Abstract

In this study we investigate the magnetic properties of magnetite fine particles using Monte Carlo simulation in the framework of a core-shell model. A single-spin movement Metropolis dynamics was implemented to compute equilibrium averages. Calculations were performed on the basis of a three-dimensional classical Heisenberg Hamiltonian, with nearest magnetic neighbour interactions, and taking into account three different superexchange integrals associated to iron cations of tetrahedral and octahedral sites. The Hamiltonian includes a surface anisotropy term applied to surface ions, and cubic anisotropy for ions belonging to the core. Different diameters were considered in order to figure out different off-stoichiometric scenarios and the influence on the magnetic properties. Results reveal a well-defined power law particle size dependence of the Curie temperature, characterized by an exponent ν = 0.82(5). No evidence for surface spin disorder was detected. Finally, susceptibility data reveal that the ferrimagnetic-to-paramagnetic transition occurs in a gradual fashion ascribed to a differentiated behaviour between the core and surface. © 2008 IOP Publishing Ltd.