Plenarvorträge - DPG-Tagungen

Oberflächenphysik Mittwoch
Cs-pads appear blurred. Subsequent reduction of the laser peak power
leads to sharper edges as are also observed when the 80MHz oscillator is
used for illumination.
O 28.75 Mi 16:00 Bereich C
Time-of-Flight Two-Photon Photoemission Spectromicroscopy
of Noble metal cluster films with Femtosecond Laser Radiation.
— •M. Cinchetti, A. Gloskovskii, D. A. Valdaitsev, S.
A. Nepijko, and G. Schönhense — Johannes Gutenberg-Universität,
Institut für Physik, 55099 Mainz, Deutschland
Time-of-Flight Two-Photon Photoemission Spectromicroscopy [1] was
used to investigate the photoemission properties of some Nobel metal
surfaces under femtosecond laser irradiation (photon energy 3.1eV, pulse
width < 200fs, laser fluence 6.4µJcm −2 ).In particular, we present the results
of investigations of Cu surface inhomogeneities and Ag nanoparticle
films deposited on a Si(111) substrate. The photoemission yield resulted
to be enhanced in presence of surface inhomogeneities with dimensions in
the nanometer range. The energy distribution curves obtained from regions
characterized by a strongly enhanced photoemission yield are shown
to have the same qualitative behaviour. They differ significantly from the
energy distribution curves from the homogeneous and clean metal surfaces.
The differences are explained in terms of (i) a reduction of surface
potential barrier connected with the appearance of local fields between
the nanoparticles and the substrate; (ii) the excitation of Localized Surface
Plasmons in the nanoparticles and the consequent modification of
the near field [2], that influences the photoemission. [1] M. Cinchetti, A.
Oelsner, G. H. Fecher, H. J. Elmers, and G.Schönhense, Appl. Phys. Lett.
83, 1503 (2003). [2] V.M. Shalaev, C. Douketis, T. Haslett, T. Stuckless,
and M. Moskovitis, Phys. Rev. B 53, 11193 (1996).
O 28.76 Mi 16:00 Bereich C
Time-Resolved Photoemission Electron Microscopy at nanostructured
surfaces — •Carsten Wiemann, Michael Bauer,
Michael Munzinger, Daniela Bayer, and Martin Aeschlimann
— TU Kaiserslautern
TR-2PPE (Time-Resolved 2-Photon-Photoemission) is a wellestablished
technique to investigate electron dynamics at surfaces on a
femtosecond time-scale. A combination of TR-2PPE with a spatially resolving
electron detection scheme such as Photoemission Electron Microscopy
(PEEM) allows in addition to analyze the precise local origin of
the emitted electrons. In this way a ’lifetime mapping’ of heterogeneous
samples is - in principle - possible. [O. Schmidt et al. Appl. Phys. B 74
(2002) 223-227] Recent time-resolved PEEM results on nanostructured
surfaces demonstrating the capabilities and limits of this setup in extending
time-resolved spectroscopy to the nanoscale will be presented.
O 28.77 Mi 16:00 Bereich C
Surface Recombination Dynamics of SiO2/Si(100) - A Combined
Laser and Synchro Radiation Study — •David Bröcker 1 ,
Tatjana Gießel 1 , and Wolf Widdra 2 — 1 Max-Born-Institut,
Max-Born-Strasse 2a, 12489 Berlin — 2 Martin-Lutter-Universtität
Halle-Wittenberg, Fachbereich Physik, Fachgruppe Experimentelle
Physik III
The charge carrier dynamics at the silicon surface has been studies by
probing the surface photovoltage. Upon excitation of electron hole pairs
in the surface-near region by picosecond laser pulses, the dynamics of the
charge carrier recombination has been determined by time-resolved Si 2p
core level photoemission. We compared several sample preparations, the
clean Si(100) surface, the hydrogen monohydride, and thin oxide layers.
The decay process could be described by a model based on thermionic
emission. The influence of the laser fluence and the temperature on the
decay process has been studied in detail.
O 28.78 Mi 16:00 Bereich C
X-ray longitudinal Kerr rotation and ellipticity spectra measured
at the 2p edges of Fe, Co, and Ni — •Sergio Valencia 1 ,
Hans-Christoph Mertins 1 , Peter M. Oppeneer 2 , Dirk Abramsohn
1 , Andreas Gaupp 1 , and Wolfgang Gudat 1 — 1 BESSY, Berlin
— 2 IFW Dresden
The first measurements of the X-ray longitudinal magneto-optical Kerr
effect (L-MOKE) at the 2p absorption edges of amorphous Fe, Co, and
Ni films are reported. The L-MOKE rotation and ellipticity spectra are
recorded by a complete polarization analysis of the X-rays reflected from
the ferromagnetic surface. At the edges large Kerr rotations of up to ±24 ◦
are detected, which are more than two orders of magnitude larger than
those in the visible energy range. Using a numerical simulation package
based on the magneto-optical refractive indices, the spectral contributions
stemming from magneto-optically active atomic transitions could
be seperated from layer interference effects. Both theoretically and experimentally
it is shown that at grazing incidence the longitudinal Kerr
rotation and Kerr ellipticity are proportional to the transversal MOKE
and the XMCD reflection spectrum, respectively.
O 28.79 Mi 16:00 Bereich C
Magnetische Röntgenstreuung an lateralen Strukturen —
•Arndt Remhof, Johannes Grabis, Alexei Nefedov,
Erik Verduijn und Hartmut Zabel — Experimentalphysik/Festkörperphysik,
Ruhr-Univeristät Bochum, 44780 Bochum
Nano- und Mikrostrukturierung von dünnen magnetischen Systemen
ermöglicht das Ummagnetisierungsverhalten, d. h. die Hystereseschleife,
gezielt zu verändern. Wir haben ein quadratisches Muster aus Co/CoO
Inseln mittels Elektronenstrahl-Lithographie hergestellt. Die Inseln haben
einen Durchmesser von 1mm und eine Periode von 2,5 mm. Die
magnetische Hysterese wurde mit Bragg-MOKE und mit resonanter magnetischer
Röntgenstreuung (XRMS) an der Co L2,3 bei verschiedener
Ordnung der Interferenz bestimmt. Bragg-MOKE liefert eine Fourieranalyse
des Magnetisierungsprofils während des Ummagnetisierungsprozesses.
Analog dazu liefern auch Hysteresen, die mit XRMS an verschiedenen
Ordnungen von Bragg-Reflexen gemessen werden, eine Fourieranalyse
der Ummagnetisierung. Im Vergleich zu Bragg-MOKE zeichnet
sich XRMS durch Elementspezifität und durch höhere Eindringtiefen aus.
Wir präsentieren erste Ergebnisse, die mit Bragg-MOKE und XRMS an
strukturierten Co/CoO Proben gewonnen wurden, und vergleichen die
Ergebnisse.
Gefördert durch SFB 491.
O 28.80 Mi 16:00 Bereich C
Faraday rotation spectrum at shallow core levels: 3p edges
of Fe, Co, and Ni — •Sergio Valencia 1 , Hans-Christoph
Mertins 2 , Peter M. Oppeneer 3 , Dirk Abramsohn 1 , Andreas
Gaupp 1 , Claus M. Schneider 4 , and Wolfgang Gudat 1 —
1 BESSY, Berlin — 2 FH Münster — 3 IFW Dresden — 4 IFF, FZ Jülich
We report measurements of the Faraday effect at the 3p edges of thin
Fe, Co, and Ni films. The Faraday rotation and ellipticity are determined
by a complete polarization analysis of the synchrotron radiation transmitted
across the samples. Surprisingly, the rotation constants k attain
large values comparable to those observed at the 2p edges. Maximal rotation
constants of 2.2 · 10 5 , 1.5 · 10 5 , and 0.8 · 10 5 deg/mm are found
for Fe, Co, and Ni, respectively. While at the 2p edges the spin-orbit
splitting of the 2p levels is considered to be mainly responsible for the
magneto-optical effects, the unexpected similarity underlines the role of
the exchange-splitting of the 3p core levels in the extreme ultra-violet
region (EUV). A full set of optical and magneto-optical constants, which
were determined from the measured rotation and ellipticity, are presented
for the three elements in the energy range of the 3p edges.
O 28.81 Mi 16:00 Bereich C
Anomalous ferromagnetism of monatomic Co wire at the
Pt(111) surface step edge — •Peter M. Oppeneer 1 , Alexander
B. Shick 2 , and Franti˘sek Máca 2 — 1 IFW Dresden — 2 Inst. of
Physics, ASCR, Prague
We report a first-principles investigation of the anomalous ferromagnetism
of a quasi-one-dimensional Co chain decorating the Pt(111) step
edge. Our calculations demonstrate that the symmetry breaking at the Pt
step leads to an easy magnetization axis at an anomalous angle of ∼ 20 ◦
towards the Pt step, in semi-quantitative agreement with the experimentally
observed peculiar easy axis at an angle of ∼ 40 ◦ [P. Gambardella
et al., Nature 416, 301 (2002)]. The computed hard magnetization axis
direction also corresponds well to the experimentally observed hard axis.
A (T = 0) magnetocrystalline anisotropy energy of 3 meV/Co is calculated
in good agreement with the experimental value of ∼ 2 meV/Co at
T = 45 K. On account of the symmetry breaking the Co spin and orbital
moments become noncollinear, even in the case of a collinear ferromagnetic
spin arrangement. We also performed LSDA+U calculations, with
which a significant enhancement of the Co orbital magnetic moment is
achieved already when modest electron correlations (small U) are taken
into account.