Plenarvorträge - DPG-Tagungen

Magnetismus Dienstag
MA 13.107 Di 15:00 Bereich A
Well-Aligned Co-Filled Carbon Nanotubes: Preparation and
Magnetic Properties — •Radinka Kozhuharova 1 , Dieter
Elefant 1 , Andreas Graff 1,2 , Albrecht Leonhardt 1 , Ingolf
Mönch 1 , Thomas Mühl 1 , Manfred Ritschel 1 , Claus M.
Schneider 1,3 , and Stefka Zotova 1 — 1 Leibniz-Institute of Solid
State and Materials Research Dresden, Helmholtzstr. 20, D-01069
Dresden — 2 present address: Max Planck Institute of Microstructure
Physics, Halle — 3 present address: Research Center Jülich GmbH,
D-52425 Jülich
Carpet-like flakes (area < 3mm x 3mm; thickness < 500nm) of wellaligned
Co-filled multi walled carbon nanotubes were grown by decomposition
of cobaltocene. The product was analysed by scanning (FE-SEM,
Leo 1530)- and transmission (TEM, Tecnai F30 with GIF200)- electron
microscopy. The phase composition and the crystalline structure were
determined by X-ray difffraction and selected area electron diffraction.
Using alternating gradient magnetometry the ferromagnetic behaviour
of the filled nanotubes was investigated. The nanotubes have outer diameter
of 50-90nm and diameter of the metal core of 15-30nm. They are
discontinuously filled with fcc Co nanowires having a length of up to a few
micrometers. Magnetometry measurements show a weak uniaxial magnetic
anisotropy with the easy axis along the nanowires. A high coercivity
of about 59mT at room temperature is observed, which is significantly
higher than in bulk Co.
MA 13.108 Di 15:00 Bereich A
PAC-Untersuchungen an amorphen Ferromagnetika — •V. Samokhvalov
1 , F. Schneider 1 , S. Unterricker 1 , M. Dietrich 2 und
die ISOLDE - Collaboration 3 — 1 Institut für Angewandte Physik,
TU Bergakademie Freiberg, D-09596 Freiberg/Sachsen — 2 Technische
Physik, Universität des Saarlandes, D-66041 — 3 CERN, CH-1211 Geneva
23, Switzerland
Amorphe ferromagnetische Materialien, wie z. B. Fe80B20, sind mit
dem Mößbauer-Effekt sehr ausführlich untersucht worden. Mit der Sonde
57 Fe wurden dabei über die Verteilungen der Hyperfeinfelder wesentliche
Aussagen zur Nahordnung in der Umgebung der Eisenatome gewonnen.
Allerdings bereitet die Interpretation der immer vorhandenen
Quadrupolwechselwirkung Probleme. Wir haben amorphes Fe80B20 mit
der Methode PAC (= perturbed angular correlations) und den Sonden
111m Cd, 111 In( 111 Cd) und 62 Zn( 62 Cu), die am ISOLDE-Separator mit einer
Energie von 50 keV implantiert wurden, untersucht. Messungen nach
unterschiedlichen Wärmebehandlungen werden vorgestellt und die auftretenden
Feldverteilungen diskutiert.
MA 13.109 Di 15:00 Bereich A
Electronic Structure of Transition-Metal Dicyanamides
Me[N(CN)2]2 (Me = Mn, Fe, Co, Ni, Cu) — •Takács Albert 1 ,
Chiuzbăian Sorin 1 , Crainic Traian Ionica 1 , Demchenko D.
O. 2 , Filkenstein L.D. 3 , Galakhov V.R. 3 , Kmety Carmen
R. 4 , Kurmaev E. Z. 3 , Liu Amy Y. 2 , Moevs A. 5 , Neumann
Manfred 1 , and Stevensson Knneth L. 6 — 1 Universität Osnabrück
— 2 Department of Physics, Georgetown University, Washington —
3 Institute of Metal Physics, Yekaterinburg — 4 Argonne National
Laboratory, USA — 5 University of Saskatchewan, Canada — 6 Purdue
University Indiana , USA
The electronic structure of Me[N(CN)2]2 (Me=Mn, Fe, Co, Ni, Cu)
molecular magnets has been investigated using x-ray emission spectroscopy
(XES) and x-ray photoelectron spectroscopy (XPS) as well as
theoretical density-functional-based methods. Both theory and experiments
show that the top of the valence band is dominated by Me 3d
bands, while a strong hybridization between C 2p and N 2p states determines
the valence band electronic structure away from the top. The
2p contributions from non-equivalent nitrogen sites have been identified
using resonant inelastic x-ray scattering spectroscopy with the excitation
energy tuned near the N 1s threshold. The binding energy of the Me 3d
bands and the hybridization between N 2p and Me 3d states both increase
in going across the row from Me = Mn to Me = Cu. Calculations
indicate that the ground-state magnetic ordering is largely dependent on
the occupation of the metal 3d shell and that structural differences in the
superexchange pathways for different compounds play a secondary role.
MA 13.110 Di 15:00 Bereich A
X-ray magnetic and natural circular dichroism above and below
the Verwey transition in a Fe3O4 single crystal — •S. Gold, E.
Goering, and G. Schütz — Max-Planck-Institut für Metallforschung,
70569 Stuttgart
Recently Fe3O4 has become an intensive studied system, due to its
proposed and experimentally observed high spin polarization at the fermi
energy and its related applicability in future spin electronic devices. In
addition, at the so called Verwey transition, which has been investigated
over many decades, a dramatic change in the electrical resistivity appears.
We show detailed X-ray magnetic circular dichroism (XMCD) experiments
at 1T applied magnetic field performed above and below the
Verwey transition and along different crystallographic directions ((100),
(110) and (111)). We have observed very small changes in the XMCDspectra
of this cubic system as a function of temperature and orientation,
which has not been observed before in a cubic system. Changes in the
differences of the projected orbital moments along the given crystallographic
axes above and below the Verwey transition are consistent to the
observed change of the easy axis. In addition a directly observable and
unexpected large nonmagnetic natural circular dichroism (XNCD) signal
has been observed by flipping the helicity of the synchrotron beam. This
is a sign of an unexpected local non centro symmetric charge distribution
at the 3d transition metal site. Our investigations give new insight to the
microscopic ground state and excited state properties of Fe3O4 related
to the magnetic behavior and the Verwey transition.
MA 13.111 Di 15:00 Bereich A
First principles electronic structure of spinel LiCr2O4: A
possible half-metal? — •Markus Lauer 1 , Roser Valenti 2 ,
H.C. Kandpal 3 , and Ram Seshadri 4 — 1 Fachrichtung Theoretische
Physik, Universitaet des Saarlandes, Postfach 15 11 50, D-66041
Saarbruecken — 2 Institut fuer Theoretische Physik, Universitaet
Frankfurt, Robert-Mayer-Strasse 8, D-60043 Frankfurt — 3 Institut
fuer Anorganische Chemie, Universitaet Mainz, Duesberg Weg 10-14,
D-55099 Mainz — 4 Materials Department, University of California,
Santa Barbara, CA 93106 USA
We have employed first-principles electronic structure calculations to
examine the hypothetical oxide spinel LiCr2O4 with the d 2.5 electronic
configuration. The cell and internal (oxygen position) structural parameters
have been obtained for this compound through structural relaxation
in the first-principles framework. Within the one-electron band picture,
we find that LiCr2O4 is magnetic, and a candidate half-metal. The
electronic structure is substantially different from the closely related rutile
half-metal CrO2. Comparisons with superconducting LiTi2O4, heavy
fermion LiV2O4 and charge-ordering LiMn2O4 suggest the effectiveness
of a nearly-rigid band picture involving simple shifts of the position of
EF in these very different materials. Comparisons are also made with the
electronic structure of ZnV2O4, a correlated insulator that undergoes a
structural and antiferromagnetic phase transition.
MA 13.112 Di 15:00 Bereich A
Magnetic properties in chromium-chalcogenide systems —
•Diana Benea 1 , Hubert Ebert 1 , Wolfgang Bensch 2 , and
Zhong-Le Huang 2 — 1 Chemistry Department, University of Munich,
Butenandtstr. 5-13, D-81377 München, Germany — 2 Institute for
Inorganic Chemistry, Olshausenstr. 40, D-24098 Kiel, Germany
A theoretical investigation of magnetic properties of the systems
Cr1+x(Se,Te)2 and (Cr,Ti)1+xTe2 is presented. The crystallographic structure
of these compounds was refined in the space group P¯3m1 with four
crystallographically different sites for both metal and chalcogen atoms.
The chalcogen layers are fully occupied, whilst in the metal layer there
are vacancies. The preferential occupation of the metal layers is discussed.
The density of states (DOS) are calculated and the influence of
the Cr/Ti content on the magnetic moments in the system is discussed.
The magnetic behaviour of the systems has been investigated using the
Korringa-Kohn-Rostoker (KKR) band structure method. The disorder in
the system has been accounted for by means of the Coherent Potential
Approximation (CPA).