Evolution of the magnetic phase transition in MnO confined to channel type matrices: Neutron diffraction study

Neutron diffraction studies of antiferromagnetic MnO confined to MCM-41 type matrices with channel diameters 24--87 {} demonstrate a continuous magnetic phase transition in contrast to a discontinuous first order transition in the bulk. The character of the magnetic transition transforms with decreasing channel diameter, showing the decreasing critical exponent and transition temperature; however, the latter turns out to be above the N\'eel temperature for the bulk. This enhancement is explained within the framework of the Landau theory, taking into consideration the ternary interaction of the magnetic and associated structural order parameters.