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Plenarvorträge - DPG-Tagungen

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Oberflächenphysik Mittwoch<br />

Cs-pads appear blurred. Subsequent reduction of the laser peak power<br />

leads to sharper edges as are also observed when the 80MHz oscillator is<br />

used for illumination.<br />

O 28.75 Mi 16:00 Bereich C<br />

Time-of-Flight Two-Photon Photoemission Spectromicroscopy<br />

of Noble metal cluster films with Femtosecond Laser Radiation.<br />

— •M. Cinchetti, A. Gloskovskii, D. A. Valdaitsev, S.<br />

A. Nepijko, and G. Schönhense — Johannes Gutenberg-Universität,<br />

Institut für Physik, 55099 Mainz, Deutschland<br />

Time-of-Flight Two-Photon Photoemission Spectromicroscopy [1] was<br />

used to investigate the photoemission properties of some Nobel metal<br />

surfaces under femtosecond laser irradiation (photon energy 3.1eV, pulse<br />

width < 200fs, laser fluence 6.4µJcm −2 ).In particular, we present the results<br />

of investigations of Cu surface inhomogeneities and Ag nanoparticle<br />

films deposited on a Si(111) substrate. The photoemission yield resulted<br />

to be enhanced in presence of surface inhomogeneities with dimensions in<br />

the nanometer range. The energy distribution curves obtained from regions<br />

characterized by a strongly enhanced photoemission yield are shown<br />

to have the same qualitative behaviour. They differ significantly from the<br />

energy distribution curves from the homogeneous and clean metal surfaces.<br />

The differences are explained in terms of (i) a reduction of surface<br />

potential barrier connected with the appearance of local fields between<br />

the nanoparticles and the substrate; (ii) the excitation of Localized Surface<br />

Plasmons in the nanoparticles and the consequent modification of<br />

the near field [2], that influences the photoemission. [1] M. Cinchetti, A.<br />

Oelsner, G. H. Fecher, H. J. Elmers, and G.Schönhense, Appl. Phys. Lett.<br />

83, 1503 (2003). [2] V.M. Shalaev, C. Douketis, T. Haslett, T. Stuckless,<br />

and M. Moskovitis, Phys. Rev. B 53, 11193 (1996).<br />

O 28.76 Mi 16:00 Bereich C<br />

Time-Resolved Photoemission Electron Microscopy at nanostructured<br />

surfaces — •Carsten Wiemann, Michael Bauer,<br />

Michael Munzinger, Daniela Bayer, and Martin Aeschlimann<br />

— TU Kaiserslautern<br />

TR-2PPE (Time-Resolved 2-Photon-Photoemission) is a wellestablished<br />

technique to investigate electron dynamics at surfaces on a<br />

femtosecond time-scale. A combination of TR-2PPE with a spatially resolving<br />

electron detection scheme such as Photoemission Electron Microscopy<br />

(PEEM) allows in addition to analyze the precise local origin of<br />

the emitted electrons. In this way a ’lifetime mapping’ of heterogeneous<br />

samples is - in principle - possible. [O. Schmidt et al. Appl. Phys. B 74<br />

(2002) 223-227] Recent time-resolved PEEM results on nanostructured<br />

surfaces demonstrating the capabilities and limits of this setup in extending<br />

time-resolved spectroscopy to the nanoscale will be presented.<br />

O 28.77 Mi 16:00 Bereich C<br />

Surface Recombination Dynamics of SiO2/Si(100) - A Combined<br />

Laser and Synchro Radiation Study — •David Bröcker 1 ,<br />

Tatjana Gießel 1 , and Wolf Widdra 2 — 1 Max-Born-Institut,<br />

Max-Born-Strasse 2a, 12489 Berlin — 2 Martin-Lutter-Universtität<br />

Halle-Wittenberg, Fachbereich Physik, Fachgruppe Experimentelle<br />

Physik III<br />

The charge carrier dynamics at the silicon surface has been studies by<br />

probing the surface photovoltage. Upon excitation of electron hole pairs<br />

in the surface-near region by picosecond laser pulses, the dynamics of the<br />

charge carrier recombination has been determined by time-resolved Si 2p<br />

core level photoemission. We compared several sample preparations, the<br />

clean Si(100) surface, the hydrogen monohydride, and thin oxide layers.<br />

The decay process could be described by a model based on thermionic<br />

emission. The influence of the laser fluence and the temperature on the<br />

decay process has been studied in detail.<br />

O 28.78 Mi 16:00 Bereich C<br />

X-ray longitudinal Kerr rotation and ellipticity spectra measured<br />

at the 2p edges of Fe, Co, and Ni — •Sergio Valencia 1 ,<br />

Hans-Christoph Mertins 1 , Peter M. Oppeneer 2 , Dirk Abramsohn<br />

1 , Andreas Gaupp 1 , and Wolfgang Gudat 1 — 1 BESSY, Berlin<br />

— 2 IFW Dresden<br />

The first measurements of the X-ray longitudinal magneto-optical Kerr<br />

effect (L-MOKE) at the 2p absorption edges of amorphous Fe, Co, and<br />

Ni films are reported. The L-MOKE rotation and ellipticity spectra are<br />

recorded by a complete polarization analysis of the X-rays reflected from<br />

the ferromagnetic surface. At the edges large Kerr rotations of up to ±24 ◦<br />

are detected, which are more than two orders of magnitude larger than<br />

those in the visible energy range. Using a numerical simulation package<br />

based on the magneto-optical refractive indices, the spectral contributions<br />

stemming from magneto-optically active atomic transitions could<br />

be seperated from layer interference effects. Both theoretically and experimentally<br />

it is shown that at grazing incidence the longitudinal Kerr<br />

rotation and Kerr ellipticity are proportional to the transversal MOKE<br />

and the XMCD reflection spectrum, respectively.<br />

O 28.79 Mi 16:00 Bereich C<br />

Magnetische Röntgenstreuung an lateralen Strukturen —<br />

•Arndt Remhof, Johannes Grabis, Alexei Nefedov,<br />

Erik Verduijn und Hartmut Zabel — Experimentalphysik/Festkörperphysik,<br />

Ruhr-Univeristät Bochum, 44780 Bochum<br />

Nano- und Mikrostrukturierung von dünnen magnetischen Systemen<br />

ermöglicht das Ummagnetisierungsverhalten, d. h. die Hystereseschleife,<br />

gezielt zu verändern. Wir haben ein quadratisches Muster aus Co/CoO<br />

Inseln mittels Elektronenstrahl-Lithographie hergestellt. Die Inseln haben<br />

einen Durchmesser von 1mm und eine Periode von 2,5 mm. Die<br />

magnetische Hysterese wurde mit Bragg-MOKE und mit resonanter magnetischer<br />

Röntgenstreuung (XRMS) an der Co L2,3 bei verschiedener<br />

Ordnung der Interferenz bestimmt. Bragg-MOKE liefert eine Fourieranalyse<br />

des Magnetisierungsprofils während des Ummagnetisierungsprozesses.<br />

Analog dazu liefern auch Hysteresen, die mit XRMS an verschiedenen<br />

Ordnungen von Bragg-Reflexen gemessen werden, eine Fourieranalyse<br />

der Ummagnetisierung. Im Vergleich zu Bragg-MOKE zeichnet<br />

sich XRMS durch Elementspezifität und durch höhere Eindringtiefen aus.<br />

Wir präsentieren erste Ergebnisse, die mit Bragg-MOKE und XRMS an<br />

strukturierten Co/CoO Proben gewonnen wurden, und vergleichen die<br />

Ergebnisse.<br />

Gefördert durch SFB 491.<br />

O 28.80 Mi 16:00 Bereich C<br />

Faraday rotation spectrum at shallow core levels: 3p edges<br />

of Fe, Co, and Ni — •Sergio Valencia 1 , Hans-Christoph<br />

Mertins 2 , Peter M. Oppeneer 3 , Dirk Abramsohn 1 , Andreas<br />

Gaupp 1 , Claus M. Schneider 4 , and Wolfgang Gudat 1 —<br />

1 BESSY, Berlin — 2 FH Münster — 3 IFW Dresden — 4 IFF, FZ Jülich<br />

We report measurements of the Faraday effect at the 3p edges of thin<br />

Fe, Co, and Ni films. The Faraday rotation and ellipticity are determined<br />

by a complete polarization analysis of the synchrotron radiation transmitted<br />

across the samples. Surprisingly, the rotation constants k attain<br />

large values comparable to those observed at the 2p edges. Maximal rotation<br />

constants of 2.2 · 10 5 , 1.5 · 10 5 , and 0.8 · 10 5 deg/mm are found<br />

for Fe, Co, and Ni, respectively. While at the 2p edges the spin-orbit<br />

splitting of the 2p levels is considered to be mainly responsible for the<br />

magneto-optical effects, the unexpected similarity underlines the role of<br />

the exchange-splitting of the 3p core levels in the extreme ultra-violet<br />

region (EUV). A full set of optical and magneto-optical constants, which<br />

were determined from the measured rotation and ellipticity, are presented<br />

for the three elements in the energy range of the 3p edges.<br />

O 28.81 Mi 16:00 Bereich C<br />

Anomalous ferromagnetism of monatomic Co wire at the<br />

Pt(111) surface step edge — •Peter M. Oppeneer 1 , Alexander<br />

B. Shick 2 , and Franti˘sek Máca 2 — 1 IFW Dresden — 2 Inst. of<br />

Physics, ASCR, Prague<br />

We report a first-principles investigation of the anomalous ferromagnetism<br />

of a quasi-one-dimensional Co chain decorating the Pt(111) step<br />

edge. Our calculations demonstrate that the symmetry breaking at the Pt<br />

step leads to an easy magnetization axis at an anomalous angle of ∼ 20 ◦<br />

towards the Pt step, in semi-quantitative agreement with the experimentally<br />

observed peculiar easy axis at an angle of ∼ 40 ◦ [P. Gambardella<br />

et al., Nature 416, 301 (2002)]. The computed hard magnetization axis<br />

direction also corresponds well to the experimentally observed hard axis.<br />

A (T = 0) magnetocrystalline anisotropy energy of 3 meV/Co is calculated<br />

in good agreement with the experimental value of ∼ 2 meV/Co at<br />

T = 45 K. On account of the symmetry breaking the Co spin and orbital<br />

moments become noncollinear, even in the case of a collinear ferromagnetic<br />

spin arrangement. We also performed LSDA+U calculations, with<br />

which a significant enhancement of the Co orbital magnetic moment is<br />

achieved already when modest electron correlations (small U) are taken<br />

into account.

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