M. Tischer

The contribution of particle core and surface to strain, disorder and vibrations in thiolcapped CdTe nanocrystals

Thiolcapped CdTe nanocrystals (18 A diameter) are investigated by extended x-ray absorption fine structure (EXAFS) measurements between 8 and 290 K at the Cd and Te K-edge. The different coordination sites of Cd in the particle core and at the surface are identified and are consistent with a CdTe particle core which is covered by a Cd–SR surface layer (R=organic rest). We are able to study individually changes of the properties of the interior and the surface of the nanoparticle with respect to bulk material. Structure and dynamics of the CdTe nanocrystals are mainly altered by the requirement of heteroepitaxial growth at the interface between the CdTe core and the Cd–SR shell. As a consequence, bond lengths and Debye temperatures of particle core and surface show a tendency to accommodate in thiolcapped CdTe nanocrystals. The trends in bond lengths variation observed in the experiment can be reproduced by calculations of the strain distribution induced by the lattice mismatch in a simplified isotropic mo...

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.

Temperature dependent MCXD measurements of thin Ni films on Cu(100)

We present soft X-ray magnetic circular X-ray dichroism (MCXD) measurements at the Ni L edges of thin Ni films on Cu(100) with a thickness between 1 and 7 monolayers. The data were measured around the L 3 edge covering the whole temperature range up to the critical temperature. Furthermore, measurements around both the L 3 and L 2 edges were performed at low temperatures. The temperature variation of the MCXD signal and the critical temperatures of the films as a function of their thickness are discussed and compared to results of other studies

Local magnetism of Co monolayers: A new type of magnetic circular x-ray dichroism measurement

Abstract We report Magnetic Circular X-ray Dichroism (MCXD) measurements of ultrathin monolayers (ML) and bulk-like Co films grown on a Cu(100) surface. No increase is observed in the orbital magnetic moment of the Co atoms for the ultrathin films in contrast to expectations due to symmetry lowering on the surface. We developed a new ac field modulated MCXD-susceptibility technique permitting accurate measurements of the critical temperature T c . At about 1.8 ML, T c suddenly increases by more than 150 K when the film structure changes from hetero-to homoepitaxial growth.

Coverage dependence of surface structures and vibrations of ClNi(100) studied by Cl K-edge SEXAFS

The coverage dependence of surface structures and vibrations of ClNi(100) have been investigated by means of Cl K-edge SEXAFS (surface extended X-ray absorption fine structure) spectroscopy. In the 0.25 ML phase, Cl was found to locate on the fourfold hollow site with little or no surface reconstructions, this being similar to the c(2 × 2) phase with 0.50 ML Cl. The ClNi bond distance was determined to be 2.31 ± 0.02 A for 0.25 ML, which is slightly shorter than that of 2.35 ± 0.02 A for 0.50 ML. The temperature dependence of the SEXAFS spectra was analyzed to yield the second- and third-order cumulants which provide harmonic and anharmonic force constants. The ClNi chemical bond was consequently found to be stiffer and less anharmonic in the 0.25 ML state than in the 0.50 ML state, this corresponding to a slightly shorter bond distance at a lower coverage. Such a trend is discussed in terms of charge transfer from substrate to adsorbate and the repulsive interaction between adsorbates.

Adsorbate-substrate dynamics for c(2 × 2)ClNi(100): a SEXAFS study

Temperature and angle dependent surface extended X-ray absorption fine structure (SEXAFS) measurements are reported for c(2 × 2)ClNi(100). A complete dynamic analysis including SEXAFS Debye-Waller factor and mean cubic relative displacement is presented for nearest neighbour (n.n.) and for the first time next nearest neighbour (n.n.n.) ClNi coordinations and compared with the data available on c(2 × 2)SNi(100). The ClNi bonding to the second layer Ni atoms seems to be stronger than for SNi. Einstein temperatures for the first and second ClNi coordinations suggest that in the present system vibrations associated with longer out-of-plane bonds are more anharmonic than the shorter and relatively stronger in-plane bonds.

Bonding on Surfaces: The SEXAFS Point of View

The temperature dependent Surface EXAFS of O and N on Ni(100) and O on Cu(100) is analyzed. The first moments of the Pair Distribution Function (PDF) are determined, including anharmonic contributions. From a systematic analysis of the pair potentials it is proposed that for the reconstructed surfaces the bond is more covalent, for the unreconstructed ones more ionic. The value of the charge transfer to the adatom is found to be ~0.63 electrons for the ionic cases. Upon the reconstruction of the Cu(100) surface the static part of the disorder is found to increase. The dissociation process of O2 molecules on Cu(100) is studied. The differences in the PDF moments between the different adsorption states are shown to correlate to the strength of chemisorption.

Enhancement of orbital magnetism at surfaces: Co on Cu(100) (abstract)

By combining MCXD experiments with first principles electronic structure calculations, we demonstrate that the orbital contribution to magnetism can be strongly enhanced at surfaces. This effect is illustrated for Co grown on a Cu(100) surface. The MCXD measurements were performed using the SX 700 plane gratingmonochromators at BESSY. The Co films were evaporated and characterized in situ, for their cleanness, thickness (1.6–50 ML), and structural order. For a film thickness up to 2.2 ML, measurements of the ac MCXD susceptibility response at a fixed photon energy allowed one to measure the critical temperature,Tc, of the films and their critical properties were characterized in situ. For several samples the MCXD response as a function of the x‐ray incidence angle was investigated as well. This allows one to quantify and correct saturation effects that can occur in the measurements. Data were taken in a temperature range between 40 and 350 K, and for many samples measurements were performed at several reduced temperatures,T/Tc. The first layer of Co on the Cu(100) surface shows an enhanced orbital moment, in contrast to the subsequent layers where the orbital moment is bulklike. The lowering of the symmetry, the enhanced spin moment, and the increased value of the density of states at the Fermi level are factors that combine to give the observed enhancements.

Local magnetism and element specific susceptibility for Ni//Cu(100)

Abstract We report magnetic circular X-ray dichroism (MCXD) measurements of ultrathin Ni films grown on Cu(100). The Curie temperature of the films is determined by means of a.c. MXCD susceptibility, a technique that allows T c to be probed precisely and unambiguously. Furthermore, the interaction of submonolayer oxygen with the films, adsorbed in physisorbed molecular and atomic states, is studied. The decrease in the magnetic response of the sample is investigated systematically for various adsorption states of increasing chemisorption strength. The oxygen induced changes in the MCXD difference and in the absorption features measured with linear polarized light provide a better characterization of the electronic states involved in the MCXD response. The magnetism of the Ni films is already influenced by O 2 molecules in a physisorbed state, a c(2 × 2) overlayer of atomic oxygen is found to neutralize the magnetism of two layers of an ultrathin Ni film.

Pair potentials in the SEXAFS analysis

An overview for a dynamical SEXAFS analysis for several systems is presented in terms of effective pair potentials. Model potentials of Madelung type are found to describe well the adsorption of N, ...