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Single-molecule magnets or SMMs are a class of metalorganic compounds, that show superparamagnetic behavior below a certain blocking temperature at the molecular scale. In this temperature range, SMMs exhibit magnetic hysteresis of purely molecular origin.〔(Introduction to Molecular Magnetism by Dr. Joris van Slageren )〕 Contrary to conventional bulk magnets and molecule-based magnets, collective long-range magnetic ordering of magnetic moments is not necessary.〔 ==Intramolecular coupling== The magnetic coupling between the spins of the metal ions is mediated via superexchange interactions and can be described by the following isotropic Heisenberg Hamiltonian: : where is the coupling constant between spin i (operator ) and spin j (operator ). For positive J the coupling is called ferromagnetic (parallel alignment of spins) and for negative J the coupling is called antiferromagnetic (antiparallel alignment of spins). * a high spin ground state, * a high zero-field-splitting (due to high magnetic anisotropy), and * negligible magnetic interaction between molecules. The combination of these properties can lead to an energy barrier so that, at low temperatures, the system can be trapped in one of the high-spin energy wells.〔 "These molecules contain a finite number of interacting spin centers (e.g. paramagnetic ions) and thus provide ideal opportunities to study basic concepts of magnetism. Some of them possess magnetic ground states and give rise to hysteresis effects and metastable magnetic phases. They may show quantum tunneling of the magnetization which raises the question of coherent dynamics in such systems. Other types of molecules exhibit pronounced frustration effects,〔(Frustrated Magnets ), Leibniz Institute for Solid State and Materials Research, Dresden, Germany〕 whereas so-called spin crossover substances can switch their magnetic ground state and related properties such as color under irradiation of laser light, pressure or heat. Scientists from various fields – chemistry, physics; theory and experiment – have joined the research on molecular magnetism in order to explore the unprecedented properties of these new compounds."〔(Molecular Magnetism Web ) Introduction page〕 "Single-molecule magnets (SMMs) have many important advantages over conventional nanoscale magnetic particles composed of metals, metal alloys or metal oxides. These advantages include uniform size, solubility in organic solvents, and readily alterable peripheral ligands, among others."〔(ScienceDaily (Mar. 27, 2000) ) article ''Several New Single-Molecule Magnets Discovered''〕 "A single molecule magnet is an example of a macroscopic quantum system. () If we could detect spin flips in a single atom or molecule, we could use the spin to store information. This would enable us to increase the storage capacity of computer hard disks. () A good starting point for trying to detect spin flips is to find a molecule with a spin of several Bohr magnetons. (electron has an intrinsic magnetic dipole moment of approximately one Bohr magneton. ) There is a very well studied molecular magnet, Mn12-acetate, which has a spin S = 10 (Figure 3). This molecule is a disc-shaped organic molecule in which twelve Mn ions are embedded. Eight of these form a ring, each having a charge of +3 and a spin S = 2. The other four form a tetrahedron, each having a charge of +4 and a spin S = 3/2. The exchange interactions within the molecule are such that the spins of the ring align themselves in opposition to the spins of the tetrahedron, giving the molecule a total net spin S = 10."〔(National Physical Laboratory (UK) ) Home > Science + Technology > Quantum Phenomena > Nanophysics > Research – article ''Molecular Magnets''〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Single-molecule magnet」の詳細全文を読む スポンサード リンク
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