Effects of a weak magnetic surface anisotropy on the magnetic phase diagram of the dipolar antiferromagnetic Heisenberg system

The results of a series of Monte Carlo simulations are presented which examine the effect of a weak magnetic surface anisotropy on the phase behavior of a system of classical three-dimensional spins on a square lattice. In addition to the magnetic surface anisotropy the spins are assumed to interact through a nearest neighbor antiferromagnetic exchange interaction (J<0) and the long range dipolar interaction. The magnetic phase diagram obtained from these simulations is presented for both a negative (planar) and positive (perpendicular) magnetic surface anisotropy constant. In both cases the JT phase diagram exhibits an in-plane dipolar phase for low values of the exchange constant and a perpendicular antiferromagnetic phase for larger values. Both the location and temperature dependence of the phase boundary separating these two phases is shown to be dependent on the sign of the magnetic surface anisotropy constant. In particular the phase boundary is found to monotonically decrease for a negative surface anisotropy and monotonically increase for a positive value. This implies the existence of a thermally induced reorientation transition in which the sequence of states is determined by the sign of the surface anisotropy constant. The results from these simulations are discussed within the context of previous studies on similar systems.