Magnetic ordering and phase transition in MnO embedded in a porous glass

We present the results of a neutron diffraction study of the antiferromagne t MnO embedded in a porous glass. The type of magnetic ordering and the str uctural distortion are similar to those of the bulk, but the ordered magnet ic moment of 3.84(4)mu (B)/ion is strongly reduced and the Neel temperature is enhanced. The magnetic transition is second order, in contrast to the f irst order transition of the bulk. The size of the magnetic region is small er than the average size of the nanoparticles. The reasons for this behavio r are discussed.