National Institute Of Materials Physics - Romania

This work concerns the preparation and characterisation of homogeneous bulk Co2Mn (Si, Ge, Ga, Sn Sb0.8Sn0.2) to be used as targets for pulsed laser deposition of magnetic contacts for spintronic devices. The samples have been prepared by direct synthesis from elemental powders followed by vertical gradient freeze. Characterisation of their structural properties and compositional homogeneity was performed by X-ray diffraction and energy-dispersive X-ray spectroscopy combined with scanning electron microscopy. The results show one-phase for the Heusler alloys. The Curie temperatures of the samples have been measured by both thermogravimetric analysis and differential scanning calorimetry and are in a good agreement with the previously published values. The saturation magnetisation values are close to the values for single crystals. (C) 2004 Elsevier B.V. All rights reserved.

Nanostructured Ni/C-60 thin films, produced by co-evaporation of Ni and fullerene, have been investigated by both X-ray diffraction (XRD) and X-ray magnetic circular dichroism (XMCD). X-ray diffraction has shown the co-existence of diffraction specific peaks for the fullerene face-centered cubic crystal structure and diffraction peaks of Ni nanoparticles in the face-centered cubic structure, which are broadened by nanoparticule finite dimensions. Analysis of peak widths shows that the Ni nanocrystallite size ranges from 4.7 to 19.2 nm. The XMCD signals for a sample series, preparated on relatively low-temperature substrates during the co-evaporation, can be interpreted as having distinct contributions due to both bulk and surface Ni atoms. These two kind of atoms are antiferromagnetic coupled. The Ni atoms which are at the interface with the fullerite matrix show ail enhanced orbital magnetic moment, while the orbital magnetic moment of Ni atoms in the bulk has very low values, as it can be expected for a crystalline field with a cubic symmetry.

A surface characterization has been performed on n-type GaAs (Si: GaAs) samples by using X-ray photoelectron spectroscopy (XPS) technique in order to get information on the degree of surface contamination during the device processing steps. The GaAs samples were etched by "in situ" ion sputtering (Ar plasma) before and after carrying out the measurements. According to XPS measurements the native oxide layer on as-received surface contains As2O3, AS(2)O(5) and Ga2O3. Large amounts of C and O are also present at surface before plasma cleaning of surface: C-C bonds, chemisorbed oxygen atoms. The XPS spectra recorded on the surface sample sputtered with 5 keV Ar+ ions were analyzed using SDP subroutines program facilities. After the first sputtering step it was observed different shifts of the principal peaks of GaAs bonding and compounds together with a modification of the relative peak intensities. The C1s and O1s peak size decreased but remained still significant. The following sputtering steps lead to a drastically decrease of C and 0 peaks down to the noise level.

The electrical and transport properties of GaAs crystals such as resistivity (rho), mobility (mu) and carrier concentration (n) depend on the purity and defects of a particular crystal. These parameters are normally obtained by performing a simple Hall effect analysis in a bar configuration. The technical procedure is connected with the design of appropriate contacts for Hall bar configuration. In order to ensure good ohmic contacts oil GaAs, thin metal layers of Au-Ge-Ni have been deposited in high vacuum followed by heat treatment in low vacuum. Hall effect measurement is important for checking the quality of the GaAs wafers. We analyzed various GaAs wafers with (100) and (110), orientation obtained by LEC and HB methods. The resistivity of the wafers varied from low resistivity, n-type doped crystals (n = 10(18) cm(-3)), to high resistivity crystals (n = 10(14) cm(-3)). This paper presents the data of Hall characterization of wafers cut from GaAs crystals grown by different methods (LEC, HB).

Interface reaction and magnetism of epitaxially-grown Fe on InAs(100) are studied by core-level photoemission (As 3d and In 4d) and Fe 2p X-ray magnetic circular dichroism using synchrotron radiation. The reactivity of Fe/InAs(100) is relatively low compared to that of other interfaces involving deposition of 3d metals on III-V semiconductors. As a consequence, we observe a magnetic signal at Fe L-2,L-3 edges for the lowest thicknesses studied (1 ML). The atomic magnetic moment reaches a value close to that of the bulk alpha-Fe (2.2 mu(B)) for Fe coverages exceeding 5 ML. A ferromagnetic compound with approximate stoichiometry of FeAs is formed at the interface. The orbital magnetism represents between 12 and 20% of the total momentum, due to 3d density of states depletion and to crystal-field modification of the electronic levels. These properties make the Fe/InAs(100) interface very promising for spin-tunneling devices.

Ru-zeolites prepared via deposition of aqueous solution of ruthenium chloride on X, L, ZSM-5 and APO-54 were investigated. The impregnated solids were characterized by the following techniques: N-2 sorption, H-2 chemisorption, TPR, XRD, XPS, EXAFS and Mossbauer spectroscopy. It was found that deposition of Ru is influenced by the pore structure and the chemical composition of the supports, in particular the concentration and strength of the protonic sites, with the formation of the different species. The packing of Ru as well as the degree of the reduction and the metal particle size are different according to the penetration depth of the metal and the nature of the zeolite. The amount of chlorine possibly remaining as adatoms of Ru is very low or inexistent. In calcined and reduced catalysts no chloride peak could be evidenced by XPS. The experiments confirmed that the use of large amounts of Ru chloride precursor leads to rather large metal particles. (C) 1999 Elsevier Science B.V. All rights reserved.

Two Pd-Dy/gamma-Al2O3 catalysts, with 1 wt.% Dy and various Pd loadings (0.1 and 0.5 wt.%) have been investigated by EXAFS at the Dy L(III)-edge, and compared with 1 % Dy/gamma-Al2O3. The Dy-carrier interaction was found to change with the Pd dispersity.