Plenarvorträge - DPG-Tagungen

Oberflächenphysik Montag
O 12 Magnetismus in reduzierten Dimensionen
Zeit: Montag 15:45–17:00 Raum: H45
O 12.1 Mo 15:45 H45
Magnetische Röntgenstreuung an lateral strukturierten
Co/CoO Filmen — •Arndt Remhof, Johannes Grabis,
Alexei Nefedov und Hartmut Zabel — Experimentalphysik/Festkörperphysik,
Ruhr-Universität 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 1µm und eine Periode von 3µm. 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 Ummagnetisierungs-prozesses. 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 12.2 Mo 16:00 H45
Spin effects in Coulomb blockade of Fe/MgO/Fe — •Wulf
Wulfhekel 1 , F. Zavaliche 1,2 , M. Klaua 1 , C.C. Kuo 1,3 , M.T.
Lin 1,3 , and J. Kirschner 1 — 1 MPI für Mikrostrukturphysik, Weinberg
2, 06120 Halle — 2 Departement de physique, Universite de Montreal,
Quebec H3C 3J7, Canada — 3 Department of Physics, National Taiwan
University, Taipei 106, Taiwan
The Coulomb blockade (CB) of nanometer sized Fe islands grown on
single crystalline MgO films on Fe(001) was studied using scanning tunneling
microscopy (STM) at 25 K in ultra high vacuum. After deposition
of 3-4 monolayers (ML) MgO, a sub-ML amount of Fe was deposited
leading to the formation of Fe islands on the MgO films as observed by
STM. Scanning tunneling spectroscopy was then used to locally measure
the tunneling characteristics for tunneling from the tip via the island
into the Fe substrate. The I(V) curves showed the characteristic steps of
CB. The position of the steps in the I(V) characteristics of periodicity
UC are proportional to the Coulomb charging energy e/C, where e is
the electron charge and C the islands capacitance. A plot of UC versus
e/C for the Fe islands, where C was estimated from the island size as
observed with STM, reveals a linear dependence in agreement with CB.
We, however, observe a constant and positive offset in UC by 1 eV, which
cannot be explained by spin independent tunneling. We propose a model
for spin-dependent CB, that relates the offset to the exchange the additional
electron feels in the Fe islands. No systematic offset in UC has
been found for identical experiments with Pd islands indicating that the
observed offset is related to the island material.
O 12.3 Mo 16:15 H45
Valence band electronic structure and chemical reactivity in
MgO/Fe(100) and MgO/Co/Fe(100) oxide-metal interfaces
grown on GaAs(100) — •Liu-Niu Tong 1 , Frank Matthes 1 ,
and C.M. Schneider 2 — 1 Leibniz Institut für Festkörper- und
Werkstoffforschung — 2 Institut für Elektronische Eigenschaften,
Forschungszentrum
O 13 Methodisches (Experiment und Theorie)
The interface electronic structure of MgO/Fe(100) and MgO/Co/
Fe(100) films epitaxially grown on GaAs(100) was studied by employing
spin-polarized photoemission spectroscopy (SPPES) and magnetic circular
dichroism (MCD) techniques. With increasing excitation energy from
30 eV to 60 eV, we observe an enhanced spectral intensity in the SPPES
data of pure Fe for transitions from bulk ∆ 1 and ∆ 5 initial states near
the Fermi energy that is accompanied with an increase in the detected
spin polarization. After covering the Fe(100) surface by 0.5 ML MgO,
the shape of the Fe ∆ 5 minority-spin peak becomes significantly broadened,
while the shape of the Fe ∆ 1 majority-spin peak is only weakly
affected. Additionally, we observed that 1 ML MgO on top of Fe(100)
suppresses almost all contribution from the Fe ∆ 5 minority transition in
the measured spin polarization data. This observed effect is especially
strong for an excitation energy of 40 eV. Furthermore, our MCD data
show that the O 2p peak is shifting towards the Fermi level as a function
of time after preparation, indicating a slow change in the film chemistry.
This chemical reaction can also be seen for MgO/Fe samples that were
immediately covered after preparation with an additional Fe film on top.
O 12.4 Mo 16:30 H45
X-ray magnetic circular dichroism sum rule correction for the
light transition metals — •Eberhard Goering — Max-Planck-
Insitut für Metallforschung, 70569 Stuttgart
Quantum mechanical mixing of the L2 and L3 edge excitations strongly
increases with reduced 2p spin-orbit-splitting. For a large number of
2p→3d absorption spectra the shape has been fitted nearly perfectly by a
model, which takes into account lifetime and jj-mixing effects. X-ray magnetic
circular dichroism (XMCD) sum rule correction factors have been
determined for V and Cr, which are consistent to complementary investigations.
This fitting procedure and the estimated correction factors are
the basis for a future light element XMCD effective spin renormalization
technique.
O 12.5 Mo 16:45 H45
Spin-resolved inverse photoemission with improved energy resolution
— •Michael Budke, Volker Renken, Helmut Liebl,
Georgi Rangelov, and Markus Donath — Physikalisches Institut,
Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster
Spin-resolved inverse photoemission (IPE) is a powerful tool that measures
the spin-dependend electronic structure above the Fermi level of
magnetic surfaces and thin films. We have developed a source for spinpolarized
electrons with variable energy resolution from 150 meV to 400
meV (FWHM) at currents between 0.5 µA and 5 µA on the sample. The
photon detector consists of a conventional Geiger tube filled with either
acetone or iodine [1,2]. MgF2 is used as entrance window, followed by a
SrF2- or CaF2-window. The transmission cutoff was shifted by varying
the temperature of the windows. With this arrangement the energy resolution
of the counters is continuously variable between 150 meV and 400
meV (FWHM). Depending on the experimental requirements the total
energy resolution of our IPE system can therefore be varied between 500
meV and nearly 200 meV. The latter value is an improvement of the
energy resolution by a factor of two compared to previous spin-resolved
IPE systems [3]. [1] Dose V., Appl. Phys. 14, 117 (1977) [2] Funnemann
D., Merz H., J. Phys. E: Sci. Instrum. 19, 554 (1986) [3] Dose V., Fauster
T., Schneider R., Appl. Phys. A 40, 203 (1986)
Zeit: Montag 17:00–17:30 Raum: H45
O 13.1 Mo 17:00 H45
Experimente mit kinematischen stehenden Röntgenwellen an
CaF2/Si(111) — •Martin Tolkiehn 1 , Dmitri V. Novikov 1 und
Cunrang Wang 2 — 1 HASYLAB am DESY, Notkestraße 85, D-22603
Hamburg — 2 Institut für Halbleiterbauelemente und Werkstoffe, Universität
Hannover, Appelstraße 11A, D-30167 Hannover
Die Methode der stehenden Röntgenwellen (XSW) ist sehr gut geeignet
zur Bestimmung der Position von Adsorbatatomen an Oberflächen
und Dotieratomen in perfekten Kristallen [1,2]. Sie beruht auf der Ver-
änderung der Phase des stehenden Wellenfelds in der Nähe eines Bragg–
Reflexes, die mit Hilfe der dynamischen Theorie der Röntgenbeugung
berechnet wird. Die Anwendung dieser Theorie auf nicht perfekte Kristalle
ist ohne weiteres nicht möglich. Das stehende Wellenfeld ist jedoch
auch noch bei größeren Abweichungen vom Braggwinkel vorhanden, bei
denen die kinematische Näherung gilt.
Wir zeigen theoretisch und experimentell am Beispiel CaF2/Si(111)
[3], daß sich der Bereich fern vom Bragg–Winkel zur Bestimmung der
Adsorbatposition nutzen läßt und dasselbe Ergebnis liefert wie die kon-