arXiv subjects

Oscar Iglesias

Publications and source records attributed to Oscar Iglesias.

28 records · Page 2Linked to original sources

arXiv2003

Role of surface disorder on the magnetic properties and hysteresis of nanoparticles

We present the results of Monte Carlo simulations of a model of a single maghemite ferrimagnetic nanoparticle including radial surface anisotropy distinct from that in the core with the aim to clarify what is its role on the magnetization processes at low temperatures. The low temperature equilibrium states are analized and compared to those of a ferromagnetic particle with the same lattice structure. We have found that the formation of hedgehog-like structures due to increased surface anisotropy is responsible for a change in the reversal mechanism of the particles.

cond-mat.mtrl-sci↗

arXiv2003

Shape and surface anisotropy effects on the hysteresis of ferrimagnetic nanoparticles

We present the results of Monte Carlo simulations of a model of a single maghemite ferrimagnetic nanoparticle with the aim to clarify the role played by the increased anisotropy at the surface and by the shape (spherical or elliptical) of the particle on the magnetization processes at low temperatures. The formation of hedgehog-like structures for high enough surface anisotropy is responsible for a change in the reversal mechanism of the particles.

cond-mat.mtrl-sci↗

arXiv2002

Effects of the Magnetic Field on the Relaxation of Small Particle Systems

We study the effect of a magnetic field on the thermal relaxation of non-interacting small monodomain particle systems particles with a distribution of anisotropy constants and random easy-axes directions. Numerical calculations of the relaxation curves for different distribution widths, and under different magnetic fields $H$ and temperatures $T$, have been performed in the framework of a two-state approximation. We show how the obtained data can be analyzed in terms of an modified $Tln(t/τ_0)$ scaling from which the field dependence of the mean relaxing energy barriers can be extracted, a microscopic information which is not easily obtainable by other methods.

cond-mat.mtrl-sci↗

arXiv2002

The magnetic structure of Li2CuO2: from ab initio calculations to macroscopic simulations

The magnetic structure of the edge sharing cuprate compound Li2CuO2 has been investigated by means of ab initio electronic structure calculations. The first and second neighbor in-chain magnetic interactions are calculated to be -142 K and 22 K, respectively. The ratio between the two parameters is smaller than suggested previously in the literature. The interchain interactions are antiferromagnetic in nature and of the order of a few Kelvins only. Monte Carlo simulations using the ab initio parameters to define the model Hamiltonian result in a Neel temperature in rather good agreement with experiment. Spin population analysis situate the magnetic moment on the copper and oxygen ions somewhere between the completely localized picture derived from experiment and the more delocalized picture based on local density calculations.

cond-mat.mtrl-sci↗

arXiv2001

Magnetic Field scaling of Relaxation curves in Small Particle Systems

We study the effects of the magnetic field on the relaxation of the magnetization of small monodomain non-interacting particles with random orientations and distribution of anisotropy constants. Starting from a master equation, we build up an expression for the time dependence of the magnetization which takes into account thermal activation only over barriers separating energy minima, which, in our model, can be computed exactly from analytical expressions. Numerical calculations of the relaxation curves for different distribution widths, and under different magnetic fields H and temperatures T, have been performed. We show how a $\svar$ scaling of the curves, at different T and for a given H, can be carried out after proper normalization of the data to the equilibrium magnetization. The resulting master curves are shown to be closely related to what we call effective energy barrier distributions, which, in our model, can be computed exactly from analytical expressions. The concept of effective distribution serves us as a basis for finding a scaling variable to scale relaxation curves at different H and a given T, thus showing that the field dependence of energy barriers can be also extracted from relaxation measurements.

cond-mat.mtrl-sci↗

arXiv2001

Finite-Size and surface effects in maghemite nanoparticles: Monte Carlo simulations

Finite-size and surface effects in fine particle systems are investigated by Monte Carlo simulation of a model of a $γ$-Fe$_2$O$_3$ (maghemite) single particle. Periodic boundary conditions have been used to simulate the bulk properties and the results compared with those for a spherical shaped particle with free boundaries to evidence the role played by the surface on the anomalous magnetic properties displayed by these systems at low temperatures. Several outcomes of the model are in qualitative agreement with the experimental findings. A reduction of the magnetic ordering temperature, spontaneous magnetization, and coercive field is observed as the particle size is decreased. Moreover, the hysteresis loops become elongated with high values of the differential susceptibility, resembling those from frustrated or disordered systems. These facts are consequence of the formation of a surface layer with higher degree of magnetic disorder than the core, which, for small sizes, dominates the magnetization processes of the particle. However, in contradiction with the assumptions of some authors, our model does not predict the freezing of the surface layer into a spin-glass-like state. The results indicate that magnetic disorder at the surface simply facilitates the thermal demagnetization of the particle at zero field, while the magnetization is increased at moderate fields, since surface disorder diminishes ferrimagnetic correlations within the particle. The change in shape of the hysteresis loops with the particle size demonstrates that the reversal mode is strongly influenced by the reduced atomic coordination and disorder at the surface.

cond-mat.mtrl-sci↗

arXiv2001

Monte Carlo Study of the Finite Size Effects on the Magnetization of Maghemite Small Particles

Monte Carlo simulations of a model for $γ$-Fe$_2$O$_3$ (maghemite) single particle of spherical shape are presented aiming at the elucidation the specific role played by the finite size and the surface on the anomalous magnetic behaviour observed in small particle systems at low temperature. The influence of the finite-size effects on the equilibrium properties of extensive magnitudes, field coolings and hysteresis loops is studied an compared to the results for periodic boundaries. It is shown that for the smallest sizes the thermal demagnetization of the surface completely dominates the magnetization while the behaviour of the core is similar to that of the periodic boundary case, independently of $D$. The change in shape of the hysteresis loops with $D$ demonstrates that the reversal mode is strongly influenced by the presence of broken links and disorder at the surface.

cond-mat.mtrl-sci↗

arXiv2000

Finite Size Effects in Small Particle Systems

We present the results of Monte Carlo simulations of a model of a $γ$-Fe$_2$ O$_3$ (maghemite) single particle of spherical shape. The magnetic Fe$^{3+}$ ions are represented by Ising spins on a spinel lattice that consists on two sublattices with octhaedral and tetrahedral coordination with exchange interactions among them and with an external magnetic field. By varying the particle diameter, we have studied the influence of the finite size of the particle on the equilibrium properties, field cooling magnetization and hysteresis loops. The simulations allow to distinguish the different roles played by the surface and the core spins of the particle on its magnetic properties. We show that for small particle sizes the core is uncoupled from the surface, that behaves as a quasi-independent layer, whereas for bigger particles the surface and the core are coupled and follow the behaviour of the bulk.

cond-mat.mtrl-sci↗

arXiv1999

Magnetic history dependence of metastable states in systems with dipolar interactions

We present the results of a Monte Carlo simulation of the ground state and magnetic relaxation of a model of a thin film consisting on a two-dimensional square lattice of Heisenberg spins with perpendicular anisotropy K, exchange J and long-range dipolar interactions g. We have studied the ground state configurations of this system for a wide range of the interaction parameters J/g, K/g by means of the simulated annealing procedure, showing that the model is able to reproduce the different magnetic configurations found in real samples. We have found the existence of a certain range of K/g, J/g values for which in-plane and out-of-plane configurations are quasi-degenerated in energy. We show that when a system in this region of parameters is perturbed by an external force that is subsequently removed different kinds of ordering may be induced depending on the followed procedure. In particular, simulations of relaxations from saturation under an a.c. demagnetizing field or in zero field are in qualitative agreement with recent experiments on epitaxial and granular alloy thin films, which show a wide variety of magnetic patterns depending on their magnetic history.

cond-mat.mtrl-sci↗

arXiv1997

Normalization factors for magnetic relaxation of small particle systems in non-zero magnetic field

We critically discuss relaxation experiments in magnetic systems that can be characterized in terms of an energy barrier distribution, showing that proper normalization of the relaxation data is needed whenever curves corresponding to different temperatures are to be compared. We show how these normalization factors can be obtained from experimental data by using the $T \ln(t/τ_0)$ scaling method without making any assumptions about the nature of the energy barrier distribution. The validity of the procedure is tested using a ferrofluid of Fe_3O_4 particles.

cond-mat.mtrl-sci↗