V. Senz

Observation of the second ordered phase of water on the MgO(100) surface: Low energy electron diffraction and helium atom scattering studies

The interaction of water with MgO (100) single crystal surfaces cleaved in situ has been studied by low energy electron diffraction and helium atom scattering in the temperature range 80 K–230 K. At Tcrystal=100–180 K water forms a layer with a c(4×2) symmetry in good agreement with previous spot profile analysis of low energy electron diffraction experiments. Adsorption at Tcrystal=185–221 K leads to the formation of a new ordered phase. The results of the low energy electron diffraction and elastic helium atom scattering experiments show that this high‐temperature phase has a (3×2) symmetry, and that the unit cell contains a glide plane. The isosteric heat of adsorption at half coverage Qst=85.3 kJ/mol has been determined from equilibrium adsorption isotherms measured between 210 and 221 K.

The properties of a two-dimensional water layer on MgO(001)

Abstract Elastic helium-atom scattering (HAS) has been employed to study the adsorption behavior of water on the in-site cleaved MgO(001) surface at 100 ≤ T crystal ≤ 180 K. At these temperatures, water forms a dense phase which consists of orthogonal domains each with a c(4 × 2) unit mesh. No differences in the structures of the H 2 O and D 2 O low-temperature phases were observed. Heating of the crystal covered with the c(4 × 2) water phase to T crystal > 185 K results in the partial desorption of the water layer and the formation of a less dense p(3 × 2) water phase. A thermodynamic study of the high-temperature water phase using low-energy electron diffraction (LEED) to measure the equilibrium adsorption isobars and isotherms enabled us to determine the isosteric heat of adsorption Q st = 85.3 ± 2.1 kJ mol −1 as well as the lateral interaction energy Q ‖ = 35.1 ± 9.6 kJ mol −1 .

Iron islands and dots on W(110) studied with polarized synchrotron radiation

Abstract Oriented iron islands of rectangular shape on tungsten were created by self-organization. These systems have been analyzed by Scanning Tunneling Microscopy (STM) with respect to their shape and concerning their electronic and magnetic behavior with angle resolved photoemission using polarized synchrotron radiation in the soft X-ray region. The magnetic information can be obtained by the technique of Magnetic Linear Dichroism in the Angular Distribution of photoelectrons (MLDAD). Furthermore, an array of iron dots on tungsten has been studied with photoemission electron microscopy (PEEM). Element-specific information is available via the characteristic absorption levels (using the tuneable synchrotron radiation) or by threshold photoemission using a mercury lamp due to the different work functions. Circularly polarized radiation enables the observation of magnetic domains.

Rotational-disordering phase transition of C60(111) epitaxial films grown on GeS(001)

The surface structures of C60 films, epitaxially grown on the GeS(001) surface, were investigated from 90 to 350 K by helium atom scattering (HAS) diffraction. The present HAS results indicate a step-flow growth mode that is consistent with the results of previous x-ray scattering studies. By monitoring the diffraction intensities, the orientational-disordering phase transition is found to be completed at Tc=235 K, which is about 25 K lower than the bulk transition temperature. This surface phase transition appears to be preempted by rotational disordering of C60 molecules at defect sites, already initiated at Ts=130 K.

Size effects in the temperature-dependent magnetization of iron clusters

The magnetic behavior of silver-capped iron clusters on tungsten (110) and iron clusters in a silver matrix on Si (111) has been investigated with respect to the temperature by means of synchrotron-based Mossbauer spectroscopy and the magneto-optical Kerr-effect. Fe clusters on W (110) of monoatomic height exhibit a perpendicular magnetic anisotropy at temperatures below 100 K. At a higher Fe coverage the strong surface anisotropy favours an in-plane magnetization. Well separated and mass-selected Fe nanoparticles with diameters of about 8.1 and 11.7 nm embedded in a silver matrix show a ferromagnetic order below 77 K. A temperature rise to 300 K leads to superparamagnetism of the 8.1 nm particles.

Structure and magnetism of hcp(0001) and fcc(001) thin cobalt films on a clean and carbon-reconstructed W(110) surface

The morphology of cobalt films on W(110) depends on the cleanness of the surface. Epitaxial hcp(0001) films are formed on a clean surface whereas the fcc(001) structure is observed in case of a carbon-induced reconstruction of the tungsten surface. Here, fcc cobalt films with two different domains are investigated with LEED and STM concerning details of their growth. Furthermore, the electronic structure of these films is studied with angle resolved valence band photoelectron spectroscopy using circularly polarized VUV synchrotron radiation. Additionally, intensity differences in the photoelectron spectra are obtained when reversing the magnetization directions giving access to the magnetic properties via the phenomenon of magnetic circular dichroism in the angular distribution of photoelectrons (MCDAD). The results are compared with those for hcp cobalt films.

TRANSVERSE MAGNETO-OPTICAL KERR EFFECT IN THE SOFT X-RAY REGIME OF ULTRATHIN IRON FILMS AND ISLANDS ON W(110)

The transverse magneto-optical Kerr effect (T-MOKE) in the soft X-ray regime is used to investigate the magnetic properties of ultrathin epitaxial Fe(110) films and thermally created epitaxial Fe islands on W(110). The measurements have been carried out at the Fe 2p core levels with tunable, linearly polarized synchrotron radiation. Besides the chemical selectivity T-MOKE at core levels is characterized by a much larger sensitivity compared to the visible regime. We have analyzed the transition from thin films to a three-dimensional island system at a thickness of 4 ML and the magnetic behavior when increasing the iron coverage to 8 ML. The Fe island system is characterized by a rotation of the easy magnetization axis in this coverage regime.