M.C. Saint-Lager

A NEW UHV DIFFRACTOMETER FOR SURFACE STRUCTURE AND REAL TIME MOLECULAR BEAM DEPOSITION STUDIES WITH SYNCHROTRON RADIATIONS AT ESRF

Abstract We describe a new surface X-ray diffractometer, which is optimized to combine surface X-ray diffraction (SXRD), absorption spectroscopy (SEXAFS) and grazing incidence small angle scattering (GISAXS). This instrument is particularly well suited for real time studies of material elaborated in-situ with molecular beam deposition (MBD) techniques. The goniometer allows for large in-plane and out-of-plane momentum transfer with high accuracy. Owing to the flipping mechanism of the sample holder, it can perform absorption experiments with polarisation directions normal and parallel to the sample surface, while keeping a grazing incidence. Several MBD sources as well as complementary surface sensitive electron techniques (RHEED and Auger spectroscopy) can be used simultaneously with X-rays. Finally, the samples can easily be inserted in the X-ray chamber via an UHV transport system and a fast entry load-lock module.

Thickness effect on alloying of ultrathin Co films on Pt(111): a real time and in situ UHV study with synchrotron X-ray diffraction

Abstract We study the alloying of ultrathin Co deposits (3 and 10 monolayer) on a Pt(111) substrate. The surface film evolution is followed in real time during annealing by X ray diffraction, which provides the depth profile with atomic resolution. We fully characterize the formation of Pt–Co surface alloys before dissolution; the kinetics is shown to depend on the Co thickness. For the 10 monolayer film, the dominant hcp phase exhibits a strong resistance to interdiffusion while for the 3 monolayer deposit the surface film is progressively enriched in Pt. Finally, whatever the annealing treatment and the film thickness, the stabilized alloy is close to Pt60Co40 bulk-like, after heating around 450°C.

X-ray anomalous diffraction and resonant magnetic scattering X-ray study of FexMn1−x/Ir(001) superlattices

Abstract X-ray anomalous diffraction in different geometries and X-ray resonant magnetic scattering have been used to investigate Fe x Mn 1− x /Ir(001) superlattices. The influence of the alloy stoichiometry on the structural and magnetic properties has been studied and compared with the bulk alloy properties. The structural results are compared with the magnetic results in order to understand the outstanding properties of these systems.

Determination of the Fe layer structure in Fe/Ru superlattices by grazing incidence angle X-ray diffraction

Abstract The hexagonal structure of ferromagnetic Fe in Fe/Ru superlattices is still controversial. The unexpected magnetic behaviour [1] as well as the X-ray absorption near-edge structure (XANES) results have led Beaudelet [2] to question the hcp structure for the Fe layers and to suggest another model of the Fe hexagonal stacking in which the local environment of Fe atoms looks like a distorted bcc one, despite the epitaxy on (0001) hcp Ru. To verify the existence of this unusual hexagonal stacking, we have undertaken an X-ray diffraction study performed with grazing incidence angle. Unexpectedly, our results support convincingly the regular hcp structure rather than the ‘bcc-like’ packing.

Simulation of Surface Resonant X-ray Diffraction

We present an ab initio numerical tool to simulate surface resonant X-ray diffraction experiments. The crystal truncation rods and the spectra around a given X-ray absorption edge are calculated at any position of the reciprocal space. Density functional theory is used to determine the resonant scattering factor of an atom within its local environment and to calculate the diffraction peak intensities for surfaces covered with a thin film or with one or several adsorbed layers. Besides the sample geometry, the collected data also depend on several parameters, such as beam polarization and incidence and exit angles. In order to account for these factors, a numerical diffractometer mimicking the experimental operation modes has been created. Finally two case studies are presented in order to compare our simulations with experimental spectra: (i) a magnetite thin film deposited on a silver substrate and (ii) an electrochemical interface consisting of bromine atoms adsorbed on copper.

LONG RANGE ORDER IN ULTRATHIN Pt–Co(111) ALLOYS

Long range order (LRO) in ultrathin Pt–Co films was studied by surface X-ray diffraction (XRD). Several fcc alloys of nanometric thickness were grown in situ by annealing at 460°C Co layers deposited onto a Pt(111) single crystal. Superstructure reflections were observed, which agreed with the extinction rules of either the L12 or the L10 chemically ordered bulk phases. The relationship between their structure factor and the atomic scattering factors was found with anomalous scattering performed near both Pt LIII and Co K edges. This method is very promising for studies of surface alloying. The films were also studied in real time during annealing. They evolved in a quite different way, depending on the initial Co thickness, but LRO always occurred by heating above 400°C. At this temperature the films became Pt-rich, with a stoichiometry close to Pt60Co40. We did not succeed in obtaining long range chemical order in a Co-rich alloy by annealing a Co/Pt(111) deposit, contrary to what happens in films grown by codeposition.

Structural investigation of metallic superlattices using X-ray anomalous scattering

Abstract X-ray anomalous scattering has been used for the selective characterisation of the structure of each sublattice in Fe/Ir (001) superlattices. The data have been analysed by comparing the simulated and experimental spectra in q-space at various wavelengths near the absorption edges. This approach, applied to symmetric reflection scans, enables a model-independent determination of the perpendicular lattice spacings of the individual layers.

Thermal bilayer monochromator crystal for a high resolution multiwavelength anomalous diffraction beamline at the European Synchrotron Radiation Facility

High intensity for diffraction experiments with high energy resolution, on an intense source like bending magnet at the European Synchrotron Radiation Facility, requires a strict control of the curvature of the optical elements placed in the beam for geometrical focusing and for wavelength monochromatization. Unwanted curvature of the first crystal of the monochromator can arise from the thermal load. Indeed, due to the size of the crystal, only cooling from the rear is conceivable. This induces a front‐to‐rear thermal gradient and, as a consequence, a strong spherical curvature. A new technique was developed for the CRG/D2AM beamline in order to control this effect. It was shown by calculation that this curvature can be exactly compensated, whatever the heat load power is, by the thermal expansion of a metallic layer coated at the rear of the crystal. First results confirm the predicted behavior and show how sensitive the technical problem of the fixation of this bilayer is on its cooling device.

CO and O2 chemisorption on Pd70Au30(110) : evolution of the surface studied by in situ STM and complementary surface analysis techniques at elevated pressures

Elevated pressure in situ STM studies were crucial to show that CO adsorption on gold surfaces strongly modifies their surface structures [1–3]. However oxygen dissociation does not occur spontaneously on such surfaces preventing efficient CO oxidation. The addition of an oxygen-dissociative metal (such as Pd) to gold may overcome this difficulty. In this work we present results concerning the behaviour of Pd70Au30(110) under elevated pressures of CO and O2 and compare them to the results obtained on Au(110) under similar conditions.