Plenarvorträge - DPG-Tagungen

Oberflächenphysik Mittwoch
O 28.82 Mi 16:00 Bereich C
Transverse magneto-optical Kerr effect at the 3p edges of Fe and
Co — •Armin Kleibert 1 , Sergio Valencia 2 , Hans-Christoph
Mertins 2 , and Joachim Bansmann 1 — 1 Universität Rostock, Fachbereich
Physik, Universitätsplatz 3, 18051 Rostock — 2 BESSY GmbH,
Albert-Einstein-Str. 15, 12489 Berlin, Germany
Soft x-ray based experimental techniques are powerful and very sensitive
methods to investigate the magnetism of systems with low dimensions
like ultrathin films or clusters, respectively. In particular, the
magneto-optical Kerr-effect (MOKE) at the 2p edges of Fe, Co, and Ni
has attracted much attention. In the transverse geometry (T-MOKE)
huge intensity changes in the resonantly enhanced reflectivity upon reversal
of the magnetization direction have been found. While effects near
the 2p core levels were extensively studied in the last years, only a few
experiments were reported concerning the vicinity of the 3p threshold.
In this contribution, we present angular dependent measurements of
the T-MOKE in the energy range of the 3p edges of thin Fe and Co
films, respectively, revealing surprisingly large asymmetries up to 40%
near the Brewster angle. From these reflectivity measurements it is possible
to obtain information about the energy dependent refractive indices
and magneto-optical Voigt constants. In order to simulate the measured
reflectivity including the influence of capping layers we applied a multilayer
formalism describing magneto-optics. A good agreement of calculation
and experiment was achieved using tabulated values of the index
of refraction and independently obtained Voigt constants.
O 28.83 Mi 16:00 Bereich C
Resonant X-Ray scattering from TbBaCo2O5.5 — •N.V.
Kasper 1 , P. Wochner 1 , G. Carbone 1 , A. Vigliante 2 , D. Mannix
3 , and G. Jakob 4 — 1 Max-Planck-Institut für Metallforschung,
Stuttgart, Germany — 2 Bruker AXS, Karlsruhe, Germany — 3 European
Synchrotron Radiation Facility, Grenoble, France — 4 Institut für
Physik, Universität Mainz, Mainz, Germany
Resonant X-ray scattering at the Co K-edge from epitaxial films and
single crystals of TbBaCo2O5.5 cobaltite with perovskite structure has
been studied in insulating and metallic states. Polarization, energy and
azimuthal dependences of the intensity of (016), (116), and (310) Bragg
peaks were measured. Strong oscillations of the resonantly scattered intensity
were observed both in σ’ and π’ channels. The scattered signal
changes very weakly during the metal-insulator phase transition.
The azimuthal oscillations of the scattered signal are attributed to
the tensor character of the anomalous scattering factor of Co. Using
this concept we have developed a model, describing very well the measured
azimuthal oscillations. On the basis of the obtained experimental
results and literature data it is argued that this anisotropy of the anomalous
scattering factor of Co is caused mainly by distortions of the local
surrounding of cobalt ions and not by anisotropies due to ordering of
electronic orbitals of Co.
O 28.84 Mi 16:00 Bereich C
X-ray magneto-optical sum rules in small Co clusters —
•Kai Fauth 1,2 , Markus Heßler 1,2 und Gisela Schütz 2 —
1 Physikalisches Institut der Universität Würzburg, Am Hubland, 97074
Würzburg — 2 MPI für Metallforschung, Heisenbergstraße 3, 70569
Stuttgart
The so-called spin and orbital sum rules [1,2] in x-ray magnetic circular
dichroism (XMCD) spectroscopy are often believed to apply with
reasonable accuracy in the case of the late 3d transition metals and their
compounds, if the magnetic dipole term Tz can be neglected. Strong deviations
can occur due to symmetry breaking such as given at surfaces,
O 29 Hauptvortrag Pentcheva
causing the magnetic dipole term to contribute significantly to the spin
sum rule, while not contributing to the magnetization. Much less is known
in the case of small clusters. We have derived magnetic moments of superparamagnetic
Co55 clusters from XMCD at various applied magnetic
fields. We find that the moments thus obtained from the magnetization
curve and those determined separately from the sum rules agree to within
10%. This result sets an upper limit for the importance of Tz in these
clusters.
[1] B. T. Thole et al. , Phys. Rev. Lett. 68, 1943 (1992)
[2] P. Carra et al. , Phys. Rev. Lett. 70, 694 (1993)
O 28.85 Mi 16:00 Bereich C
X-ray resonant magnetic scattering of Fe/Cr superlattices
— •Alexei Nefedov, Johannes Grabis, Andre Bergmann,
Florin Radu, and Hartmut Zabel — Institut für Experimentalphysik/Festkörperphysik,
Ruhr-Universität Bochum, D-44780 Bochum,
Germany
We have studied the magnetic and structural properties of an antiferromagnetically
(AF) coupled Fe/Cr(001) superlattice by soft X-ray
resonant magnetic scattering. Strong magnetic Bragg peaks are observed
at the half order positions in reciprocal space parallel to the [001] growth
direction and in between the structural Bragg reflections from the superlattice
periodicity. The magnetic hysteresis loops measured at the structural
Bragg peak and at the half-order peak clearly demonstrate the
AF coupling of the Fe/Cr multilayer. Transverse scans and off-specular
reflectivity measurements confirm an AF domain structure of the superlattice
in remanence with large perpendicular correlation. In addition,
the transverse scan of the half-order Bragg peak exhibits a Lorentzian
line shape at zero field, which diminishes in higher fields, indicative for
a remanent multidomain state which approaches a single domain state
towards saturation.
This work was supported by the German Federal Ministry of Education
and Research (BMBF) under Contract No.03ZAE7BO.
O 28.86 Mi 16:00 Bereich C
Inducing spin crossover in metallo-supramolecular polyelectrolytes
through an amphiphilic phase transition — •Yves
Bodenthin 1 , U. Pietsch 1 , D. Kurth 2 , and H. Möhwald 2 —
1 University of Potsdam, Institute of Physics, Am Neuen Palais 10,
14469 Potsdam, Germany — 2 MPI-KG Golm, Am Mühlenberg 1, 14476
Potsdam-Golm
In the last years resonant magnetic X-ray reflectivity experiments
(XMCD) become important for the investigation of molecular magnetism
in thin films.
In the present work we report a novel approach to affect spin crossover
by introducing mechanical strain through an amphiphilic phase transition
in a metallo-supramolecular polyelectrolyte-amphiphile complex
(PAC). The investigated system consists of thin layers deposited in Yconformation
on a silicon substrate by means of Langmuir-Blodgett technique.
It was observed by EXAFS, x-ray reflectivity and in-plane diffraction
experiments that the structural phase transition in the amphiphilic
mesophase is responsible for the distortion of the coordination geometry
of the tightly coupled transition metal centres. Consequently, the energetic
separation of the subsets of the iron d-orbitals changes and gives
rise to spin crossover. Using temperature resolved SQUID and XMCD
measurements we observe a diamagnetic to paramagnetic phase transition
above room temperature which is connected to a structural phase
transition between two liquid crystalline phases.
Zeit: Donnerstag 09:30–10:15 Raum: H36
Hauptvortrag O 29.1 Do 09:30 H36
Beyond the (p=0 Pa, T=0 K) limit: Surface Phase Diagrams and
Growth Kinetics from First Principles — •Rossitza Pentcheva
— Section Crystallography, Dept. for Earth and Environmental Sciences,
LMU München — Fritz-Haber-Institut der MPG, Berlin
Combining density functional theory (DFT) with methods from statistical
mechanics (ab initio kinetic Monte Carlo (kMC)) or thermodynamics
(ab initio atomistic thermodynamics, see e.g. [1]) enables us to extend
the predictive power of DFT to meso- and macroscopic length scales
and/or to finite pressures and temperatures. Two examples for their application
are presented: In the initial growth of Co on Cu(001) DFT-kMC
simulations reveal that the activation of atomic exchange leads to substantial
deviations from the predictions of standard nucleation theory,
namely a bimodal growth mode and a nonmonotonic scaling behavior of
island density [2]. Using ab initio atomistic thermodynamics we construct
a phase diagram of Fe3O4(001), a promising material for the development
of spintronic devices. A hitherto ignored termination with octahedral
iron and oxygen forming a wave-like structure along the [110]-direction