Orbital moment of a single Co atom on a Pt(111) surface - a view from correlated band theory

The orbital magnetic moment of a Co adatom on a Pt(111) surface is calculated in good agreement with experimental data making use of the LSDA+U method. It is shown that both electron correlation induced orbital polarization and structural relaxation play essential roles in orbital moment formation. The microscopic origins of the orbital moment enhancement are discussed

Orbital magnetic moment and extrinsic spin Hall effect for iron impurity in gold

We report electronic structure calculations of an iron impurity in gold host. The spin, orbital and dipole magnetic moments were investigated using the LDA+$U$ correlated band theory. We show that the {\em around-mean-field}-LDA+$U$ reproduces the XMCD experimental data well and does not lead to formation of a large orbital moment on the Fe atom. Furthermore, exact diagonalization of the multi-orbital Anderson impurity model with the full Coulomb interaction matrix and the spin-orbit coupling is performed in order to estimate the spin Hall angle. The obtained value $\gamma_S \approx 0.025$ suggests that there is no giant extrinsic spin Hall effect due to scattering on iron impurities in gold.Comment: 5 pages, 2 figure

Anomalous Ferromagnetism of Monatomic Co Wire at the Pt(111) Surface Step Edge

A first-principles investigation of the anomalous ferromagnetism of a quasi-one-dimensional Co chain at the Pt(111) step edge is reported. Our calculations show that the symmetry breaking at the step leads to an easy magnetization axis at an odd angle of $\sim20^{\circ}$ {\em towards} the Pt step, in agreement with experiment [P. Gambardella {\em et al.}, {\em Nature} {\bf 416}, 301 (2002)]. Also, the Co spin and orbital moments become noncollinear, even in the case of a collinear ferromagnetic spin arrangement. A significant enhancement of the Co orbital magnetic moment is achieved when modest electron correlations are treated within LSDA+$U$ calculations.Comment: Presented at MRS Meeting in Boston, Dec. 2003; 4 pages including 3 figure

Coulomb Correlations and Magnetic Anisotropy in ordered L10L1_0 CoPt and FePt alloys

We present results of the magneto-crystalline anisotropy energy (MAE) calculations for chemically ordered $L1_0$ CoPt and FePt alloys taking into account the effects of strong electronic correlations and spin-orbit coupling. The local spin density + Hubbard U approximation (LSDA+U) is shown to provide a consistent picture of the magnetic ground state properties when intra-atomic Coulomb correlations are included for both 3$d$ and 5$d$ elements. Our results demonstrate significant and complex contribution of correlation effects to large MAE of these material.Comment: revised version; 4 pages, 2 figure

Probing magnetism in the vortex phase of PuCoGa5 by X-ray magnetic circular dichroism

We have measured X-ray magnetic circular dichroism (XMCD) spectra at the Pu M4;5 absorption edges from a newly-prepared high-quality single crystal of the heavy fermion superconductor 242PuCoGa5, exhibiting a critical temperature Tc = 18.7 K. The experiment probes the vortex phase below Tc and shows that an external magnetic field induces a Pu 5f magnetic moment at 2 K equal to the temperature-independent moment measured in the normal phase up to 300 K by a SQUID device. This observation is in agreement with theoretical models claiming that the Pu atoms in PuCoGa5 have a nonmagnetic singlet ground state resulting from the hybridization of the conduction electrons with the intermediate-valence 5f electronic shell. Unexpectedly, XMCD spectra show that the orbital component of the 5f magnetic moment increases significantly between 30 and 2 K; the antiparallel spin component increases as well, leaving the total moment practically constant. We suggest that this indicates a low-temperature breakdown of the complete Kondo-like screening of the local 5f moment.JRC.G.I.5-Advanced Nuclear Knowledg

Surface interaction of PuO2, UO2+x and UO3 with water ice

The interaction of PuO2, UO2+x and UO3 with water ice was studied using ultraviolet photoelectron spectroscopy (UPS). Water was adsorbed at 80–120 K as thick ice multilayers. Surface modification after desorption of the ice around 200 K was investigated. Main information on the surface oxidation state was obtained by highly surface sensitive UPS-HeII spectra, probing primarily the first monolayer. The oxidation state was directly deduced from the intensity of the actinide 5f levels. The surface character of the phenomenon was further confirmed by comparing HeII spectra with the more bulk sensitive HeI spectra. Spectral interpretation was done using the cross-section variations in HeI and HeII spectra and by comparing the spectra with theoretical density of states curves, obtained by LSDA + U calculations. It was shown previously that reduction to Pu2O3 takes place, when the ice covered PuO2 films are warmed up under UV light and ice is desorbed. In this paper, this effect was investigated in further detail. It was shown that only the top surface layer is reduced. Reduction is inhibited by surface diffusion of oxygen trapped in the films during sputter deposition and not incorporated in the lattice. UO2+x and UO3 also undergo reduction, but to a significant lesser extent than PuO2. Reoxidation of surface U by bulk oxygen was much slower than that of surface Pu. It was shown that for all oxides, reduction needs the illumination of an ice overlayer by UV light. Surface reduction by atomic hydrogen was investigated to check for possible influence of ice photolysis products. UO3 was shown to be reduced to UO2 while PuO2 is not further reduced. The observations are explained by photochemical decomposition of water by the UV light (used for UPS) at the oxide-ice interface. It is thought that the oxide acts as photocatalyst, absorbing light and splitting adsorbed water. The thick ice layer traps the reaction products on the surface, thereby enabling them to react and reduce the surface. Why only the reductants (probably H) and not the concomitant oxidants react with the surface is still unknown.JRC.E.6-Actinide researc