  We have studied by Monte Carlo simulations the thermal behaviour of a small
(N=13 particles) cluster described by a Heisenberg model, including
nearest-neighbour ferromagnetic interactions and radial surface anisotropy, in
an applied magnetic field. We have studied three different lattice structures:
hexagonal close packed, face centered cubic and icosahedral. We show that the
zero-field thermal behaviour depends not only on the value of the anisotropy
constant but also on the lattice structure. The behaviour in an applied field,
additionally depends, on the different orientations of the field with respect
to the crystal axes. According to these relative orientations, hysteresis
cycles show different step-like characteristics.
