Plenarvorträge - DPG-Tagungen

Magnetismus Donnerstag
additional weak interaction along the legs is treated in mean field approximation.
Moessbauer spectroscopy on the Fe compound revealed a
temperature dependent electric field gradient (EFG) due to crystal field
splitting and prove a Fe(II) S=2 configuration. No magnetic order is
found down to 2K.
MA 21.6 Do 11:30 H22
ESR analysis of the cyclic cluster [Na@Fe6(tea)6]·ClO4·2MeOH
— •Ilka Keilhauer 1 , Bernd Pilawa 1 , Danira Marinov 2 , Arthur
Grupp 3 , and Stefan Knorr 3 — 1 Physikalisches Institut, Universität
Karlsruhe (TH), D-76128 — 2 Anorganische Chemie, LMU, D-81377
München — 3 2. Physikalisches Institut, Universität Stuttgart,D-70550
Stuttgart
The six Fe III -ions of Na@Fe6(tea)6 (1) are antiferromagnetically coupled.
The magnetic anisotropy can be analysed by ESR measurements.
The crystal structure of (1) is triclinic with two magnetically nonequivalent
Fe6 clusters per unit cell. This gives rise to complicated spectra
composed of the resonances of the two clusters X and Y. It will be shown
that the splittings of the first excited states S=1,2 and 3 and the orientation
of the molecular symmetry axes can be determined for both clusters
by the analysis of the ESR spectra taken at 7K and 20K. The parameters
of the single ion on-site anisotropy are d/kB=−0.2875±0.0023K and
d/kB=−0.2921±0.0035K, respectively.
The relative angle between the cluster axes of 69.7 ◦ ±1 ◦ determined at
low temperature deviates considerably from the room temperature value
of 54 ◦ . This might indicate a structural phase transition which can be
caused by the temperature dependent motion of the ClO4 tetraeders.
MA 21.7 Do 11:45 H22
1 H-NMR on Fe2(hpdta), Fe4(hpdta)2 and Fe6(hpdta)3 clusters
— •Roland Leppin and Bernd Pilawa — Physikalisches Institut,
Universität Karlsruhe (TH)
1 H-NMR measurements on cyclic Fe6 clusters have shown the importance
of the energy level scheme for the nuclear relaxation. The investigation
of the Fe2(hpdta) dimer is an interesting approach for a better
understanding of the spindynamics, as it can be used as a building block
MA 22 Spinabhängiger Transport I
for more complex clusters like the Fe4(hpdta)2 and Fe6(hpdta)3 systems,
where the metal ions form a square and roof-like structure, respectively.
The 1H T −1
1 rate measurements at 52 MHz in a magnetic field of 1.2
T show differences between the systems. For the Fe2 dimer, the T −1
1 rate
increases monotonously. For Fe4 it starts at a comparatively high value
at low temperatures, goes down to a minimum around 15 K and increases
again with increasing temperature. For the Fe6 sample there is a broad
maximum around 30 K which reminds the results obtained for the cyclic
Fe6(tea)6 samples.
MA 21.8 Do 12:00 H22
Magnetic Resonance Spectroscopic Studies of Magnetization
Relaxation in Single-Crystalline Mn12Ac — •Joris van
Slageren 1 , Suriyakan Vongtragool 1 , Nadeschda Kirchner 1 ,
Martin Dressel 1 , Alexander Mukhin 2 , and Boris Gorshunov 2
— 1 1. Physikalisches Institut, Universität Stuttgart, Stuttgart, Germany
— 2 General Physics Institute, Russian Academy of Sciences, Moscow,
Russia
New, spectroscopic experiments on the magnetization relaxation of the
single molecule magnet Mn12Ac are presented. These studies were performed
using frequency domain magnetic resonance spectroscopy on single
crystalline samples. Special attention is paid to the consequences of
the known distribution of zero-field splitting parameters on the relaxation
dynamics. It is also shown, that the resonance spectrum of the
magnetized sample recorded in zero external field has a peculiar shape in
Faraday geometry, due to a combination of the resonance phenomenon
and the Faraday effect. The nonmagnetized sample shows a normal spectral
shape. In this way it is possible to study magnetization relaxation
in zero external field. Finally, we have succeeded in performing selective
magnetodipolar excitations on one side of the potential energy double
well, by using circularly polarized radiation. This offers another possibility
for studying magnetization relaxation in zero external field, which is
not possible when using linearly polarized radiation. These studies are
important to investigate the role of accidental transverse magnetic field
on the relaxation behaviour of single molecule magnets.
Zeit: Donnerstag 12:15–13:00 Raum: H22
MA 22.1 Do 12:15 H22
Imaging tunnel magnetoresistance related to magnetic microdomain
distribution along a La2/3Ca1/3MnO3 grain boundary
— •Wagenknecht Michael 1 , Timo Nachtrab 1 , Markus Turad
1 , Dieter Koelle 1 , Reinhold Kleiner 1 , Jan Boris Philipp 2 ,
and Rudolf Gross 2 — 1 Physikalisches Institut-Experimentalphysik
II, Universität Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen,
Germany — 2 Walther-Meißner-Institut, Bayerische Akademie der
Wissenschaften, Walther-Meißner Straße 8, D-85748 Garching, Germany
We present electrical transport measurements across an artificial grain
boundary in a La2/3Ca1/3MnO3 thin film grown on a 24 ◦ bicrystal SrTiO3
substrate. In the ferromagnetic-metallic phase below Tc ≈ 220K the grain
boundary acts as a tunneling barrier. Applying an in-plane magnetic field
up to 40mT parallel to the grain boundary we observe several discrete
jumps in the magnetoresistance across the barrier. We explain the resistance
steps by formation of microdomains along the grain boundary.
Using low temperature laser scanning microscopy allows us to image the
tunnelling conductance distribution along the grain boundary for the
discrete resistance steps. By this we are able to map the fraction of parallel/antiparallel
aligned microdomains versus magnetic field and to correlate
the domain distribution with the overall resistance across the grain
boundary.
MA 22.2 Do 12:30 H22
Magnetoresistance and resistance relaxation of La0.7Ca0.3MnO3
step-edge junctions in pulsed magnetic fields up to 50 T — •K.
Dörr 1 , N. Kozlova 1 , M. Ziese 2 , M. Kozlov 1 , K.-H. Müller 1 ,
and L. Schultz 1 — 1 IFW Dresden, Postfach 270116, 01171 Dresden
— 2 Division of Superconductivity and Magnetism, University of Leipzig,
Linnéstrasse 5, 04103 Leipzig
The magnetoresistance (MR) of step-edge junctions in La0.7Ca0.3MnO3
films [1] has been investigated in magnetic fields up to 47 T between
4.2 K and 125 K. Step-edge junctions show similar electrical properties
as grain boundaries [1], including the low-field MR attributed to spin-
polarized tunneling. A tendency towards saturation in high fields and a
large hysteresis with pronounced remanent MR are characteristic results.
Time-dependent resistance data R(t) recorded after field pulses of 7 T
follow a logarithmic law for 30 ms < t