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Programm und Abstracts der Sitzung MA 21

Physics of a new copper-based triangular chain

•Jürgen Schnack¹, Hiroyuki Nojiri², Paul Kögerler³ und Leroy Cronin^4
2Dept. of Physics, Okayama University, Tsushimanaka 3-1-1, Okayama, 700-8530 Japan
³Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA
⁴Dept. of Chemistry, The University of Glasgow, Glasgow, G12 8QQ, UK
¹Universität Osnabrück, Fachbereich Physik, D-49069 Osnabrück, Germany

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.

High-field magnetization study of the S = 1/2 antiferromagnetic Heisenberg chain [PM Cu(NO₃)₂(H₂O)₂]_n with field-induced gap

•S. Süllow¹, A.U.B. Wolter¹, H. Rakoto², M. Costes², A. Honecker³, W. Brenig³, A. Klümper⁴, H.-H. Klauss¹, F.J. Litterst¹, R. Feyerherm⁵ und D. Jérome^6
1IMNF, TU Braunschweig, Braunschweig
²Laboratoire National des Champs Magnétique Pulsés, Toulouse, France
³ITP, TU Braunschweig, Braunschweig,
⁴FB Physik, Bergische Universität Wuppertal, Wuppertal
⁵HMI GmbH, Berlin
⁶Université 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/k_B along the chain directions to 36.3(5) K and determine the field dependence of the staggered magnetization component m_s. 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.

Observation of a transverse magnetization in the S=1/2 antiferromagnetic chain [PM·Cu(NO₃)₂·(H₂O)₂]_n by ¹³C-NMR

A.U.B. Wolter¹, P. Wzietek², D. Jerome², S. Süllow¹, F.J. Litterst¹, A. Honecker³, W. Brenig³, R. Feyerherm⁴ und •H.-H. Klauss^1
2Laboratoire de Physique des Solides, Universite Paris-Sud, Orsay, France
³Institut für Theoretische Physik, TU Braunschweig
⁴Hahn-Meitner-Institut, Berlin
¹Institut für Metallphysik und Nukleare Festkörperphysik, TU Braunschweig

We present ¹³C-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.

Charge-induced modulation of magnetic interactions in a [2 ×2] metallo-organic grid-complex

•Christian Romeike¹, Maarten R. Wegewijs¹, Wolfgang Wenzel² und Herbert Schoeller^1
1Institut für Theoretische Physik A, RWTH Aachen, 52045 Aachen
²Institut 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 Fe²⁺-[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 Co²⁺-[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 Co²⁺ 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)

Magnetic correlations in oxalate spin ladders

•C. Mennerich¹, M. Bröckelmann¹, J. Kreitlow¹, A. Wolter¹, S. Süllow¹, F.J. Litterst¹, R. Klingeler², B. Büchner², D.-J. Price³ und H.-H. Klauss^1
1Institut für Metallphysik und Nukleare Festkörperphysik, TU Braunschweig
²Leibniz-Institut für Festkörper- und Werkstoffforschung, Dresden
³Department 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 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.

ESR analysis of the cyclic cluster [Na@Fe₆(tea)₆]·ClO₄·2MeOH

•Ilka Keilhauer¹, Bernd Pilawa¹, Danira Marinov², Arthur Grupp³ und Stefan Knorr^3
32. Physikalisches Institut, Universität Stuttgart,D-70550 Stuttgart
¹Physikalisches Institut, Universität Karlsruhe (TH), D-76128
²Anorganische Chemie, LMU, D-81377 München

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 7 K and 20 K. The parameters of the single ion on-site anisotropy are d/k_B = -0.2875±0.0023 K and d/k_B = -0.2921±0.0035 K, 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.

¹H-NMR on Fe₂(hpdta), Fe₄(hpdta)₂ and Fe₆(hpdta)₃ clusters

•Roland Leppin und Bernd Pilawa
Physikalisches Institut, Universität Karlsruhe (TH)

¹H-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 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 T₁^-1 rate measurements at 52 MHz in a magnetic field of 1.2 T show differences between the systems. For the Fe₂ dimer, the T₁^-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.

Magnetic Resonance Spectroscopic Studies of Magnetization Relaxation in Single-Crystalline Mn₁₂Ac

•Joris van Slageren¹, Suriyakan Vongtragool¹, Nadeschda Kirchner¹, Martin Dressel¹, Alexander Mukhin² und Boris Gorshunov^2
11. Physikalisches Institut, Universität Stuttgart, Stuttgart, Germany
²General 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.

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