Phase behavior of antiferromagnetic ultrathin magnetic films

The phase diagram of a system of classical spins on a square lattice, interacting through a nearest-neighbor antiferromagnetic exchange and a dipolar interaction, is presented. The phase diagram is based on results from a series of Monte Carlo simulations. The phase diagram shows a parallel antiferromagnetic phase, in which the spins are aligned in the x-y plane, and an antiferromagnetic perpendicular phase, in which the spins are aligned perpendicular to the x-y plane. The critical value of the exchange interaction, J(c)(T), on the phase boundary separating the two phases shows only a weak dependence on temperature (J(c) approximate to J(0)), while the transition appears to be first order with an extremely small latent heat. The Monte Carlo data also indicate that the parallel phase separates into two distinct phases, although further work is required to determine the precise nature of the phase boundary separating the two regions. Finally, the low-temperature magnetization data suggest a softening of the spin-wave stiffness close to the phase boundary, The Neel temperature of the perpendicular antiferromagnetic phase is found to be consistent with earlier predictions of spin-wave calculations.