Magnetic Nanostructures by Molecular Design: Synthetic Chemistry in Materials Research
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Magnetic Nanostructures by Molecular Design: Synthetic Chemistry in Materials Research Jenn L. H. Sandifer and George Christou Department of Chemistry, University of Florida, Gainesville, FL 32611, U.S.A. ABSTRACT In the field of nanostructured magnetic materials, the top-down approach of making eversmaller magnetic particles has been successful in decreasing the minimum size of magnetic nanoparticles made in this way. Simultaneously, synthetic chemists have employed a "bottomup" approach to the same problem through discovery of increasingly large molecular magnetic clusters that behave as individual magnets at low temperature. These molecules are called singlemolecule magnets, and they have been discovered in a variety of sizes, up to 4 nanometers in diameter. Some display interesting solid-state properties in the crystalline state: bistability through hydrogen bonding or crystallization in potentially useful patterns, such as tubes. The advantage of discrete molecules as nanoscale magnets is the monodispersity of particle size. As the molecular approach and the materials approach to nanoscale magnets brings the possible sizes of particles by each method within range of the other, interdisciplinary research into the complementarity of the two approaches becomes necessary. The magnetism displayed by nanoparticles and molecules alike at this size shows both classical and quantum properties, uniting the two types of production as a window into this area bridging the two major ways of describing the physical world. A discussion of the progress of the synthetic chemistry approach and physics in this area will be made, as well as a discussion of how molecular chemistry may fit in to the field of magnetic nanostructures. INTRODUCTION The trend towards miniaturization in magnetic nanostructure design has been accompanied by a trend in synthetic inorganic chemistry towards larger and larger molecular magnets. The ability of individual molecules to act as independent magnets was discovered in the early 1990s [1] and has been followed by years of synthetic work by chemists attempting to create new and interesting single-molecule magnets (SMMs) and characterization and theoretical work by physicists studying the unique magnetic properties of these molecules. The result is a diverse collection of magnetic nanostructures with a wide range of structural and magnetic properties. Some of the synthetic strategies used and recent highlights of research in this field will be discussed herein. THEORY A single-molecule magnet is a molecule with a large spin ground state and a significant barrier to the reversal of the magnetization direction (see figure 1), resulting in a blocking temperature, TB, below which relaxation of the magnetization occurs very slowly. The magnitude of this energy barrier to the reversal of magnetization direction is governed by two parameters: the ground state spin of the complex, S, and the easy-axis anisotropy, D. Figure 1 depicts the energies of the ms states in the S = 10 complex Mn12O12(O2CCH3
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