Finite temperature magnetization reversal in ultrathin magnetic films

We study the magnetization reversal of Heisenberg spins in a two-dimensional plane under an external reversing field at finite temperatures. In the Ising limit dominated by uniaxial anisotropy and exchange, we observe nucleation of domains in Monte Carlo simulations. Analysis of the probability distribution in the energy provides for an estimate of the free energy for domain nucleation that is much smaller than that obtained from the wall energy at zero field because the energy of the nucleus approaches zero at the threshold of coherent rotation. The coercive field depends linearly on the temperature. When the dipolar interaction becomes important, reversed spins domains form quickly but do not grow. There is an intermediate regime when most spins are aligned parallel to the plane. (C) 1996 American Institute of Physics.