Ferromagnetic frozen structures from the dipolar hard spheres fluid at moderate and small volume fractions

We study the magnetic phase diagram of an ensemble of dipolar hard spheres (DHS) with or without uniaxial anisotropy and frozen in position on a disordered structure by tempered Monte Carlo simulations. The crucial point is to consider an anisotropic structure, obtained from the liquid state of the dipolar hard spheres fluid, frozen in its polarized state at low temperature. The freezing inverse temperature $\beta_f$ determines the degree of anisotropy of the structure which is quantified through a structural nematic order parameter, $\lambda_s$. The case of the non zero uniaxial anisotropy is considered only in its infinitely strong strength limit where the system transforms in a dipolar Ising model (DIM). The important finding of this work is that both the DHS and the DIM with a frozen structure build in this way present a ferromagnetic phase at volume fractions below the threshold value where the corresponding isotropic DHS systems exhibit a spin glass phase at low temperature.Comment: 12 pages, 13 figure

Size and polydispersity effect on the magnetization of densely packed magnetic nanoparticles

The magnetic properties of densely packed magnetic nanoparticles (MNP) assemblies are investigated from Monte Carlo simulations. The case of iron oxide nanoparticles is considered as a typical example of MNP. The main focus is put on particle size and size polydispersity influences on the magnetization curve. The particles are modeled as uniformly magnetized spheres isolated one from each other by a non magnetic layer representing the organic coating. A comparison with recent experimental results on $\gamma-$Fe$_2$O$_3$ powder samples differing by their size is given.Comment: To be published in the Journal of Applied Physics, to be found at http://jap.aip.org

Conferencia: "Phase diagram of magnetic nanoparticles ensembles from Monte Carlo simulations in the framework of an effective macro spin model."

ConferenciaIn this presentation, after a brief overview of the topic we present Monte Carlo simulations aimed at model an actual experimental situation, namely the magnetic phases of well ordered superlattices made of either Co or Fe2O3. The model considered is the simplest effective macro-spin model, namely a monodisperse ensemble of dipolar hard spheres located on the nodes of a fcc lattice, with uniaxial MAE. We focus on the characterization of the low temperature phase of the system and consequently on the conditions under which the SFM can be obtained in the framework of the finite size scaling. The results are discussed with respect to the the disorder control variable which is either the amplitude of the MAE or the variance of the easy axes orientational distribution.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. Vicerrectorado de Investigación de la UM

Contribution a la description theorique de l'interface metal noble / solution electrolytique

SIGLECNRS T 59785 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Magnetization in uniaxial spherical nanoparticles: consequence on the interparticle interaction

4 pages, proceeding of the JEMS 2008. To be published in the Journal of Magnetism and Magnetic Materials (available at http://www.sciencedirect.com/science/journal/03048853 )We investigate the interaction between spherical magnetic nanoparticles which present either a single domain or a vortex structure. First the magnetic structure of a uniaxial soft sphere is revisited, and then the interaction energy is calculated from a micromagnetic simulation. In the vortex regime the orientation of the vortex relative to the easy axis depends on both the particle size and the anisotropy constant. We show that the leading term of the interaction is the dipolar interaction energy between the magnetic moments. For particles presenting a vortex structure, we show that the polarization due to the dipolar field must be included. The parameters entering in the dipolar interaction are deduced from the magnetic behavior of the isolated particle

Phase diagram of a three-dimensional dipolar Ising model with textured Ising axes.

International audienceWe study from tempered Monte Carlo simulations the magnetic phase diagram of a textured dipolar Ising model on a face centered cubic lattice. The Ising coupling of the model follow the dipole-dipole interaction. The Ising axes are distributed with a uniaxial symmetry along theẑ direction with a gaussian probability density of the polar angles. This distribution provides a quenched disorder realization of the dipolar Ising model making a continuous link between the parallel axes dipoles and the random axes dipole models. As expected the phase diagram presents three distinctive phases: paramagnetic, ferromagnetic and spin-glass. A quasi long range ferromagnetic and a reentrant spin-glass phases are obtained in the vicinity of the ferromagnetic spin-glass line. This model provides a way to predict the magnetic phases of magnetic nanoparticles supracrystals in terms of the texturation of the easy axes distribution in the strong anisotropy limit

On the phase diagram of a three-dimensional dipolar model

The magnetic phase diagram at zero external field of an ensemble of dipoles with uniaxial anisotropy on a FCC lattice has been investigated from tempered Monte Carlo simulations. The uniaxial anisotropy is characterized by a random distribution of easy axes and its magnitude $\lambda_u$ is the driving force of disorder and consequently frustration. The phase diagram, separating the paramagnetic, ferromagnetic and spin-glass regions, was thus considered in the temperature, $\lambda_u$ plane. Here we interpret this phase diagram in terms of the more convenient variables namely the bare dipolar interaction and anisotropy energies $\epsilon_d$ and $\epsilon_u$ on the one hand and the volume fraction $\Phi$ on the other hand and compare the result with that corresponding to the random distribution of particles in the absence of anisotropy. We also display the nature of the ordered phase reached at low temperature by the ensemble of dipoles on the FCC lattice in terms of both the dipolar coupling and the texturation of the easy axes distribution when the latter is no more random. This system is aimed at modeling the magnetic phase diagram of supracrystals of magnetic nanoparticles

A statistical field theory approach applied to the liquid vapor interface

10 pages, 2 figuresInternational audienceLast years, there has been a renewed interest in the utilization of statistical field theory methods for the description of systems at equilibrium both in the vicinity and away from critical points, in particular in the field of liquid state physics. These works deal in general with homogeneous systems, although recently the study of liquids in the vicinity of hard walls has also been considered in this way. On the other hand, effective Hamiltonian pertaining to the $\phi^4$ theory family have been written and extensively used for the description of inhomogeneous systems either at the simple interface between equilibrium phases or for the description of wetting. In the present work, we focus on a field theoretical description of the liquid vapor interface of simple fluids. We start from the representation of the grand partition function obtained from the Hubbard-Stratonovich transform leading to an exact formulation of the problem, namely neither introducing an effective Hamiltonian nor associating the field to the one-body density of the liquid. Using as a reference system the hard sphere fluid and imposing the coexistence condition, the expansion of the Hamiltonian obtained yields a usual $\phi^4$ theory without unknown parameter. An important point is that the so-called capillary wave theory appears as an approximation of the one-loop theory in the functional expansion of the Hamiltonian, without any need to an underlying phenomenology

Liquid metals for nuclear applications

International audienceNuclear applications of liquid metals concern waste transmuters and liquid metal cooled fast reactors of Generation IV. One section of this paper is devoted to a short review of the motivations for liquid metals as reactor coolant or spallation target material and to historic aspects. The next two sections are dedicated to the neutron and physical properties of liquid metals, with special emphasis on sodium, lead and lead–bismuth eutectic, their respective advantages and drawbacks as reactor coolants. The question of the structure of liquid metals in relation with the liquid metal coolant technology is briefly addressed. The last section concerns the compatibility of structural materials with sodium, lead and lead–bismuth eutectic, with attention being paid to the impurities control in the different cases. We conclude briefly

Modeling of the influence of coarsening on viscoplastic behavior of a 319 foundry aluminum alloy

International audienceBoth metallurgical and mechanical behaviors of a 319 foundry aluminum alloy have been modeled by means of a multiscale approach. The nano-scale, represented by the coarsening of Al2Cu precipitates, has been modeled according to the Lifshitz-Slyozov-Wagner (LSW) law in a range of temperature going from 23 °C to 300 °C up to 1000 h aging time. Results were then compared to transmission electron microscope (TEM) observations and are in good agreement with the experimental measurements. The model allows us to know the critical radius, the volume fraction and the number of particles per μm3 in a α-phase representative volume element (RVE). The increase in yield stress generated by the interaction of dislocations with precipitates, lattice and solid solution, is modeled on the microscale. The yield stress becomes thus a function of the precipitation state, and is time/temperature dependent. These two models were then combined into a mechanical macroscale model in order to represent the Low Cycle Fatigue (LCF) behavior of the material. An elasto-viscoplastic law has been used and all the material parameters were experimentally determined with LCF stress/strain loops for the first cycle and for the mechanical steady state. The simulation results are in good agreement with the experiments