Plenarvorträge - DPG-Tagungen

Magnetismus Donnerstag
MA 20.11 Do 12:45 H10
Epitaxial growth and ferroelectric and ferromagnetic properties
of La0.7Sr0.3MnO3 / Pb(Zr,Ti)O3 bilayers — •C. Thiele, K.
Dörr, T. Walter, K.-H. Müller, and L. Schultz — IFW Dresden,
Postfach 270116, 01171 Dresden
Magnetic and electrical properties of ferromagnetic manganites crucially
depend on lattice constants. Thus, it was suggested to control epitaxial
strain of thin films using the piezoeffect of a Pb(Zr,Ti)O3 (PZT)
toplayer [1]. Thereby, it seems possible to modify the resistivity in a wide
temperature range including 300 K, the magnetoresistance and magne-
MA 21 Bio- und molekularer Magnetismus
tization behavior of the ferromagnetic films. Pulsed laser deposition has
been utilized to investigate the growth of La0.7Sr0.3MnO3 / PZT bilayers
on single-crystalline substrates in order to obtain high structural quality
and undisturbed magnetic / ferroelectric properties. We discuss measurements
of resistance, magnetization and ferroelectrical polarization in
relation to the stoichiometry and microstructure of the bilayers. The latter
was investigated by x-ray diffraction, atomic force microscopy and
x-ray photoelectron spectroscopy.
This work is supported by DFG, FOR 520.
[1] H. Tabata et al., IEICE Trans. Electron. E80-C, 918 (1997).
Zeit: Donnerstag 10:15–12:15 Raum: H22
MA 21.1 Do 10:15 H22
Physics of a new copper-based triangular chain — •Jürgen
Schnack 1 , Hiroyuki Nojiri 2 , Paul Kögerler 3 , and Leroy
Cronin 4 — 1 Universität Osnabrück, Fachbereich Physik, D-49069
Osnabrück, Germany — 2 Dept. of Physics, Okayama University,
Tsushimanaka 3-1-1, Okayama, 700-8530 Japan — 3 Ames Laboratory,
Iowa State University, Ames, Iowa 50011, USA — 4 Dept. of Chemistry,
The University of Glasgow, Glasgow, G12 8QQ, UK
We report the synthesis and magnetic characterization of a new class
of one-dimensional chains of antiferromagnetically coupled copper triangles.
Since the copper triangles are coupled via hydrogen bonds, which
naively are assumed to be weak exchange pathways, one expects that
the chain shows a pronounced plateau at 1/3 of the saturation magnetization.
The high field magnetization curve shows, however, no plateau.
The observed magnetization and susceptibility curves are successfully interpreted
by a theoretical model, in which the inter- and intra-triangle
exchange parameters are of similar size. In effect the infinite chain turns
out to be an interesting example of a frustrated spin system with competing
interactions.
MA 21.2 Do 10:30 H22
High-field magnetization study of the S = 1/2 antiferromagnetic
Heisenberg chain [PM Cu(NO3)2(H2O)2]n with field-induced
gap — •S. Süllow 1 , A.U.B. Wolter 1 , H. Rakoto 2 , M. Costes 2 ,
A. Honecker 3 , W. Brenig 3 , A. Klümper 4 , H.-H. Klauss 1 , F.J.
Litterst 1 , R. Feyerherm 5 , and D. Jérome 6 — 1 IMNF, TU Braunschweig,
Braunschweig — 2 Laboratoire National des Champs Magnétique
Pulsés, Toulouse, France — 3 ITP, TU Braunschweig, Braunschweig,
— 4 FB Physik, Bergische Universität Wuppertal, Wuppertal — 5 HMI
GmbH, Berlin — 6 Université Paris-Sud, Orsay
We present a high-field magnetization study of the S=1/2 antiferromagnetic
Heisenberg chain [PM Cu(NO3)2(H2O)2]n. For this material, as
result of the Dzyaloshinskii-Moriya interaction and a staggered g-tensor,
the ground state is characterized by an anisotropic field induced spin
excitation gap and a staggered magnetization. Our data reveal the qualitatively
different behaviour in the directions of maximum and zero spin
excitation gap. The data are analyzed via exact diagonalization of a linear
spin chain with up to 20 sites and on basis of the Bethe ansatz equations,
respectively. For both directions we find very good agreement between
experimental data and theoretical calculations. We extract the magnetic
coupling strength J/kB along the chain directions to 36.3(5) K and determine
the field dependence of the staggered magnetization component
ms. This work has partially been supported by funds of the European
contract no. HPRI-CT-1999-40013 and by the DFG under contract no.
SU229/6-1.
MA 21.3 Do 10:45 H22
Observation of a transverse magnetization in the S=1/2 antiferromagnetic
chain [PM·Cu(NO3)2·(H2O)2]n by 13 C-NMR —
A.U.B. Wolter 1 , P. Wzietek 2 , D. Jerome 2 , S. Süllow 1 , F.J. Litterst
1 , A. Honecker 3 , W. Brenig 3 , R. Feyerherm 4 , and •H.-H.
Klauss 1 — 1 Institut für Metallphysik und Nukleare Festkörperphysik,
TU Braunschweig — 2 Laboratoire de Physique des Solides, Universite
Paris-Sud, Orsay, France — 3 Institut für Theoretische Physik, TU Braunschweig
— 4 Hahn-Meitner-Institut, Berlin
We present 13 C-NMR studies of the static and dynamic magnetic properties
of the S=1/2 antiferromagnetic chain [PM·Cu(NO3)2·(H2O)2]n. In
this system the spin-orbit interaction gives rise to an anisotropic magnetic
field induced spin excitation gap. The microscopic model used to
describe the thermodynamic properties (M. Oshikawa and I. Affleck, PRL
79 2883) proposed the existence of a staggered magnetization component
transverse to the applied field which cannot be measured macroscopically.
We measured the local susceptibility via the NMR frequency shift
at three different carbon sites in the pyrimidine molecule as a function
of temperature and magnetic field orientation. The transverse staggered
magnetization is identified as a low temperature deviation from the linear
correlation between local and global susceptibility. The observed magnitude
and temperature dependence are consistent with the theoretical
model. The spin excitation gap is determined from the temperature dependence
of the T1 relaxation rate. These measurements reveal additional
excitations in the gap.
MA 21.4 Do 11:00 H22
Charge-induced modulation of magnetic interactions in a
[2 × 2] metallo-organic grid-complex — •Christian Romeike 1 ,
Maarten R. Wegewijs 1 , Wolfgang Wenzel 2 , and Herbert
Schoeller 1 — 1 Institut für Theoretische Physik A, RWTH Aachen,
52045 Aachen — 2 Institut für Nanotechnologie, Forschungszentrum
Karlsruhe, Postfach 3640, 76021 Karlsruhe
We study the electronic spin and charge degrees of freedom of a [2×2]transition
metal-grid complex cation at different stages of its reduction[1]
in a phenomenological low temperature model. We show that extra electrons
on the bridging ligands can drastically change the coupling of the
spins on the metal sites and lead to maximal total spin states of the
molecular complex (spin of unpaired electrons on the metals and ligands)
due to the Nagaoka mechanism [2]. For a low-spin Fe 2+ -[2×2]-grid
electrons can move between the ligands using an empty orbital on the
metal as a bridge. In contrast to this, for a low-spin Co 2+ -[2 × 2]-grid
the metal bridge contains a localized electron which can interact or exchange
with the passing electrons. The presence of a localized spin on
each bridging metal-ion in the case of Co 2+ has two main effects: (1) the
total spin achievable in the fully polarized Nagaoka state is larger and (2)
the threshold Coulomb interaction required to achieve this state can be
diminished to values close to those of Fe only if direct exchange coupling
between the metal-ions and ligands is taken into account. It has to be of
the same order as the exchange coupling obtained in perturbation theory.
[1] M.Ruben et al., Chem.Eur.J. 2003, 9, No.1
[2] Y. Nagaoka, Phys. Rev.147, 392 (1966)
MA 21.5 Do 11:15 H22
Magnetic correlations in oxalate spin ladders — •C. Mennerich
1 , M. Bröckelmann 1 , J. Kreitlow 1 , A. Wolter 1 , S.
Süllow 1 , F.J. Litterst 1 , R. Klingeler 2 , B. Büchner 2 , D.-J.
Price 3 , and H.-H. Klauss 1 — 1 Institut für Metallphysik und Nukleare
Festkörperphysik, TU Braunschweig — 2 Leibniz-Institut für Festkörperund
Werkstoffforschung, Dresden — 3 Department of Chemistry, Univ.
of Southampton, United Kingdom
We present studies of the magnetic behaviour in the isostructural spin
ladder materials Na2T2(C2O4)3(H2O)2 with T= Ni, Co, Fe, Mn (J. Chem.
Soc., Dalton Trans. 2000, 3566 ). Here the oxalate (C2O4) 2− molecule acts
as the bridging element in the rungs and legs of the transition metal ladder.
We performed magnetic susceptibility and high field magnetization
as well as Moessbauer spectroscopy (Fe system only). The susceptibility
of all systems shows a pronounced maximum between 10 and 25 K indicating
an antiferromagnetic interaction. The data can be described using
a dimer model representing two metal centers in the 2+ high spin state
with a dominant magnetic interaction along the rungs of the ladder. An