Surface effects in nanoparticles: Monte Carlo simulations
Abstract
We present a microscopic model for nanoparticles and perform Monte Carlo simulations of their magnetic properties. On account of Mossbauer spectroscopy and high-field magnetization results, we consider a particle as containing a magnetically ordered core and a relatively disordered surface. The magnetic state in the particle is described by the anisotropic classical Dirac-Heisenberg model including exchange and dipolar interactions and bulk and surface anisotropy. We consider the case of ellipsoidal (or spherical) particles with open boundary conditions at the surface. Using a surface shell of constant thickness (about 0.35 nm) we vary the particle size and study the effects of surface disorder on the thermal and spatial behaviors of the net magnetization of the particle. We study the shift in the surface critical region for different surface-to-core ratios of the exchange coupling constants. It is also shown that the profile of the local magnetization exhibits strong temperature dependence. Thermal behaviour of the specific heat is also obtained.Keywords
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