  We present the results of Monte Carlo simulations of the magnetic properties
of individual spherical nanoparticles with the aim to explain the role played
by surface anisotropy on their low temperature magnetization processes. Phase
diagrams for the equilibrium configurations have been obtained, showing a
change from quasi-uniform magnetization state to a state with hedgehog-like
structures at the surface as $k_S$ increases. Through the simulated hysteresis
loops and the analysis of spin configurations along them, we have identified a
change in the magnetization reversal mechanism from quasi-uniform rotation at
low $k_S$ values, to a non-uniform switching process at high $k_S$. Results for
the dependence of the coercive field and remanence on $k_S$ and particle size
are also reported.
