Order-Disorder Transitions under Confinement

The phase transitions in Ising thin films with antiferromagnetic interactions are studied as a function of the surface field h for symmetry-preserving surface orientations. It is found that for very large h an N-layer system undergoes a separation between its surfaces and the (N-2) inner layers. The system comprised of the surfaces and the inner layers displays a rather unusual phase diagram with a disordered phase lying between the ordered phases down to zero temperature. For large but moderate h, both ordered regions merge at high temperatures, isolating a low-temperature disordered region. Cluster variation calculations are used to compute the finite-temperature behavior.