Surface effects in nanoparticles: application to maghemite gamma-Fe2O3

We present a microscopic model for nanoparticles, of the maghemite (gamma-F e2O3) type, and perform classical Monte Carlo simulations of their magnetic properties. On account of Mossbauer spectroscopy and high-field magnetisat ion results, we consider a particle as composed of a core and a surface she ll of constant thickness. The magnetic state in the particle is described b y the anisotropic classical Dirac-Heisenberg model including exchange and d ipolar interactions and bulk and surface anisotropy. We consider the case o f ellipsoidal (or spherical) particles with free boundaries at the surface. Using a surface shell of constant thickness (similar to 0.35 nm) we vary t he particle size and study the effect of surface magnetic disorder on the t hermal and spatial behaviors of the net magnetisation of the particle. We s tudy the shift in the surface "critical region" for different surface-to-co re ratios of the exchange coupling constants. It is also shown that the pro file of the local magnetisation exhibits strong temperature dependence, and that surface anisotropy is responsible fur the non saturation of the magne tisation at low temperatures.