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

We present the results of a neutron diffraction study of the antiferromagnet MnO embedded in a porous glass. The type of magnetic ordering and the structural distortion are similar to those of the bulk, but the ordered magnetic moment of 3.84(4)muB/ion is strongly reduced and the Néel temperature is enhanced. The magnetic transition is second order, in contrast to the first order transition of the bulk. The size of the magnetic region is smaller than the average size of the nanoparticles. The reasons for this behavior are discussed.