espanolEn este trabajo investigamos las propiedades magneticas de nanoparticulas de magnetita usando simulacion Monte Carlo en el marco de un modelo �core-shell�. Los calculos se realizan con base en un Hamiltoniano Heisenberg clasico tridimensional con interaccion a primeros vecinos y la dinamica de Metropolis. El Hamiltoniano incluye interacciones de superintercambio entre iones de Hierro, un termino de anisotropia de Neel para atomos de superficie y anisotropia cubica magnetocristalina para los atomos del interior. Se implementaron condiciones de frontera libres y se simularon nanoparticulas de diferentes diametros con el fin de estudiar el efecto del cambio en la estequiometria y del tamano sobre el comportamiento magnetico. Para el sistema en �bulk� se implementaron condiciones de frontera periodicas, y los resultados se comparan con los correspondientes a las nanoparticulas con el objetivo de notar el comportamiento de la temperatura critica al cambiar el tamano del sistema, y el rol que juega la superficie sobre las propiedades magneticas. EnglishIn this study we investigate the magnetic properties of magnetite fine particles by means of Monte Carlo simulation in the framework of a core-shell model. Calculations are performed on the basis of a three-dimensional classical Heisenberg Hamiltonian, with nearest magnetic neighbor interactions, and the Metropolis dynamics. The Hamiltonian includes interactions between Fe ions through the superexchange integrals for the different crystallographic sites, a Neel�s surface anisotropy term applied to surface ions, and a cubic magnetocrystalline anisotropy term for those ions belonging to the core. Free boundary conditions were implemented and nanoparticles of different diameters were simulated in order to study the effect of the change in the stoichiometry and particle size on the magnetic behavior. Periodic boundary conditions were used to simulate a large enough bulk system and results were used to compare with those of nanoparticles in order to evidence the behavior of the critical temperature with the system size and the role played by the surface on the magnetic properties.