Finite-size versus surface effects in nanoparticles

Abstract

We study the finite-size and surface effects on the thermal and spatial behaviours of the magnetisation of a small magnetic particle. We consider two systems: (1) A box-shaped particle of simple-cubic structure with either periodic or free boundary conditions. This case is treated analytically using the isotropic model of D-component spin vectors in the limit D→∞, including the magnetic field. (2) A more realistic particle (γ-Fe 2O 3) of ellipsoidal (or spherical) shape with open boundaries. The magnetic state in this particle is described by the anisotropic classical Dirac–Heisenberg model including exchange and dipolar interactions, and bulk and surface anisotropy. This case is dealt with by the classical Monte Carlo technique.