FLUCTUATION EFFECTS IN ULTRATHIN FILMS

We discuss finite temperature fluctuations of the magnetization ill in ultrathin magnetic films where spins interact with short-range exchan ge (J), anisotropy (K) and long-range dipolar (g) couplings. Phase bou ndaries for a triangular lattice are obtained from Monte Carlo simulat ions as a function of J, g and K. For nonzero g, the magnetization can rotate from the z to the in-plane direction. The transition temperatu re as a function of the effective anisotropy field K-g/0.1208 is essen tially unchanged as g becomes zero. This suggests a new physical pictu re in which, as the temperature is increased, the change in the direct ion of M for g not equal 0 is closely connected with the Ising transit ion for g = 0 when the M(2), disappears. Mean field calculations sugge st three transitions where only M(z) is finite at low temperature. As the temperature is increased, (1) M(x) becomes nonzero, (2) M(z) becom es zero and eventually (3) M(x) becomes zero.