1,721,171 research outputs found
SILS 2004
No full textOrganizzazione del Convegno
Eleonora Paris, Dipartimento di Scienze della Terra, Università di Camerino
Tel. 0737-402607; Fax 0737-402644
Segreteria del Convegno
Gabriele Giuli, Dipartimento di Scienze della Terra, Università di Camerino
Tel. 0737-402610; Fax 0737-402644;
e-mail: [email protected]
Invited Speakers
D. I. Svergun
``Synchrotron small-angle X-ray scattering studies of biological macromolecules in solution''
M. Kiskinova
``Spectromicroscopy at ELETTRA: recent achievements and new projects''
A. Zecchina
``Zeolitic and silica supported supported catalysts studied by synchrotron radiation''
F. Farges
``XAS of crystalline and amorphous silicatic systems''
R. Rueffer
``Application of nuclear resonance scattering to the study of nanoscale materials''
Comitato Scientifico
G. Artioli, Università di Milano
C. M. Bertoni, Università di Modena
M. Bolognesi, Università di Genova
A. Franciosi, INFM-TASC, Trieste
B. Martorana, Università di Palermo
S. Mobilio, Università Roma Tre
C. R. Natoli, LNF, Frascati
G. Paolucci, ELETTRA
F. Parmigiani, Università di Brescia
M. N. Piancastelli, Università di Roma "Tor Vergata"
P. Perfetti, ISM-CNR, Roma
S. Quartieri, Università di Messina
F. Sette, ESRF
G. Stefani, Università di Roma Tre
G. Zanotti, Università di Padova
Comitato Organizzatore
A. Di Cicco, Università di Camerino
R. Gunnella, Università di Camerino
R. Marassi, Università di Camerino
E. Paris, Università di Camerino
S. Stizza, Università di Camerino
F. Boscherini, Università Bologna
R. Felici, INFM-OGG c/o ESRF, Grenoble
A. Pavese, Università di Milano
I. Pettiti, Università di Roma "La Sapienza"
G. Stefani, Università di Roma Tre
N. Zema, ISM-CN
Local bonding geometry of oxygen implanted in GAN: a depth – dependent study
No full textThe bonding environment of oxygen implanted in GaN is studied using near edge X-ray absorption
fine structure spectroscopy at the O-K-edge. The 70 keV oxygen ions form a 200 nm-thick subsurface
layer that is highly defective or amorphous depending on the implantation fluence which
ranges from 1×1015 cm−2 to 1×1017 cm−2. The information depth of the fluorescence photons
varies from 50 to 63 nm, depending on the angle of incidence. The spectra are simulated using
the FEFF8 code and assuming various models, e.g., O interstitial, O substitutional in N sites, Ga
and N vacancies, and various polymorphs of Ga2O3. The lattice disorder is modelled by displacing
atoms from their equilibrium positions by adding to their Cartesian coordinates random numbers
that belong to normal distributions. The simulation results reveal that at the low fluence limit, the
O atoms occupy interstitial sites preferentially in the empty channels aligned to the c-axis and in the
atomic planes containing the Ga atoms. When the fluence is equal to 1×1016 cm−2 the O atoms
substitute N while at 1×1017 cm−2 they form mixed GaOxNy phases with the N/O ratio decreasing
with increasing depth, i.e., as we approach the peak of the implanted O profile
Atomic and electronic structure of ultrathin fluoride barrier layers at the oxide/Si interface
No full textA SrF2 ultrathin barrier layer on Si(001) is used to form a sharp interface and block reactivity and intermixing between the semiconductor and a Yb2O3 overlayer. Yb2O3/Si(001) and Yb2O3/SrF2/Si(001) interfaces grown in ultra high vacuum by molecular beam epitaxy are studied by photoemission and x-ray absorption fine structure. Without the fluoride interlayer, Yb2O3/Si(001) presents an interface reacted region formed by SiOx and/or silicate compounds, which is about 9 A ̊ thick and increases up to 14–15 A ̊ after annealing at 500–700 ◦C. A uniform single layer of SrF2 molecules blocks intermixing and reduces the oxidized Si region to 2.4 A ̊ after deposition and to 3.5 A ̊ after annealing at 500 ◦C. In both cases we estimate a conduction band offset and a valence band offset of ∼1.7 eV and 2.4 eV between the oxide and Si, respectively. X-ray absorption fine structure measurements at the Yb LIII edge suggest that the Yb oxide films exhibit a significant degree of static disorder with and without the fluoride barrier. Sr K edge measurements indicate that the ultrathin fluoride films are reacted, with the formation of bonds between Si and Sr; the Sr–Sr and Sr–F interatomic distances in the ultrathin fluoride barrier film are relaxed to the bulk value
Spin Polarization of the Photoelectrons and Photon Polarization of X-Ray Absorption: Spectroscopy and Magnetometry
No full textWe introduce here how to perform spectroscopies that are sensitive to the spin polarization of the electron states, via the direct measure of the P vector of a (photo)electron beam in a suitable experimental set-up and how to perform photoemission/photoabsorption experiments that, by exploiting the polarization of the exciting photon beam, are directly sensitive to the spin order in the initial state revealing, for example, the magnetization state of solids and surfaces or the effects of spin-orbit interaction in the surface electronic structure
Structure of Ultrathin CeO2Films on Pt(111) by Polarization-Dependent X-ray Absorption Fine Structure
No full textWe present a study of the structure of cerium oxide ultrathin films supported on Pt(111), focused on the evolution of the epitaxial strain in films of different thickness. The stoichiometry and oxidation state of the films are determined by X-ray photoemission spectroscopy, and the surface structure, measured by low-energy electron diffraction, is compared with the results obtained by the analysis of X-ray absorption fine structure measurements at the Ce L3 edge, exploiting the polarization dependence of the cross section to probe the in-plane and the out-of-plane atomic correlations. The obtained results allow to establish the epitaxial relation between the cerium oxide film and the Pt substrate and give an accurate evaluation of the cerium oxide layer structure. The 2 ML films have a fluorite structure which is compressed in the (111) plane. The measured compression is compatible with the assumption of a coincidence lattice between overlayer and substrate, in which three CeO2 surface unit cells match four Pt unit cells. The films’ three-dimensional structure is compared with the one expected assuming the bulk-phase elastic constants. The strain is released when the film thickness is increased to 10 ML, and the lattice parameters assume the bulk values
Magnetic disorder and exchange bias in nanogranular metal / metal-oxide systems
No full textThe exchange bias (EB) effect has been studied in Ni/NiO nanogranular samples obtained by an original method which combines mechanical milling and hydrogen partial reduction of NiO. In this procedure, precursor NiO powders are ball-milled to reduce the grain size to the nanometric scale; subsequently, the milled powder is subjected to high temperature treatments in H2, inducing the reduction to metallic Ni. The prepared samples typically consist of Ni nanoparticles dispersed in a nanocrystalline NiO matrix, as observed by electron microscopy (TEM). The advantage of this method is that by varying the temperature and duration of the H2-treatment, we can well control the amount of Ni that forms. Moreover, we can predetermine, to some extent, the structural features of the NiO phase, by varying the milling parameters (milling time, ball-to-powder weight ratio).
This presentation reports about the EB properties of a series of Ni/NiO samples obtained by annealing in H2, at different temperatures (200 < T <300 °C), 20 hours-milled NiO powder. The exchange field closely depends on the Ni amount, being maximum (600 Oe) in the sample annealed at 250 °C, with 15 % Ni. In all the samples, EB vanishes at ~200 K, irrespective of the Ni content.
The structural features of the samples have been investigated by X-ray diffraction, electron microscopy and X-ray Absorption Fine Structure Spectroscopy and the low-temperature magneto-thermal behaviour has been thoroughly analysed. The results allow us to propose an EB mechanism based on the existence of a structurally and magnetically disordered NiO component, which mediates the exchange interaction between the ferromagnetic Ni nanoparticles and the NiO nanocrystalline matrix
The Fe/NiO interface studied by polarization dependent X – ray absorption spectroscopy
No full textIn order to provide a structural basis for the modelling of the electronic and magnetic properties of the Fe/NiO(001) interface we haveperformed polarization dependent Fe K-edge X-ray absorption measurements. A multi-shell fit of the data is presented and discussed.We find that a 2 ML Fe film exhibits a complete tetragonal distortion of the unit cell and demonstrate the formation of a planar FeOlayer with expanded Fe–O distances perpendicular to the growth plane. We discuss a model for the interface with the FeO layer at theoxide–metal interface. At 10 ML thickness the tetragonal strain of the Fe film is partially released._ 2005 Elsevier B.V. All rights reserved
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