FLUCTUATION EFFECTS IN ULTRATHIN FILMS

We discuss finite temperature fluctuation of the domain walls and the magnetization M in ultrathin magnetic films where spins interact with short range exchange (J), anisotropy (K), and long range dipolar (g) couplings. Phase boundaries for a triangular lattice are obtained from Monte Carlo simulations as a function of J, g, and K. The transition temperature for the disappearance of the z magnetization as a function of the effective anisotropy field K-g/0.1208 is essentially unchanged as g becomes zero. This suggests a new physical picture that the change in the direction of M is closely connected with the Ising transition for g=0. Mean field calculation suggests three transitions where only Mz is finite at low temperature. As the temperature is increased (1) Mx becomes nonzero, (2) Mz becomes zero and eventually, (3) Mx becomes zero. To investigate fluctuation of domain walls their elastic energy are calculated. For Bloch walls perpendicular to the x axis separating spins along the z axis this energy is negative for a small enough wave vector. The competition of the stabilizing long range dipolar interaction and low dimensional fluctuation suggests the possibility of a finite temperature roughening of an array of one-dimensional Néel walls in the film.