G.C. Gazzadi

The BEAR Beamline at Elettra

The BEAR (Bending Magnet for Emission Absorption and Reflectivity) beamline is installed at the right exit of the 8.1 bending magnet at ELETTRA. The beamline — in operation since January 2003 — delivers linear and circularly polarized radiation in the 5 – 1600 eV energy range. The experimental station is composed of a UHV chamber for reflectivity, absorption, fluorescence and angle resolved photoemission measurements and a UHV chamber for in‐situ sample preparation.

Growth and structure of cobalt oxide on (001) bct cobalt film

Abstract The interaction of oxygen with the (001) surface of a metastable cubic cobalt film, 10 monolayers (ML) thick, epitaxially grown on the Fe(001) surface, has been studied at 300 and 470xa0K by means of primary-beam diffraction modulated electron emission, X-ray photoelectron spectroscopy and Auger electron spectroscopy. At 300xa0K the formation of 7xa0ML of CoO has been observed, in the typical rocksalt structure, rotated by 45° with respect to the square unit cell of the unreacted, underlying cobalt film. A relevant fraction of cobalt and oxygen atoms with reduced coordination has been detected which does not affect significantly the local order in the oxide film. At 470xa0K, the oxide extends in thickness involving almost the full cobalt film. For both room-temperature and high-temperature growth conditions, the resulting oxide layer is tetragonally distorted.

Epitaxial growth of ultrathin Fe films on Ni(001): a structural study

Abstract We have investigated the atomic geometry of Fe films on a Ni(001) crystal in the thickness range 0–25xa0ML with the aim of following the transition from the fcc pseudomorphic structure to the equilibrium bcc phase. The structural techniques used are primary-beam diffraction modulated electron emission (PDMEE) and low-energy electron diffraction (LEED). For film thicknesses up to 5xa0ML the films are fcc, strained in the direction of film growth. For higher Fe coverages, the films begin a transition to the bulk bcc structure which is complete after 14xa0ML of Fe coverage. We have obtained evidence for intermixing between Fe and Ni in the first 3xa0ML. The orientational relationship between the bcc phase and the underlying fcc substrate has also been investigated, finding a bcc (110) phase with four possible domains, each with one of the 〈111〉 directions parallel to an fcc 〈110〉 direction.

Hall nano-probes fabricated by focused ion beam

Hall magnetometers with active areas down to 100 × 100 nm2 were fabricated patterning gold and Si-doped GaAs films by focused ion beam. For GaAs probes, electrical characterization shows that the magnetic flux sensitivity is better than 10−2Φ0 at room temperature. Hall nano-probes made of gold can work down to liquid helium temperature with magnetic flux sensitivity 10−1Φ0.

Growth of Fe ultrathin films on Ni(111): structure and electronic properties

Abstract We investigated the evolution during film growth of the electronic properties of Fe/Ni(111) ultrathin films with electron energy loss spectroscopy from the valence band and from the Fexa0L 2,3 ionization edges, relating them with structural modifications monitored with low energy electron diffraction. At ca. 4 monolayers of Fe a fcc to bcc phase transition was observed with the major changes in the electronic structure occurring in thexa03d band region.

Paradoxical Enhancement of the Power Factor of Polycrystalline Silicon as a Result of the Formation of Nanovoids

Hole-containing silicon has been regarded as a viable candidate thermoelectric material because of its low thermal conductivity. However, because voids are efficient scattering centers not just for phonons but also for charge carriers, achievable power factors (PFs) are normally too low for its most common form, i.e. porous silicon, to be of practical interest. In this communication we report that high PFs can, indeed, be achieved with nanoporous structures obtained from highly doped silicon. High PFs, up to a huge 22 mWxa0K−2xa0m−1 (more than six times higher than values for the bulk material), were observed for heavily boron-doped nanocrystalline silicon films in which nanovoids (NVs) were generated by He+ ion implantation. In contrast with single-crystalline silicon in which He+ implantation leads to large voids, in polycrystalline films implantation followed by annealing at 1000°C results in homogeneous distribution of NVs with final diameters of approximately 2 nm and densities of the order of 1019xa0cm−3 with average spacing of 10 nm. Study of its morphology revealed silicon nanograins 50 nm in diameter coated with 5-nm precipitates of SiBx. We recently reported that PFs up to 15 mWxa0K−2xa0m−1 could be achieved for silicon–boron nanocomposites (without NVs) because of a simultaneous increase of electrical conductivity and Seebeck coefficient. In that case, the high Seebeck coefficient was achieved as a result of potential barriers on the grain boundaries, and high electrical conductivity was achieved as a result of extremely high levels of doping. The additional increase in the PF observed in the presence of NVs (which also include SiBx precipitates) might have several possible explanations; these are currently under investigation. Experimental results are reported which might clarify the reason for this paradoxical effect of NVs on silicon PF.

Epitaxial growth of CoO on the (001) surface of bct cobalt

Abstract Adsorption of oxygen on a (001) surface of 10 monolayers thick, metastable cubic Co film epitaxially grown on the Fe(001) surface was studied at 300 and 470xa0K by means of primary-beam diffraction modulated electron emission, X-ray photoelectron spectroscopy and Auger electron spectroscopy. At 300xa0K, the formation of CoO in the very first layer was observed, in the typical rock salt structure, 45° rotated with respect to the square unit mesh of the remaining, underlying Co film. At 470xa0K, the oxide extends in thickness involving the whole Co film. Similar behavior has been observed for the (11 2 0) surface of the hcp phase. On both surfaces, the oxide layer appears tetragonally distorted.

ELEMENT-SPECIFIC, SURFACE AND SUBSURFACE STRUCTURAL ANALYSIS BY SCATTERING-INTERFERENCE OF PRIMARY ELECTRONS

Abstract We investigated the effects of scattering-interference of primary electrons on the secondary electron emission intensity fron ordered surfaces and interfaces. Because of the focusing-defocusing of the primary wave along atomic chains, maxima in the electron yield occur when the exciting beam is aligned with low index axes. Therefore the electron intensity distributions as a function of the angle of incidence of the primary beam can be interpreted as projected images of real space, local atomic arrangement. The process can be modelled in a single scattering cluster (SSC) approximation. Potential of primary-beam diffraction modulated electron emission (PDMEE) technique for surface and subsurface structural characterization is shown, for Co epitaxy on Fe(001).

Application of a HEPE-oriented 4096-MAPS to time analysis of single electron distribution in a two-slits interference experiment

The Young-Feynman two-slit experiment for single electrons has been carried out by inserting in a conventional transmission electron microscope two nanometric slits and a fast recording system able to measure the electron arrival-time. The detector, designed for experiments in future colliders, is based on a custom CMOS chip of 4096 monolithic active pixels equipped with a fast readout chain able to manage up to 106 frames per second. In this way, high statistic samples of single electron events can be collected within a time interval short enough to guarantee the stability of the system and coherence conditions of the illumination. For the first time in a single electron two-slit experiment, the time distribution of electron arrivals has been measured.

Local modifications of magnetism and structure in FePt (001) epitaxial thin films by focused ion beam : Two-dimensional perpendicular patterns

Focused ion beam was utilized to locally modify magnetism and structure of L10 FePt perpendicular thin films. As a first step, we have performed a magnetic, morphological, and structural study of completely irradiated FePt films with different Ga+ doses (1×1013–4×1016u2002ions/cm2) and ion beam energy of 30 keV. For doses of 1×1014u2002ions/cm2 and above a complete transition from the ordered L10 to the disordered A1 phase was found to occur, resulting in a drop of magnetic anisotropy and in the consequent moment reorientation from out-of-plane to in-plane. The lowest effective dose in disordering the structure (1×1014u2002ions/cm2) was found not to affect the film morphology. Taking advantage of these results, continuous two-dimensional (2D) patterns of perpendicular magnetic structures (250 nm dots, 1u2002μm dots, 1u2002μm-large stripes) were produced by focused ion beam without affecting the morphology. The 2D patterns were revealed by means of magnetic force microscopy, that evidenced peculiar domain structures in the ca...