We study the magnetization reversal of Heisenberg spins in a two-dimen
sional 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 coe
rcive field depends linearly on the temperature. When the dipolar inte
raction 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.