  The mechanisms of thermally activated magnetization switching in small
ferromagnetic particles driven by an external magnetic field are investigated.
For low uniaxial anisotropy the spins rotate coherently while for sufficiently
large uniaxial anisotropy they behave Ising-like, i.e. the switching then is
due to nucleation. The crossover from coherent rotation to nucleation is
studied for the classical three-dimensional Heisenberg model with uniaxial
anisotropy by Monte Carlo simulations. From the temperature dependence of the
metastable lifetime the energy barrier of a switching process can be
determined. For the case of infinite anisotropy we compare numerical results
from simulations of the Ising model with theoretical results for energy
barriers for both, single-droplet and multi-droplet nucleation. The simulated
barriers are in agreement with the theoretical predictions.
