DIPOLAR-INDUCED PLANAR ANISOTROPY IN ULTRATHIN MAGNETIC-FILMS

Monte Carlo simulations are used to study the role of the dipolar inte raction in determining the properties of a monolayer of magnetic dipol es on a square lattice. The dipoles are constrained to lie in the plan e but are free to rotate within it. Comparison of the Monte Carlo resu lts with those from spin-wave theory indicate that the system orders b elow a nonzero critical temperature. The effect of the thermally induc ed anisotropy on the spin-wave spectrum and its role in stabilizing th e ordered phase at low temperatures is discussed. Results for the corr elation function. both above and below the transition temperature, are compared with the corresponding results for the standard XY model.