Single-molecule magnet

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A single-molecule magnet is an object that is composed of molecules each of which behaves as a magnet. To be a single-molecule magnet, the object must show a net magnetic spin and have no magnetic interaction between molecules. Single-molecule magnets represent a molecular approach to nanomagnets (nanoscale magnetic particles).

Although the term single-molecule magnet was first employed by David Hendrickson, a chemist at the University of California, San Diego and coworkers in 1996 (J. Am. Chem. Soc. 1996, 118, 7746-7754), the first single-molecule magnet reported dates back to 1991. (A. Caneschi et al. in J. Am. Chem. Soc. 1991, 113(15), 5873-5874) The european researchers discovered that a Mn₁₂O₁₂(MeCO₂)₁₆(H₂O)₄ complex first synthesized in 1980 exhibits slow relaxation of the magnetization at low temperatures. Since then, many groups have placed many resources and time into synthesis of additional single molecule magnets and a fundamental study dates 1993: J. Am. Chem. Soc. 1993, 115(5), 1804-1816.; however, this Mn₁₂O₁₂ complex and analogous complexes remain the most effective single molecule magnet with a staggering 50 cm^-1 as the spin barrier, U. Single-molecule magnets have been realized only at very low critical temperatures, below which the compound exhibits ferromagnetism, antiferromagnetism, or ferrimagnetism.

If an object that shows single-molecule magnet behavior at room temperature is realized, it will be utilized for molecular devices. Molecular devices usually work at room temperature, so a key issue is how to raise the critical temperature of single-molecule magnets.

A compound which shows a high critical temperature exhibits high S (spin quantum number) and high negative D (zero-field splitting parameter). To make a compound exhibiting high S and high negative D, many metal ions having spins must be assembled, their interactions must be ferromagnetic or ferrimagnetic, and the spins must be in anisotropic conditions.

Many researchers are making efforts to raise the critical temperature of single-molecule magnets and to make compounds which show single-molecule magnet behavior at room temperature. The metal mainly used in SMMs is manganese, and the Christou (University of Florida) and Winpenny (University of Manchester) groups are possibly the two main contributors to the growing list of single-molecule magnets.