  We present the results of Monte Carlo simulations of the magnetic properties
of a model for a single nanoparticle consisting in a ferromagnetic core
surrounded by an antiferromagnetic shell. The simulations of hysteresis loops
after cooling in a magnetic field display exchange bias effects. In order to
understand the origin of the loop shifts, we have studied the thermal
dependence of the shell and interface magnetizations under field cooling. These
results, together with inspection of the snapshots of the configurations
attained at low temperature, show the existence of a net magnetization at the
interface which is responsible for the bias of the hysteresis loops.
