We study theoretically the magnetization reversal of Heisenberg spins
in a 2D plane under an external reversing field at finite temperatures
. Nucleation is observed in a Monte Carlo simulation for an array of s
pins interacting with exchange, anisotropic and dipolar interactions.
The nucleus is elliptical in shape. The eccentricity is smaller the la
rger the dipolar interaction is with respect to the exchange. The coer
cive field seems to depend linearly on the temperature.