Disentangling coordination and alloy effects in transition metal nanoalloys from their electronic structure

International audienc

Atomistic mechanisms for the ordered growth of Co nano-dots on Au(788): comparison of VT-STM experiments and multi-scaled calculations

Hetero-epitaxial growth on a strain-relief vicinal patterned substrate has revealed unprecedented 2D long range ordered growth of uniform cobalt nanostructures. The morphology of a Co sub-monolayer deposit on a Au(111) reconstructed vicinal surface is analyzed by Variable Temperature Scanning Tunneling Microscopy (VT-STM) experiments. A rectangular array of nano-dots (3.8 nm x 7.2 nm) is found for a particularly large deposit temperature range lying from 60 K to 300 K. Although the nanodot lattice is stable at room temperature, this paper focus on the early stage of ordered nucleation and growth at temperatures between 35 K and 480 K. The atomistic mechanisms leading to the nanodots array are elucidated by comparing statistical analysis of VT-STM images with multi-scaled numerical calculations combining both Molecular Dynamics for the quantitative determination of the activation energies for the atomic motion and the Kinetic Monte Carlo method for the simulations of the mesoscopic time and scale evolution of the Co submonolayer

Kondo screening of the spin and orbital magnetic moments of Fe impurities in Cu

We use x-ray magnetic circular dichroism to evidence the effect of correlations on the local impurity magnetic moment in an archetypal Kondo system, namely, a dilute Cu:Fe alloy. Applying the sum rules on the Fe L2,3 absorption edges, the evolution of the spin and orbital moments across the Kondo temperature are determined separately. The spin moment presents a crossover from a nearly temperature-independent regime below the Kondo temperature to a paramagneticlike regime above. Conversely, the weak orbital moment shows a temperature-independent behavior in the whole temperature range, suggesting different Kondo screening temperature scales for the spin and orbital moments

Magnetism of CoPd self-organized alloy clusters on Au(111)

Magnetic properties of gold-encapsulated CoxPd1-x self-organized nano-clusters on Au(111) are analyzed by x-ray magnetic circular dichroism for x = 0.5, 0.7, and 1.0. The clusters are superparamagnetic with a blocking temperature decreasing with increasing Pd concentration, due to a reduction of the out-of-plane anisotropy strength. No magnetic moment is detected on Pd in these clusters, within the detection limit, contrary to thick CoPd films. Both reduction of anisotropy and vanishing Pd moment are attributed to strain. (C) 2013 AIP Publishing LLC

In-situ measurements of the lattice parameter of supported palladium clusters

Palladium clusters have been epitaxially grown under ultra-high vacuum (UHV) on MgO(100) surfaces cleaved in situ. The lattice parameter was measured in situ by surface electron energy-loss fine-structure spectroscopy (SEELFS) above the palladium N23 edge. The size distribution of the clusters was obtained from ex-situ transmission electron microscopy. An expansion of the palladium lattice, increasing on decrease in the mean cluster size, has been observed. From the UHV in-situ measurements the lattice dilatation is attributed to pseudomorphism. With the SEELFS technique the Debye-Waller factor was also determined. An increase in the mean square displacement of the atoms was observed when the cluster size decreased; the latter was explained by the increase in the proportion of surface atoms and by the effect of the lattice dilatation

Rules for tight-binding calculations in bi-metallic compounds based on density functional theory: the case of Co–Au

International audienceEven though recent developments in electronic structure calculations based on density functional theory (DFT) allow us to use them for more and more realistic systems, they still remain unsuited for comprehensive studies of complex transition metal compounds involving intricate structural and chemical effects. In that case, the tight-binding approximation (TBA) is a good compromise to get reliable results with only a minimal set of parameters, provided that clear rules enable a proper self-consistent treatment of charge transfers between inequivalent sites. Thus, in the case of the Co-Au system, DFT calculations demonstrate that a local neutrality rule is obeyed per orbital and per chemical species. Shifting the atomic levels accordingly in TBA calculations is then sufficient to accurately determine the local densities of states whatever the chemical configuration. In addition, this also allows us to justify the derivation from TBA of pairwise ordering pair interactions and to determine them self-consistently