D. Arvanitis

An angle-dependent magnetic circular X-ray dichroism study of Co/Cu(100): experiment versus theory

Values for the magnetic moments of 3d transition metals have been calculated from magnetic circular X-ray dichroism measurements using sum rules. In this work we reveal that these calculations are sensitive to a damping effect resulting from a decrease in photon penetration at the L3.2 peaks. Known as saturation effects, we present a simple correction to compensate for this phenomenon. We also show that the magnetic moments, as determined using sum rules, are relatively insensitive to the step function fitted to the absorption spectra. The magnetic moments derived from our data differ from the known ground-state moments. The corresponding collective factors appear to be transferable between cobalt and nickel.

C 1s ionisation potential and energy referencing for solid C60 films on metal surfaces

We present a measurement of the C Is ionisation potential (IP) for solid C-60, as well as a reliable determination of the highest occupied molecular orbital (HOMO) IP. The difference between the gas phase and solid state measurements for the HOMO agrees w

Ac susceptibility measurements of magnetic monolayers: MCXD, MOKE, and mutual inductance

Abstract A refined analysis of dispersive and absorptive contributions to the complex magnetic susceptibility yields detailed information on magnetic and structural properties of ultrathin films. The new ac modulated magnetic circular X-ray dichroism (MCXD) technique is described in detail. Experimental results from measurements at the L2,3 edges of Ni and Co films are discussed in a more general framework including modulated magneto-optic Kerr effect and mutual inductance measurement of Gd films. A comparison of the experimentally determined small demagnetizing factors (N| ∼ 10−3) of monolayers allows us to draw conclusions on the structure of the film. In addition, we show by element specific ac MCXD susceptibility measurements that in a 1.3 ML Co/5.3 ML Ni bilayer the coupled films become ferromagnetic at the same Curie temperature.

Homogeneous and heterogeneous magnetism in (Zn,Co)O : From a random antiferromagnet to a dipolar superferromagnet by changing the growth temperature

For more than a decade ZnO doped with Mn and Co has remained as one of the most prospected diluted magnetic semiconductor for spintronic applications with conflicting outcome concerning the genuineness of its room temperature ferromagnetism. In order to clarify this issue we investigate (Zn,Co)O layers grown by atomic layer deposition at low temperatures. We employ and relay on wide range of extensive material characterization, which in combination with superconducting quantum interference device magnetometry allow us decisively exemplify the growth temperature as the key factor discriminating between paramagnetic (obtained at 160 °C) and various forms of ferromagnetic responses, seen when the grows is carried out at 200 °C and above.

Vibronic coupling in the photoemission bands of condensed C60

We present photoelectron spectra for the highest occupied molecular orbital (HOMO) of C60 in the gas phase and a two-dimensional (2D) condensed layer. For low temperatures the similarity in the spectra is striking and places a strict upper limit of 0.07 eV on the HOMO electronic bandwidth for the 2D solid, which is a fraction of the theoretically predicted value. This is shown to be consistent with expectations of a polaronic model for the bands and suggests the need fora reassessment of the band dispersions in 3D C60.

Enantiospecific Spin Polarization of Electrons Photoemitted Through Layers of Homochiral Organic Molecules

Electrons photoemitted through layers of purely organic chiral molecules become strongly spin-polarized even at room temperature and for double-monolayer thicknesses. The substitution of one enantiomer for its mirror image does not revert the sign of the spin polarization, rather its direction in space. These findings might lead to the obtention of highly efficient spin filters for spintronic applications.

Description of the new I1011 beamline for magnetic measurements using synchrotron radiation at MAX-lab

We report on the characterization of the new I1011 beamline at the MAX-II storage ring, in the MAX-lab synchrotron radiation laboratory and give examples of first results. This beamline is using an Elliptically Polarizing Undulator source, producing soft x-rays of a variable polarization state. It delivers high flux and high brightness circularly polarized x-rays in the energy range 0.2 to 1.7 keV, covering the L-edges of the late 3d elements. The new beamline will operate with an octupole magnet endstation. It is specially engineered to solve the problem of the limited optical access typically associated with magnetic fields and synchrotron radiation endstations. Eight water-cooled magnets allow the application of the magnetic field of up to 1 T in any direction. X-ray absorption spectroscopy, X-ray resonant reflectivity and the corresponding magnetic variants, i.e., XMCD, XMLD and XRMS experiments are possible also under an applied magnetic field. The high flux allows working with dilute magnetic systems such as ultra-thin films and nano structures.

Adsorbate substrate bonding and dynamics as determined by SEXAFS

Abstract A review is given of surface extended X-ray absorption fine structure (SEXAFS) data taken at the N and O K-edges of atomic and molecular adsorbates on Cu and Ni surfaces. The theory of the dynamic analysis of SEXAFS data is briefly presented. We conclude that a dynamic analysis is also necessary in many cases for a reliable structural determination. This result is particularly important in the case of reconstructed surfaces where the structural and dynamic contributions in the SEXAFS amplitude and phase can be of comparable strength. An analysis of the dynamic part of the disorder allows one to distinguish between reconstructed and unreconstructed systems. Furthermore, the use of model pair potentials allows for a discussion of surface reconstruction in terms of simple physical parameters. Finally a short discussion is given of the photon flux and stability required for a state of the art SEXAFS experiment, in view of the availability of the improved third generation soft X-ray sources.

A study of the self-absorption effect in the fluorescence yield of NiO at the oxygen K-edge

Abstract We demonstrate geometry-dependent self-absorption effects in the fluorescence yield photoabsorption spectra of bulk NiO. We observe a decrease of the atomic like smooth part of the data above the oxygen K edge and a damping of the EXAFS wiggles. For a cubic substance (NiO single crystal) self-absorption is studied systematically in the energy range above the O K absorption edge by varying the angle between the incident photons and the sample surface over a wide range. If self-absorption is not considered in the data analysis, the amplitudes of the EXAFS modulation can be damped significantly and the errors for coordination numbers can reach 50%.

Element-specific characterization of heterogeneous magnetism in (Ga,Fe)N films

We employ x-ray spectroscopy to characterize the distribution and magnetism of particular alloy constituents in (Ga,Fe)N films grown by metal organic vapor phase epitaxy. Furthermore, photoelectron ...