Dipolar interaction effects in the thermally activated magnetic relaxation of two-dimensional nanoparticle ensembles

The thermally activated magnetic relaxation in two-dimensional ensembles of dipolar interacting nanoparticles with large uniaxial perpendicular anisotropy is studied by a numerical method and within the mean-field approximation. The role that the correlation effects in the presence of a bias magnetic field and taking into account the lattice structure play in magnetic relaxation is revealedComment: 10 pages, 3 figure

Thermal decay of the magnetization in two-dimensional nanoparticle ensembles

A method to numerically simulate the thermally induced magnetic relaxation in two-dimensional (2D) nanoparticle ensembles is generalized for the case of applied perpendicular magnetic fields. The influence of the correlations of the nanoparticle magnetic moments and of the external field on the relaxation law and on the relaxation rate is studied. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/261

Elucidating Individual Magnetic Contributions in Bi-Magnetic Fe3O4/Mn3O4 Core/Shell Nanoparticles by Polarized Powder Neutron Diffraction

Heterogeneous bi-magnetic nanostructured systems have had a sustained interest during the last decades owing to their unique magnetic properties and the wide range of derived potential applications. However, elucidating the details of their magnetic properties can be rather complex. Here, a comprehensive study of Fe3O4/Mn3O4 core/shell nanoparticles using polarized neutron powder diffraction, which allows disentangling the magnetic contributions of each of the components, is presented. The results show that while at low fields the Fe3O4 and Mn3O4 magnetic moments averaged over the unit cell are antiferromagnetically coupled, at high fields, they orient parallel to each other. This magnetic reorientation of the Mn3O4 shell moments is associated with a gradual evolution with the applied field of the local magnetic susceptibility from anisotropic to isotropic. Additionally, the magnetic coherence length of the Fe3O4 cores shows some unusual field dependence due to the competition between the antiferromagnetic interface interaction and the Zeeman energies. The results demonstrate the great potential of the quantitative analysis of polarized neutron powder diffraction for the study of complex multiphase magnetic materials

A New Approach to Include Surface Contributions in Micromagnetic Simulations of Nanoparticles

AbstractIn this work a micromagnetic model is presented for ferromagnetic nanoparticles where the surface is treated as a single effective layer and not as a separate shell. The model consists of two coupled Partial Differential Equations (PDE), one for the magnetization vector of the bulk volume and the second for the outer nodes. The strength of the coupling depends on the effective width of the layer. Simulations were made by means of the Finite Element Method (FEM). For a comparison FEM for core/shell type and atomistic Monte Carlo simulations were also performed. Our results show that Hs, the field where reversal takes place, varies as ∼1/D, where D is the particle's radius, with the anisotropy strength for any anisotropy direction. Moreover the computational cost of the effective layer model is lower than the core shell one, thus can be easily extended to larger particles where dipolar interactions should also be taken into account

Glassy dynamics in the exchange bias properties of the Fe/FeOxide nanogranular system

We have observed a glassy dynamics of the exchange bias properties in a granular system composed of Fe nanoparticles dispersed in a structurally and magnetically disordered (cluster glass-like) Fe oxide matrix. The exchange field, measured at T = 5 K, increases with increasing the time tw spent at T = 50 K, after applying the cooling field. During tw, the oxide phase evolves towards a lower energy configuration, resulting in a stronger interface exchange coupling with the Fe particle moments. Monte Carlo simulations on core/shell nanoparticles reproduce such aging effect, provided that a shell random anisotropy is assumed

Exchange bias in disordered granular systems

The exchange bias properties of a granular system composed of Fe nanoparticles (mean size ~6 nm) embedded in a structurally and magnetically disordered oxide have been investigated. The exchange bias field, resulting from the exchange coupling between the ferromagnetic particles and the frozen spin-glass-like oxide, strongly depends on the magneto-thermal history and ageing of the sample. Such dependence is a consequence of the magnetically disordered nature of the oxide phase and it is explained by considering that different frozen spin configurations of the oxide are selected on varying the temperature and the field-cooling process and that they evolve with the application time of the cooling field. Monte Carlo simulations on core/shell nanoparticles (ferromagnetic core/disordered ferrimagnetic shell) well reproduce the ageing effect

Charge distribution on the water/γ-Fe 2 O 3 interface

International audienceWe have studied the charge distribution in the γ-Fe 2 O 3 interface with H 2 O, for two different structures (films and spherical nanoparticles) with Density functional (DFT) molecular dynamics calculations. Our results show that the adsorption energy depends on the shape of the surface and in the case of the films also on the orientation of the crystal and that the ionic state of iron atoms increases with the addition of water in both structures while the magnetic moments of the structures do not show any significant change. The mean displacement of the charge with temperature is significant only in the spherical nanoparticles. The average electrostatic potential decreases with the addition of water and shows an oscillatory behavior near the surface.

Exchange bias in a magnetic ordered/disordered nanoparticle system: A Monte Carlo simulation study\u2019

We have employed the Monte Carlo (MC) simulation method to gain information on the exchange bias (EB) effect in nanoparticles composed of a ferromagnetic core and a disordered ferrimagnetic shell. The magnetic disorder of the shell affects the EB properties to the extent that they exhibit aging and training effects. The results of our MC simulations are in very good agreement with the experimental findings in a granular system composed of Fe nanoparticles (mean size not, vert, similar6 nm) embedded in a Fe oxide matrix confirming that the glassy nature of the shell is responsible for the observed aging and training effects

F-sum rule for magnetic neutron-electron scattering (2) Electrons in a magnetic field

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