Finite-size versus surface effects in nanoparticles

We study the finite-size and surface effects on the thermal and spatial beh aviours of the magnetisation of a small magnetic particle. We consider two systems: (1) A box-shaped particle of simple-cubic structure with either pe riodic or free boundary conditions. This case is treated analytically using the isotropic model of D-component spin vectors in the limit D --> infinit y. including the magnetic field. (2) A more realistic particle (gamma -Fe2O 3) of ellipsoidal (or spherical) shape with open boundaries. The magnetic s tate in this particle is described by the anisotropic classical Dirac-Heise nberg model including exchange and dipolar interactions, and bulk and surfa ce anisotropy. This case is dealt with by the classical Monte Carlo techniq ue. (C) 2000 Elsevier Science B.V. All rights reserved.