D. Chandesris

Crystallographic structure of cobalt films on Cu(001): elastic deformation to a tetragonal structure

Abstract The stable structure of cobalt is hexagonal closed-packed (hcp), but cobalt can be stabilized in a distorted face-centered cubic structure (fcc) by epitaxy on Cu(001). The deviation from the isotropic fcc structure is determined by surface-extended X-ray absorption fine structure (EXAFS). A polarization dependent first shell analysis of the EXAFS spectra shows that the Co/Cu(001) films have a face-centered tetragonal structure (fct): the mean nearest-neighbour distance parallel to the surface is 2.55A(same value as in bulk copper) and the interlayer bond length is 2.50A: the films are in perfect epitaxy on copper (001) with a contraction of the lattice parameter perpendicular to the surface of 4%, in agreement with the continuum elasticity theory. A constant tetragonalization is observed for films of 2 to 15 monolayers. A simulation of the EXAFS spectra is also done using the FEFF code. In order to simulate the polarization dependence of the experimental spectra, we have introduced in this theoretical calculation a polarization dependence of the paths (single scattering and multiple scattering ones) based on a plane wave approximation. This simple model gives a good agreement between calculated and experimental polarization dependent EXAFS spectra both on bulk hcp cobalt and on the face-centered tetragonal films Co/Cu(001); this agreement confirms the structure determined from the first shell analysis.

A surface EXAFS study of thin nickel deposits on (110) TiO2 surfaces

Abstract The first stages of nickel deposition on a clean (110) TiO 2 surface were studied using surface extended X-ray absorption fine structure (EXAFS). Experiments were performed on two kinds of nickel deposits, one of 0.8 equivalent monolayers and another of 1.6 equivalent monolayers. Depositions were performed on well characterized TiO 2 (110)-p(1 × 1) surfaces. The correlation of Auger experiments with EXAFS results showed that no islands were present on the surface; only two-dimensional growth of nickel was observed. Moreover, it was shown, in the case of the thinner deposit, that a model with nickel atoms forming chains in the channels determined by oxygen atoms on the surface is consistent with the EXAFS data.

EXAFS study of the crystallographic structure of cobalt thin films on Pt(111)

Abstract The growth and structure of cobalt on a platinum (111) single crystal is studied using X-ray absorption spectroscopy at the cobalt K-edge. A polarisation dependent analysis of the EXAFS spectra shows first neighbours distances in Co Pt (111) films identical to those in bulk cobalt for thicknesses between 2 and 10 monolayers. The full shell spectra polarisation dependence indicates the hexagonal stacking of the thicker film. Influences of annealing and recovering by platinum on the structure of the cobalt films are also described.

Structure of epitaxial ferromagnetic layers by multishell analysis of polarization dependent XAFS data

Abstract X-ray absorption fine structure (XAFS) has been often used to determine the structure of metastable magnetic films and multilayers deposited on single crystal substrates. These metastable phases provide important extensions of the family of simple elemental magnetic crystals and give new elements for the comparison with the predictions of electronic structure calculations. Further the well-known strengths of XAFS, a quantitative simulation of the spectra including distant shells contributions allows to identify unambiguously stacking sequences and the symmetry of the adsorption sites at the interface. This is now possible by using multiple scattering calculations which take the polarization of the light into account. It is illustrated by the CoCu system. The stable structure of cobalt is hexagonal closed packed (hcp), but cobalt can be stabilized in a distorted face centered cubic structure (fcc) by epitaxy on Cu(001). The deviation from the isotropic fcc structure is here determined precisely by polarization dependent XAFS. On the other hand, for a cobalt film grown on a (111) oriented copper single crystal, a complete simulation of the X-ray absorption spectra, using a multiple scattering formalism, clearly shows that the thick films have a local hcp stacking. The analysis of the thinner films reveals details of the growth of cobalt on Cu(111): the first two deposited layers follow the fcc (111) copper lattice, whereas the third one already starts an hcp (0001) stacking.

Structure and local order of thin cobalt films on Cu(1 1 1)

Abstract We have studied thin cobalt films deposited on Cu(1 1 1), from 0.5 to 8 ML (monolayer). We have found that for all the thicknesses, the in-plane nearest neighbour distance is 2.53 A, and the out-of-plane one is 2.50 A. Moreover, we have used the FEFF code to simulate the polarization-dependent shape of the Fourier transforms of HCP and FCC crystals. We conclude that below 3 ML, the cobalt is clearly prolonging the copper lattice, but that beyond this thickness, HCP domains are appearing.

X-ray absorption resonance spectroscopy as a local probe of the electronic structure at metal-semiconductor interfaces and silicide: Pt/Si, Pd/Si, and Yb/Si

Abstract Measurement of the distribution of the empty band states is one of the key problems for reaching a more complete understanding of the electronic structure of metal/semiconductor systems. X-ray absorption resonance on transition-metal (TM) L 2,3 edges are a probe of the presence and of the approximate distribution of d-like empty states. The results provide evidence of the existence of empty antibonding d-like states in the electronic structure of near-noble-metal/silicon interfaces and silicides; Yb L 2.3 resonances provide evidence of mixed valency in the Yb/Si system even for thin underreacted Yb/Si interfaces.

Resonant 3d photoemission of La and Ce around the L3 absorption edge in LaPd and CePd

Abstract We have measured the 3 d -photoemission of La and Ce at photon energies around their L 3 absorption edges in LaPd and CePd. We have observed a modification of the photoemitted intensity of these deep core levels due to quantum interference between the direct photoemission channel and de-excitation of the core excited state, as first described by Fano. We discuss, in a qualitative way, the observed differences between La and Ce, as well as the possible role of quadrupolar transitions to the resonant process.

Crystallographic properties and adsorbate geometries by surface EXAFS

Abstract We describe briefly the principles of the extended X-ray absorption fine structure (EXAFS) technique and discuss in what conditions it can be applied to the local crystallographic study of a clean surface or a surface covered with an adsorbate. Then we present two experiments: a total yield SEXAFS study of cobalt adsorbed on copper(111) and Auger SEXAFS study of an amorphized clean surface of silicon. In both cases we discuss what kind of information can be deduced.

Structure and growth mode of epitaxial Co/Au(111) magnetic thin films.

Abstract The growth mode and the structure of Co layers deposited at room temperature (RT) on a Au(111) single crystal has been studied using core level photoemission spectroscopy (CL-PES) and surface X-ray absorption spectroscopy (SEXAFS). The analysis of the Au 4f 7/2 photoemission line for different Co coverages provides evidence of an island growth mode with the presence of a slight interdiffusion process at the interface. The polarization dependence of the XANES and EXAFS spectra, taken at the Co K-edge, shows clearly a hexagonal stacking for films thicker than 4ML. The analysis of the first nearest neighbour shell shows that this hexagonal structure is very close to that of the Co hcp bulk: the cobalt films do not grow in a coherent epitaxy on the Au(111) surface and they are then not strongly strained. Thus, we can exclude a magnetoelastic contribution to the perpendicular magnetic anisotropy existing in this system.

GROWTH OF ULTRATHIN Mn FILMS ON Ag(100)

Mn films with a thickness of one monolayer have been deposited, in ultrahigh vacuum, on a Ag(100) single crystal at room temperature, and analyzed by X-ray photoelectron diffraction (XPD) and surface extended X-ray absorption fine structure (SEXAFS). The strong enhancement of the X-ray photoelectron Mn2p3/2 signal along the Ag[1100] direction indicates that a substantial fraction of the Mn atoms is located within the second topmost atomic layer. The SEXAFS data demonstrate that the Mn–Ag and Mn–Mn first neighbor distances are the same as the Ag–Ag distances in the Ag lattice; as a result, the atoms occupy Ag sites on the fcc lattice. Comparison of the experimental data with FEFF calculations indicates that a superficial Mn–Ag alloy is formed. If the film is mildly (330 K) annealed, the contrast in the XPD Mn 2p3/2 modulations versus polar angle increases and reaches a maximum value of 60%. As confirmed by single scattering simulations, these results mean that atomic place exchange occurs between Mn and Ag atoms, and eventually the second atomic plane of the sample is constituted mainly by Mn atoms, whereas the first atomic layer is a pure Ag plane, i.e. a buried atomic Mn monolayer is formed.