About Spins

Molecular Magnetism: Controlled Nanoscale Magnetism

1. Motivation

The targeted synthesis of tailor-made magnetic molecules was long time an unrealized dream of chemists and physicists. Only in the last ten years it succeeded to develop and utilize the required processes for the production [1]. Meanwhile these processes attained a degree of quality which permits almost without limitation but with surprising systematic the creation of any magnetic molecules within the scope of "chemical engineering" [2, 3]. With the appearance of the first magnetic molecules, a completely new and unique area of research arised for the physicists. The reason for that is, above all, that the chemical structure of these molecules permits to observe and measure the magnetic characteristics of individual molecules. The physicists owe this fact the distinctive structure of the molecules:The magnetic ions are embedded into an organic matrix and and are surrounded by large ligand complexes, so that a magnetic interaction with neighbor molecules can be disabled to a large extent. That means in particular that measurements of (poly-) crystalline samples consisting of a large quantity of molecules, immediately detect the intra-molecular interactions within a molecule. This fact is particularly interesting and hardly conceivable that the produced molecules are partially highly symmetrical concerning the geometrical arrangement of the magnetic ions [13]. Small, quasi-one-dimensional systems with two [4] to ten iron ions are to be encountered as well as magnetic cages with 30 and more paramagnetic centers [5]. These almost ideal "Laboratory systems" enable it to answer fundamental questions to the magnetism: Particularly systems with a small quantity of magnetic ions and small spin quantum numbers permit an accurate quantum-mechanical handling, which can be directly compared with experimental results [6]. Beyond that the variation of size, symmetry and spin quantum number (by substitution of the ions) enables an evaluation of quantum-mecanical accurate and classical aproximated ideas of the models. Basic questions regarding the modification of magnetic characteristics with transition from the molecule to the solid state can be just as well beiing tested [7] as technical applications in the nano and biotechnology [2].

The goal of this research project is to increase the basic understanding in the field of the molecular magnetism as well as to undertake a push to new application areas. In addition both theoretical and experimental investigations with teams at the Ames Laboratory and at other locations are being performed presently.

In order to take into account the increasing significance of computer science in physics, at several German universities already corresponding courses of studies ("physics with informatic") were established. The here described research projects are directed to deepen the linkage of both disciplines in research and study. The following therefore shows project-descriptions, which emphasize physics, some others focus computer science.

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2. Preparation

While the "production" of magnetic molecules in chemical laboratories is running at full speed, on the physics side suitable experimental and theoretical research methods need to be developed, in order to be able to achieve a comprehensive understanding of the physical characteristics of these new molecules. In the context of my thesis [8] and in the following studies, some fundamental knowledge could be collected over real- and model Spin systems [5, 9, 10, 11]. The physical model descriptions are based on the classical Heisenberg model. Under assistence of examined and developed methods in the Heisenberg model for an optimized coupling of a heating bath to a classical spin system, it is possible in a very effective way to determine fundamental constants as for instance the dynamic spin-spin-correlation function for arbitrary temperatures and arbitrary system geometries [12]. A comparison with experimental data revealed, that for small [5], as well as for smallest systems [8] a classical description shows amazing good results. Theoretical investigations with antiferromagnetically feed back control systems have shown that at low temperatures a slow collective precession of the spin system occurs. It could be shown that this "collective behavior" can be described by a scale-law [10]. In addition first forecasts of scattering neutron experiments were delivered, however, their verification is still pending [8]. Even the classical description has their limits, one must realize that this way of describing in most cases is the only one possible at all to give statements about physical characteristics. Regarding the next already foreseeable technological steps as for instance the two and three-dimensional arrangement of a large number of molecules, this instrument provides the only possible way to gather quantitative statements. Under the classical way of describing it is possible, without any problem, to simulate, temperature-independent, some 10000 or more interacting magnetic stages in their timly dynamic. In this relationship first investigations of magnetization reversal and the dynamics of domain-walls at complex magnetic structures [8, 14] already had been accomplished. The use of a deterministic and stochastic coupled heating bath while maintaining the basic spin dynamics is thereby an excellent approach [8, 15].

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3. Research subjects


3.1 Spindynamic in magnetic molecules

In the context of the current simultaneous projects with the Universities of Osnabrück and Bielefeld and the Ames Laboratory in Iowa in particular Simulations for the determination of static characteristics such as susceptibility and specific heat are performed. The results revealed the fact that the classical description provides amazingly good conformance with the experiments and thus represents a proven instrument in this area of research [5]. Apart from the static magnetic characteristics in particular the dynamic characteristics play a large role in relation with questions of principle for the physical understanding of magnetic molecules. Fundamental magnitude in this context is the time- and temperature dependant Spin-Spin-Correlation function, which "codes" all dynamic characteristics like e.g. spin wave stimulations. With the help of this magnitude, prognosis for Neutron-Scattering and NMR-Experiments can be made. Presently actual researches being performed conjoint with Prof. Marshal Luban (Ames), Dr. Jürgen Schnack (Osnabrück), deal with the influence of magnetic fields against the static and dynamic physical characteristics of magnetic molecules. The systems tested so far could be excellent described by the Isotropic Heisenberg-Model in consideration of the Next-neighbor-interaction. The influence of further effects such as anisotropies, Dipole-Dipole-Interactions etc. received in so far little consideration and are supposed to be the subject of further research. Although the classical spin dynamics provides valuable results, it is necessary to perform quantum-mechanical computations, especially for systems whose paramagnetic ions contain a small quantity of spin quantum numbers. Corresponding assignments are established in the team of Dr. Jürgen Schnack (Osnabrück) who deal with the possibility of a heating bath connection to a quantum-mechanical system [16].

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3.2 Efficient simulation methods for thermodynamic analysis at complex Spinsystems

In the research of material, In the field of molecular-dynamic simulation methods there is a series of known and new promising approaches [8]. The experience indicated that the most efficient methods are those, which arise as a result of a combination of different techniques. These hybrid methods combine the benefits of several techniques, in order to be able to determine even more precise and faster thermodynamic characteristics. In the context of this major point of research further developments and optimizations of the existing deterministic and stochastic techniques are to performed and new techniques being developed and analyzed. Objective is to provide a collection of optimized techniques for the different problem areas. The emphasis , beside the question of physical modelling of heating bath effects, is on the side of computer-science oriented aspects: Strategies for the parallelism of techniques and new options of utilizing distributed computing are to be analyzed. As informative example the SETI@home project is mentioned [17].

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3.3 Approximate description of complex molecular arrangements

The work performed so far is essentially of a fundamental nature but nevertheless already appear applicabilities in diverse areas. First and foremost the possibility is enticing, to utilize magnetic molecules for new highly integrated electronic components. Both, suitable experimental and theoretical investigations must be executed. The prerequisites for experimental work implies in particular the development of a reproducible technology to manipulate and create test-structures e.g. regular molecular arrangements on surfaces. The measurement of these structures by raster-tunnel or raster-force-microscopy finally permits their characterization. From theoretical view the modelling of such complex structures will represent a challenge. Because of the preliminary achievements, there are very promising approaches to be progressed.

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    3.3.1 Micro magnetism in complex structures

    Micro magnetism represents a tremendous important and challenging scope within "the Computational Physics". In the context of this area of research methods are to be established in order to analyze static and dynamic micro-magnetic phenomena. Both, on the part of the basic research and from applied side (keyword: Magneto-electronics) a large interest exists in the forecast of magnetic structures (e.g. domain walls) as well as in the dynamic investigation of magnetic resetting processes. The theory of the micro magnetism represents an important link between processes on atomic scale and mesoskopic and macroscopic phenomena.

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    3.3.2 Visualization of Spindynamik-Simulations

    The scope of the ongoing assignments exposed, that the visualization of static and dynamic results crucially contributes to the understanding of the basic physics trials. So for example three-dimensional representations of the spin arrangements could supply knowledge over "the basic state" of magnetic molecules [5, 8]. In order to examine the dynamics of domain-walls at a model, the visualization is an indispensable tool to analyze micro-magnetism [8, 14]. In addition an entire set of descriptive diagrams and animations for the principles of the magnetism by means of already existing visualization routines could be generated [19]. The techniques used so far are not very effective, however on the other hand a whole string of methods about the visualization of three-dimensional data-sets can be found in the literature, which are to be examined and used for these purposes. Besides the aspects of research in particular results for the support of lectures and practical courses can be found. Suitable "tools" e.g. in the scope of a "simulation practical course" could be applied.

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3.4 Investigations to Quantum-Informatics

In the early 80's Richard Feynman realized that certain quantum-mechanical effects cannot be effectively simulated using "classical" computers [20]. This observation led to the speculation that basically calculations perhaps can be performed a lot more effective, when taking advantage of quantum effects. Recently this speculation has itself proven to be true when Peter Shor presented his quantum algorithm for the factorizing of integers [21]. Recently IBM succeded even in the experimental breakthu in quantum-informatics. Chemists at IBM developed a magnetic molecule, which contains five fluorine and two carbon ions, whose nuclear spin interact together in such a manner that they form a so-called "Qubit". By means of a NMR device, this "quantum bit" can be quasi "programmed"[22].
Over all this fascinating complex of themes covers fundamental research, both in the area of physics and computer science, however likewise provide a spectacular utilization of magnetic molecules. Both, theoretical and experimental assignments provide a doorway to this area of research.

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4. Procedure and work schedule

As starting point a fully functioning and easy to use application is already available. That allows to continue existing projects and to begin new projects in all areas of research as specified above. The described questioning allows to extract diploma and doctoral theses with varying degrees of difficulty, that for students of all courses of studies attractive possibilities of cooperation can be offered. In particular areas were computer-science is defined as a main topic, latest methods and techniques of System- and Software development will be used. The presented research topics are thematically and technically closely related. The goal is, to set-up a structured "knowledge-database" utilizing the respective physical, methodical and technical findings of each research topic in order to guarantee, that appropriate knowledge is being transferred and not lost due to the departure of staff members.

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5. International co-operation

The work specified above is performed within the scope of international co-operation. The Ames Laboratory in Iowa belongs to the leading research institutions in the USA. It is associated to the Iowa State University and complies besides its research mission corresponding instructing tasks. Based on the consisting intensive co-operation with the Ames Laboratory for many yeas, the possibility exists to exercise fundamental and application orientated diploma and doctor theses in a very open and interdisciplinary fashioned environment. A promotion and a deepening of the cultural and scientific-technical interchange with the American institutions as well as the integration into existing courses of studies in terms of abroad study semesters could represent a large enrichment for the current options to study and advance in all its internationalization.

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6. Literature

[1] D. Gatteschi, Molecular magnetism: A basis for new materials, Adv. Mater. 6 (1994) 635

[2] Molecular magnetism: From molecular assemblies to the devices, ed. by E. Coronado, P. Delhaes, D. Gatteschi, J.S. Miller, NATO ASI Series E, vol. 321, Kluwer Academic Publishing (1996)

[3] Maßgebliche Arbeiten hierzu werden von Prof. Achim Müller von der Universität Bielefeld durchgeführt (siehe auch [13]).

[4] A. Lascialfari, F. Tabak, G.L. Abbati, F. Borsa, A. Cornia, D. Gatteschi, Spin dynamics and energy gap of a Fe dimer from susceptibility and 1H NMR, J. Appl. Phys. 85 (1999) 4539

[5] A. Müller, M. Luban, C. Schröder, R. Modler, P. Kögerler, M. Axenovich, J. Schnack, P. Canfield, S. Bud’ko, N. Harrison, Classical and Quantum Magnetism in Giant Keplerate-type Magnetic Molecules, ChemPhysChem 2 (2001) 517

[6] M. Luban, F. Borsa, S. Bud'ko, P. Canfield, S. Jun, J. K. Jung, P. Kögerler, D. Mentrup, A. Müller, R. Modler, D. Procissi, B. J. Suh, M. Torikachvili, Heisenberg spin-triangles in {V6}-type magnetic molecules: Experiment and theory, Phys. Rev. B (submitted)

[7] D. Gatteschi, A. Caneschi, L. Pardi, R. Sessoli, Large clusters of metal ions: The transition from molecular to bulk magnets, Science 265 (1994) 1054

[8] C. Schröder, Numerische Simulationen zur Thermodynamik magnetischer Strukturen mittels deterministischer und stochastischer Wärmebadankopplung, Dissertation 1999, available at

[9] M. Luban, C. Schröder, Z. Jang, F. Borsa, Spin dynamics and 1H spin-lattice relaxation in the molecular magnetic ring Fe10, to be published

[10] M. Luban, C. Schröder, Collective precessional spin modes and scaling in molecular magnets, in preparation

[11] J. K. Jung, D. Procissi, R. Vincent, B. J. Suh, F. Borsa, P. Kögerler, Chr. Schröder, M. Luban, Proton NMR in the Giant Paramagnetic Molecule {Mo72Fe30}, submitted to Journal of Magnetism and Magnetic Materials

[12] C. Schröder, An adaptive heat bath coupling method for efficient constant temperature spin dynamics simulations, in preparation

[13] A. Müller, S. K. Das, E. Krickemeyer, C. Kuhlmann, Polyoxomolybdate Clusters: Giant Wheels and Balls, Inorganic Synthesis (in press)

[14] C. Schröder, V. Antropov, B. N. Harmon, Magnetization reversal of small particles, available at

[15] C. Schröder, V. P. Antropov, G. Borstel, B. N. Harmon, Application of Stochastic and Deterministic Methods to Finite Temperature Description of Extended Spin Systems, March Meeting of the American Physical Society 1998, Los Angeles, CA, USA

[16] D. Mentrup, J. Schnack, Nose-Hoover dynamics for coherent states, Physica A 297 (2001) 337-347

[17] W. T. Sullivan, III, D. Werthimer, S. Bowyer, J. Cobb, D. Gedye, D. Anderson, A new major SETI project based on Project Serendip data and 100,000 personal computers, Astronomical and Biochemical Origins and the Search for Life in the Universe, Proc. of the Fifth Intl. Conf. on Radioastronomy. 1997

[18] C. Schröder, Putting some Spin on it … A moving picture tour through the thermodynamics of classical spin systems, Multimedia contribution to the Medienfestival "Bilder aus der Physik", Göttinger Medienhaus IWF Wissen und Medien, November 2001

[19] R. Feynman, Simulating physics with computers, International Journal of Theoretical Physics 21, 6&7, (1982) 467-488

[20] P. Shor, Algorithm for quantum computation: Discrete log and factoring, Proceedings of the 35th Annual Symposium on Foundations of Computer Science (1994) 124-134

[21] see http://www.research.ibm.com/resources/news/20011219_quantum.shtml, submitted to Nature

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Copyright © 2005 - 2014 Dipl.-Ing. (FH) Thomas Hilbig, Prof. Dr. rer. nat. Christian Schröder
for the University of Applied Sciences Bielefeld - Department of Engineering Sciences and Mathematics