Finite Size Effects in Small Particle Systems

We present the results of Monte Carlo simulations of a model of a $\gamma$-Fe$_2$ O$_3$ (maghemite) single particle of spherical shape. The magnetic Fe$^{3+}$ ions are represented by Ising spins on a spinel lattice that consists on two sublattices with octhaedral and tetrahedral coordination with exchange interactions among them and with an external magnetic field. By varying the particle diameter, we have studied the influence of the finite size of the particle on the equilibrium properties, field cooling magnetization and hysteresis loops. The simulations allow to distinguish the different roles played by the surface and the core spins of the particle on its magnetic properties. We show that for small particle sizes the core is uncoupled from the surface, that behaves as a quasi-independent layer, whereas for bigger particles the surface and the core are coupled and follow the behaviour of the bulk.