Reorientation of Spin Density Waves in Cr(001) Films induced by Fe(001) Cap Layers

Proximity effects of 20 \AA thin Fe layers on the spin density waves (SDWs) in epitaxial Cr(001) films are revealed by neutron scattering. Unlike in bulk Cr we observe a SDW with its wave vector Q pointing along only one {100} direction which depends dramatically on the film thickness t_{Cr}. For t_{Cr} < 250 \AA the SDW propagates out-of-plane with the spins in the film plane. For t_{Cr} > 1000 \AA the SDW propagates in the film plane with the spins out-of-plane perpendicular to the in-plane Fe moments. This reorientation transition is explained by frustration effects in the antiferromagnetic interaction between Fe and Cr across the Fe/Cr interface due to steps at the interface.Comment: 4 pages (RevTeX), 3 figures (EPS

Phase Diagram of Multilayer Magnetic Structures

Multilayer "ferromagnet-layered antiferromagnet" (Fe/Cr) structures frustrated due to the roughness of layer interfaces are studied by numerical modeling methods. The "thickness-roughness" phase diagrams for the case of thin ferromagnetic film on the surface of bulk antiferromagnet and for two ferromagnetic layers separated by an antiferromagnetic interlayer are obtained and the order parameter distributions for all phases are found. The phase transitions nature in such systems is considered. The range of applicability for the "magnetic proximity model" proposed by Slonczewski is evaluated.Comment: 8 pages, 8 figure

Electronic structure of Fe/semiconductor/Fe(001) tunnel junctions

The electronic ground-state properties of Fe/semiconductor/Fe(001) tunnel junctions are studied by means of the ab initio screened Korringa-Kohn-Rostoker method. We focus on the magnetic properties, charge transfer, local, and q(parallel to)-resolved density of states of these systems. We consider in detail Fe/ZnSe/Fe(001) tunnel junctions and compare their electronic properties with junctions with Si and GaAs barriers. We discuss the results in connection to the spin-dependent transport properties expected for these systems

GGA+U study of the incorporation of iodine in uranium dioxide

71.15.Mb Density functional theory, local density approximation, gradient and other corrections, 71.27.+a Strongly correlated electron systems; heavy fermions,

Atomic scale investigation of the diffusion of defects and fission gases in uranium dioxide

International audienceFuel behaviour under irradiation is extremely complex due to the combined effect of radiation and temperature. Actinide fission produces large quantities of defects and fission products, which have a significant influence on the structural, thermal and mechanical properties of nuclear fuels and claddings. A better understanding of atomic transport properties in these materials is central to getting further insight into the irradiation driven micro-structural changes. An efficient approach to unravel the basic mechanisms governing these properties is to couple separate effect experiments and modelling at the atomic scale (1-3). We will present the study of the diffusion of defects and krypton in uranium dioxide (UO2) using atomic scale calculations (4). The migration barriers of the elementary mechanisms are calculated in the DFT+U framework using the nudged elastic band method. The attempt frequencies are obtained from the phonon modes of the defect at the initial and saddle points using empirical potential methods. The preferred mechanisms for defect and Kr migration and the corres-ponding diffusion coefficients as a function of the oxygen chemical potential or nonstoichiometry are then calculated by combining this data with diffusion models adapted to the systems studied

First-principles investigation on the bulk properties of americium dioxide and sesquioxides

International audienceWe report an investigation of the ground state properties of americium dioxide and its sesquioxides using first-principles calculations. In order to take into account strong 5f electronic correlations, we apply the generalized gradient approximation (GGA)+U as well as the hybrid functionals to these compounds. We present a systematic study of several bulk properties such as structural, elastic, energetic, electronic and magnetic properties as a function of the U and J GGA+U parameters in the range of 0.0-8.0 eV and 0.00-0.75 eV respectively. The values (U,J) = (6.00 eV,0.75 eV) for onsite Coulomb interaction U and exchange J parameters are those providing the best description of bulk properties by comparison to the available experimental data. We discuss the effect of loss of symmetry caused by DFT+U on several properties. We also discuss the effect of the spin-orbit coupling on these properties. Furthermore, we highlight that in order to reach the ground state of americium dioxide and sesquioxides using GGA+U, the monitoring of the occupation matrices of 5f correlated orbitals is crucial. Our computational scheme provides reliable and accurate results concerning several bulk properties that had not been studied experimentally yet. For instance, our elastic constants calculated for AmO2 follow the same trend as those of other actinide oxides (UO2, NpO2 and PuO2) and can be used as reference. We have also computed the dielectric properties of americium dioxide. Finally, we find the A-type hexagonal structure to be the most stable structure at low temperature for americium sesquioxides. For this phase, we predict the internal structural parameters as well as the bulk modulus. We provide the first data concerning the formation enthalpy of the Am2O3 bcc C-type

DFT + U investigation of charged point defects and clusters in UO2

International audienc

Advances in first-principles modelling of point defects in UO 2 : f electron correlations and the issue of local energy minima

International audienceOver the last decade, a significant amount of work has been devoted to point defect behaviour in UO 2 using approximations beyond density functional theory (DFT), in particular DFT + U and hybrid functionals for correlated electrons. We review the results of these studies from calculations of bulk UO 2 properties to the more recent determination of activation energies for self-diffusion in UO 2 , as well as a comparison with their experimental counterparts. We also discuss the efficiency of the three known methods developed to circumvent the presence of metastable states, namely occupation matrix control, U-ramping and quasi-annealing

Oxidation of plutonium dioxide an X-ray absorption spectroscopy study

International audienc