Abstract
In this study we investigate the magnetic properties of magnetite fine particles using MonteCarlo simulation in the framework of a core–shell model. A single-spin movementMetropolis dynamics was implemented to compute equilibrium averages. Calculations wereperformed on the basis of a three-dimensional classical Heisenberg Hamiltonian, withnearest magnetic neighbour interactions, and taking into account three differentsuperexchange integrals associated to iron cations of tetrahedral and octahedralsites. The Hamiltonian includes a surface anisotropy term applied to surface ions,and cubic anisotropy for ions belonging to the core. Different diameters wereconsidered in order to figure out different off-stoichiometric scenarios and theinfluence on the magnetic properties. Results reveal a well-defined power lawparticle size dependence of the Curie temperature, characterized by an exponentν = 0.82(5). No evidence for surface spin disorder was detected. Finally, susceptibility data reveal thatthe ferrimagnetic-to-paramagnetic transition occurs in a gradual fashion ascribed to adifferentiated behaviour between the core and surface.
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