  We study the influence of surface anisotropy on the zero-temperature
hysteretic properties of a small single-domain magnetic particle, and give an
estimation of the anisotropy constant for which deviations from the
Stoner-Wohlfarth model are observed. We consider a spherical particle with
simple cubic crystalline structure, a uniaxial anisotropy for core spins and
radial anisotropy on the surface, and compute the hysteresis loop by solving
the local Landau-Lifshitz equations for classical spin vectors. We find that
when the surface anisotropy constant is at least of the order of the exchange
coupling, large deviations are observed with respect to the Stoner-Wohlfarth
model in the hysteresis loop and thereby the limit-of-metastability curve, due
to the non-uniform cluster-wise reversal of the magnetisation.
