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Two new ordered monolayer phases of ${\mathrm{C}}_{60}$ on Al surfaces have been studied using electron spectroscopies and low-energy electron diffraction (LEED). On Al(111), in addition to the previously reported (6\ifmmode\times\else\texttimes\fi{}6) phase formed by evaporating with ${\mathrm{T}}_{\mathrm{sample}}$=620 K, a metastable $(2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ phase can be produced with ${\mathrm{T}}_{\mathrm{sample}}$=300 K. This phase exhibits the first LEED pattern reported for an unannealed ${\mathrm{C}}_{60}$ overlayer. On Al(110), when evaporations are also made with ${\mathrm{T}}_{\mathrm{sample}}$=620 K, LEED shows the presence of a monolayer with a pseudo-$c(4\ifmmode\times\else\texttimes\fi{}4)$ structure. Al $2p$ photoemission for ${\mathrm{C}}_{60}$/Al(110) and for $(2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ ${\mathrm{C}}_{60}$/Al(111) reveals no evidence of strong substrate reconstruction. The perturbation of the geometric and electronic structure of the ${\mathrm{C}}_{60}$ molecule due to the bonding interaction with the Al surface increases in the order ${\mathrm{C}}_{60}$/Al(110), $(2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ ${\mathrm{C}}_{60}$/Al(111), (6\ifmmode\times\else\texttimes\fi{}6) ${\mathrm{C}}_{60}$/Al(111), as demonstrated using element-specific probing of the valence band with x-ray absorption and C $1s$ shakeup. The bond has covalent character in all three cases. Symmetry-induced splitting in the ${5h}_{u}$-derived level is observed using valence photoemission, and is particularly clear for ${\mathrm{C}}_{60}$/Al(110). The stability of the equilibrium structures can be qualitatively understood from considerations of the energetics of the overlayer compression and the chemical bond between adsorbate and substrate. Work-function measurements for these and other ${\mathrm{C}}_{60}$ overlayer systems cannot, in general, be understood within a simple description involving the addition of a dipole to the surface potential.

We report near-edge x-ray-absorption fine-structure (NEXAFS) and magnetic circular x-ray dichroism (MCXD) measurements of 4-ML Ni films grown on Cu(100). The films were exposed to oxygen which then adsorbed in different adsorption states. The changes in the density of states as probed at the Ni ${\mathit{L}}_{2,3}$ edges, the oxygen K edge, as well as the magnetic response of the sample are investigated simultaneously for the various adsorption states that range from ${\mathrm{O}}_{2}$/Ni at 38 K to an ultrathin film of NiO. The NEXAFS spectra provide a characterization of the changes in the different nickel 3d, 4sp, and oxygen 2p density of states above the Fermi level. These states dominate the interaction of the oxygen and nickel atoms and can be studied separately using the element specificity of x-ray absorption. We find that 0.5 ML of atomic oxygen suppresses the magnetization corresponding to one layer of Ni, and for 1.5 ML no ferromagnetic response from a 4-ML film at 38 K could be detected. \textcopyright{} 1996 The American Physical Society.

We present photoemission and C 1s photoabsorption data for an annealed monolayer ${\mathrm{C}}_{60}$/Al(111), which show that a strong covalent bond is formed between the ${\mathrm{C}}_{60}$ molecules and the substrate. Low-energy electron-diffraction and scanning tunneling microscopy data reveal a highly symmetric overlayer structure commensurate with the substrate. Al 2p photoelectron spectra show that the chemical environment at the surface is greatly modified compared to clean Al(111), and indicate that a significant fraction of the surface atoms have moved, consistent with a picture of covalent bonding to the overlayer. The strength of the interaction is such that the fullerenes are distorted more than on other metal surfaces or in compounds with alkali metals.

The structural and dynamical properties of molecular oxygen in submonolayer coverage adsorbed on Cu(100) were investigated by means of oxygen K-edge photoabsorption. The adsorption geometry of the O 2 molecules, as well as the disorder and anharmonic contributions to the pair distribution function for the first-nearest-neighbor O-Cu shell were quantitatively determined for well-defined sequential molecular states of physisorption and chemisorption

Etude de l'evolution au voisinage de la temperature critique des rendements electroniques totals et de fluorescence, determines par absorption de rayons X dans des configurations de type NEXAFS et EXAFS

We have made in-situ investigations of ultrathin films of Fe grown on a Ag(100) single crystal. This system is known to exhibit a reorientation of the magnetization, dependent on both Fe thickness and temperature. X-ray absorption fine structure has been recorded for samples prepared in-situ, both in the near edge and in the extended x-ray absorption fine structure (EXAFS) region. Using x-ray magnetic circular dichroism we found that Fe films between 2-5 monolayers show an out-of-plane remanent magnetization. For thicker films the magnetization lies in-plane. By applying the magneto-optical sum rules we found an enhancement of the orbital moment (m(l)) for the out-of-plane phase, whereas the spin moment (m(s)) is unchanged. From the Fe L-edge EXAFS, we obtain information on the local crystallographic structure. A strong correlation between the local crystallographic structure and the magnetic properties is found for the Fe/Ag(100) system. Variations in the nearest neighbor distance of similar to 0.15 angstrom are observed when comparing 3 ML and 25 ML films. By comparison to simulations performed with the FEFF 8.10 code, we can identify a bcc structure for 6-25 ML Fe on Ag(100). For the 3 ML Fe film our simulations indicate strong deviations from any structure derived along the Bain path. We conclude that intermixing and distortion are important to describe the structure in this thickness range. (Less)

Arrays of magnetic nanodots have been prepared by evaporation in ultrahigh vacuum onto self-organized templates consisting of nanometric pyramids of SiGe alloy previously grown on Si(001). Co was deposited by molecular beam epitaxy at grazing incidence, so that preferential deposition occurs on the island facets facing the Co flux. The films' structural and magnetic morphology has been characterized by means of mirror electron microscopy and x-ray photoemission electron microscopy with magnetic circular dichroism. The morphology of the Co deposit is dictated by the surface template. The Co dots are elongated along the direction perpendicular to deposition due to geometrical factors of the surface at grazing incidence and via steering by the surface steps, and this promotes an uniaxial magnetic anisotropy in this direction. The dots are ferromagnetic and single domain at room temperature. Macroscopic magnetic domains composed of several dots are observed. This indicates that the dipolar interaction between them is stronger than the coercivity and the exchange interaction is minimized.

We have probed the electronic structure of La1-xSrxMnO3 using x-ray absorption spectroscopy, and compared our results to theory. It is shown that the O projected spectrum reveals a signal that is c ...

In this Thesis the magnetic and structural properties of thin epitaxial Fe, Co, and Ni films are discussed. Some of the in-situ prepared samples were used to characterize the degree of circular polarization of the newly installed beamline D1011 at MAX-lab. By means of x-ray magnetic circular dichroism (XMCD) and utilizing the associated magneto optic sum rules, the orbital (ml) and spin (ms) moments are determined directly in mB/atom with elemental specificity. The extended x-ray absorption fine structure (EXAFS) measurements yield site specific information on the local crystallographic structure.These measurements were performed using the circular x-rays of several beamlines. The influence of the degree of spatial source coherence lspat of the x-rays was characterized by means of Fresnel diffractometry. A correlation between enhanced XAS white line intensities and higher values of lspat was established for 20 ML Fe, Co, and Ni films on Cu(100).The degree of circularly polarized x-rays (Pc) at beamline D1011 at MAX-lab was characterized by studying Fe films on Cu(100) by means of XMCD. The maximum value of Pc is experimentally determined to Pc =0.85.The Au/Co/Au trilayer system was studied as a function of Co thickness, temperature, and Au cap thickness. A 10 mono-layer (ML) Co film, with an Au cap of 20 A, shows a spin reorientation transition (SRT) from an in-plane to an out-of-plane easy direction as the temperature is lowered from 300 K to 200 K. The magnetic properities of these Co films are very different to what is found for bulk samples due to, in particular, the broken symmetry at the interfaces.The thickness dependent spin reorientation transition in the Fe/Ag(100) system was characterized by means of XMCD and EXAFS measurements. 3 ML Fe films show an out-of-plane easy direction with an 125% enhanced orbital moment as compared to the 25 ML Fe in-plane film. Simulations of the Fe L-edge EXAFS indicate the bulk Fe bcc structure for film thicknesses of 6-25 ML Fe. For 3 ML Fe strong deviations from this bcc phase is observed.Ultrathin Co films deposited on flat and vicinal Cu(111) in the thickness region 1-25 ML were studied by means of XMCD and scanning tunneling microscopy (STM). The vicinal Cu(111) Co deposition leads to the formation of elongated islands preferentially oriented along the step edges. In connection to this particular Co growth mode we observe an increase of both the orbital and the spin moment on the vicinal Cu(111) of about 25% relative to what was observed for Co on flat Cu(111).

Les spectres de la structure fine de l'absorption de rayons X proches de la limite, a partir de l'excitation de la couche K des molecules de N 2 adsorbees sur une surface de Fe(111) (etat τ), revelent des structures qui, comparees aux donnees de la phase gazeuse correspondante, sont identifiees comme etant des excitations de Rydberg et d'electrons multiples π•. La dependance angulaire des ces excitations est mesuree pour la premiere fois