J. Restrepo

Surface anisotropy, hysteretic, and magnetic properties of magnetite nanoparticles: A simulation study

In this study we address the role of surface anisotropy on the hysteretic properties of magnetite Fe3O4 nanoparticles and the circumstances yielding both horizontal and vertical shifts in the hysteresis loops. Our analysis involves temperature dependence and particle size effects. Different particle sizes ranging from 2 up to 7 nm were considered. Our theoretical framework is based on a three-dimensional classical Heisenberg model with nearest magnetic neighbor interactions involving tetrahedral (A) and octahedral (B) irons. Cubic magnetocrystalline anisotropy for core spins, single-ion site anisotropy for surface spins, and interaction with a uniform external magnetic field were considered. Our results revealed the onset of low temperature exchange bias field, which can be positive or negative at high enough values of the surface anisotropy constant (KS). Susceptibility data, computed separately for the core and the surface, suggest differences in the hard-soft magnetic character at the core-surface inte...

Effect of surface anisotropy on the magnetic properties of magnetite nanoparticles: A Heisenberg–Monte Carlo study

In this study, we analyze the effect of surface anisotropy on the magnetic properties of magnetite Fe3O4 nanoparticles on the basis of a core-shell model. Magnetization, magnetic susceptibility, and specific heat are computed over a wide range of temperatures. In our model, we stress on magnetite nanoparticles of 5nm in diameter which consist of 6335 ions. Our theoretical framework is based on a three-dimensional classical Heisenberg Hamiltonian with the nearest magnetic neighbor interactions between iron ions involving tetrahedral (A) and octahedral (B) sites. Terms dealing with cubic magnetocrystalline anisotropy for core ions, a single-ion site surface anisotropy for those Fe ions belonging to the shell, and the interaction with a uniform external magnetic field are considered. To compute the equilibrium averages, a single-spin movement Monte Carlo–Metropolis dynamics was used. Results reveal the occurrence of low-temperature spin configurations different from those expected for a collinear single-doma...

Influence of non-stoichiometry on the magnetic properties of magnetite nanoparticles

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 ? = 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.