Trigonal bipyramidal spin clusters with orbitally degenerate 5d cyano complexes [OsIII(CN)6]3−, prototypes of high-temperature single-molecule magnets

Magnets are widely used in a large number of applications, and their market is larger than that of semiconductors. Information storage is certainly one of the most important uses of magnets, and the lower limit to the size of the memory elements is provided by the superparamagnetic size, below which information cannot be permanently stored because the magnetization freely fluctuates. This occurs at room temperature for particles in the range of 10–100 nm, owing to the nature of the material. However, even smaller particles can in principle be used either by working at lower temperatures or by taking advantage of the onset of quantum size effects, which can make nanomagnets candidates for the construction of quantum computers.

Clusters of metal ions are a class of compounds actively investigated for their magnetic properties, which should gradually change from those of simple paramagnets to those of bulk magnets. However, their interest lies in a number of different disciplines: chemistry, which seeks new synthetic strategies to make larger and larger clusters in a controlled manner; physics, which can test the validity of quantum mechanical approaches at the nanometer scale; and biology, which can use them as models of biomineralization of magnetic particles.

Molecules comprising a large number of coupled paramagnetic centers are attracting much interest because they may show properties which are intermediate between those of simple paramagnets and classical bulk magnets and provide unambiguous evidence of quantum size effects in magnets. To date, two cluster families, usually referred to as Mn12 and Fe8, have been used to test theories. However, it is reasonable to predict that other classes of molecules will be discovered which have similar or superior properties. To do this it is necessary that synthetic chemists have a good understanding of the correlation between the structure and properties of the molecules, for this it is necessary that concepts such as quantum tunneling, quantum coherence, quantum oscillations are understood. The goal of this article is to review the fundamental concepts needed to understand quantum size effects in molecular magnets and to critically report what has been done in the field to date.