Project within the Priority Program
(DFG-Schwerpunkt)

_{Molecular Magnetism - SPP1137}

Abstract of the collaborative project application:

Miniaturization is a key technological aspect; therefore it is expected that, in the future, single molecules acting as nanosized magnets will gain considerable importance in practical applications. The priority programme "Molecular Magnetism" focusses on the nvestigation of all aspects of magnetic properties associated with molecules or solids derived from molecular entities. The specific topics are (i) magnetic exchange interactions between magnetic centers in discrete molecules (coordination compounds, organic radicals and biomolecules); (ii) magnetic properties of supramolecular solids; (iii) spin-crossover systems; and (iv) physical and theoretical studies of molecular magnetism. Special attention is given to the synthesis of new magnetic materials, to the study of quantum-effects in nanosized molecular magnets and of the magnetic cooperativity and ordering occurring in supramolecular magnetic solids as well as to multifunctional spin-crossover systems. The priority programme also addresses basic theoretical aspects of molecular magnetism, that is, on establishing the fundamental principles underlying the magnetic behavior of isolated molecules containing interacting magnetic centers as well as of assemblies of molecules. Although the results of this research programme may potentially have a high impact on the future development of devices with molecular dimensions, the priority programme is intended to focus primarily on fundamental research of molecular magnetism and not on its practical applications.

Sprecher / Spokesperson: Professor Dr. Hans-Jörg Krüger
Fachbereich Chemie der Universität Kaiserslautern
E-Mail: krueger@chemie.uni-kl.de

Further informations: www.molmag.de

Abstract and Introduction to our project

Lattice dynamics contribution to the magnetic properties of molecular magnets:
Raman and ultrasonic studies

This proposal concerns high resolution Raman scattering and ultrasonic studies of two classes of molecular magnetic compounds:

    I) spin-crossover systems, which demonstrate a change of spin state as function of temperature, pressure as well light irradiation;

    II) single-molecular magnets consisting of polynuclear transition metal ion complexes bound by organic ligands.

The main goal of this project is the investigation of the role of intermolecular vibrations for the cooperative behavior of spin crossover transitions for case I systems. Spin phonon interactions and the determination of phonon modes which strongly affect the efficiency of magnetic exchange paths will be the main aim for case II systems. Raman scattering and ultrasonic studies will be performed in the magnetic field-temperature phase space and under hydrostatic pressure up to 2GPa. The role and influence of lattice degrees of freedom on the spin state crossover mechanisms in the case I and in quantum tunneling mechanism of magnetization in the case II will be elucidated.

The second goal of the project is the search of the spin state gap as well as its temperature evolution for case I systems and the manifestation of inelastic many spin effects in Raman spectra in case II systems. Lattice dynamical calculations will be performed and the results compared with experimental data. The stability of different spin states against different type of lattice distortions will be analyzed with help of modified crystal approach with inclusion of spin-orbital interaction.

The crystals to be investigated are tetranuclear and hexanuclear coordinated Fe  and Mn and spin crossover systems. 

The joint Project leaders are Peter Lemmens (IPKM, TU-Braunschweig, Germany) and Yurii G. Pashkevich (Donetsk Physics & Technology Institute, NASU, Ukraine) and results of our work have been reported during meetings of the priority program in Bad Dürkheim and the German Phys. Soc Meeting.

Publications:

1) Spectral sensitive phonon wipeout in a two-site Fe²⁺ coordination polymer, V. Gnezdilov, P. Lemmens, P. Scheib, M. Ghosh, Yu.G. Pashkevich, H. Paulsen, V. Schünemann, J. A. Wolny, G. Agustí, J. A. Real, submitted to Phys. Rev. B (2008).

2) Anomalous frequency and intensity scaling of collective and local modes in a coupled spin tetrahedra system, K. Y. Choi, H. Nojiri, N. S. Dalal, H. Berger, W. Brenig, P. Lemmens, Phys. Rev. B, in print (2008).

3) Tuning the pore wall morphology of mesoporous silicon from branchy to smooth, tubular by chemical treatment, P. Kumar, T. Hoffman, P. Huber, P. Scheib, and P. Lemmens, Journ. Appl. Physics 103, 024303 (2008).

4) Spin state transformation of a 3d ion in the pyramidal environment and under lattice distortions, E.S. Zhitlukhina, K.V. Lamonova, S.O. Orel, P. Lemmens, and Yu.G. Pashkevich, Journ. Phys.: Cond. Matter 19, 156216 (2007).

5) Crystal structure and magnetic properties of the coupled spin dimer compound SrCu₂(TeO₃)₂Cl₂, R. Takagi, M. Johnsson, R. K. Kremer, P. Lemmens, Jour. Solid State Chem. 179, 3763 (2006).

p.lemmens at tu-bs.de, last change: 19.04.2006

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