Evidence for a reorientation transition in the phase behaviour of a two-dimensional dipolar antiferromagnet

The results from a series of Monte Carlo simulations are presented for the two-dimensional Heisenberg model consisting of classical spin vectors arranged on the vertices of a square lattice in which the spins interact through an antiferromagnetic exchange interaction, a magnetic surface anisotropy and the dipolar interaction. The simulations focus on the exchange dominated regime in which the strength of the exchange interaction is significantly greater than both the dipolar interaction and the magnetic surface anisotropy. The results from the simulations show that there exists a range of the magnetic surface anisotropy parameter values in which the system exhibits a reorientation transition from a planar antiferromagnetic phase at low temperature to a perpendicular antiferromagnetic phase at higher temperature. The phase diagram, obtained from the Monte Carlo calculations, is determined as a function of both temperature and magnetic surface anisotropy parameter for a fixed value of the exchange constant. In addition, the low temperature magnetization data suggests a softening of a spin wave stiffness close to the phase boundary between the two ordered states.