  We consider a simple model of maghemite nanoparticles and study their
magnetic properties using Monte Carlo methods. The particles have a spherical
geometry with diameters ranging from 3 nm to 8 nm. The interior of the
particles consists of core spins with exchange interactions and anisotropy
given by the values in the bulk material. The outer layer of the particles
consists of surface spins with weaker exchange interactions but an enhanced
anisotropy. The thermal behaviour of the total, core and surface magnetizations
are calculated as well as the hysteresis loops due to the application of an
applied field. The effect of the surface anisotropy on the blocking
temperature, the coercive and exchange bias fields is studied.
