like thermal fluctuations of the AF-N\'eel vector inside the AF core and a
strong magnetic coupling to a ferrimagnetic Mn$_2$O$_3$ or Mn$_3$O$_4$ shell.
In addition, we have studied heterodimer ('Janus') particles, where a FM FePt
particle is attached to the AF MnO particle. Via the exchange bias effect, the
magnetic moment of the FePt subunit is stabilized by the MnO.
