Molecule-based magnets

Molecule-based magnets are materials with magnetic properties similar to those of common ferromagnets (e.g. iron, cobalt, nickel), but in which these properties originate from the electronic characteristics of the molecules that compose it.

The difference from traditional magnets is that in the latter, the magnetic behavior is based on a collective property of atoms, with non-zero spin moment, that constitute the solid, and is therefore related to the organization of a large number of atomic magnetic moments.

Molecular chemical systems exhibiting magnetic behavior can be classified into two distinct categories: molecule-based magnets and single molecule magnets (SMM). In the first case, the mechanism by which the magnetic properties are determined is not very different from that of traditional magnets, being attributable to the interactions between several spin centers and therefore to their collective behavior.

The difference is that the material is composed of molecules, with localized and directional chemical bonds, rather than atoms and ions as in common ferromagnetic crystals. Chemically, these are organic or coordination compounds.

The interesting aspect of these systems lies in the fact that they combine magnetic properties with the typical characteristics of molecules, such as the fact that they can be produced by chemical methods in solution at modest temperatures (where common magnets require metallurgical or electrochemical processes); in addition, they may have physical properties (density, optical characteristics, electrical, etc.) different from common magnets and adjustable during design and synthesis.

The Curie temperatures of these materials, however, are generally very low. In single-molecule magnets, which represent the systems that research looks at with most interest, the magnetic behavior is due to the presence of a limited number of paramagnetic atoms within the single molecule.

The molecule then manifests its magnetism on an individual scale, creating a virtually zerodimensional magnet. Such molecules, endowed with high spin momentum, have the property, once magnetized, to retain their magnetization for a long time if maintained at the temperature of a few kelvin. The type of magnetism shown by SMMs, associated with the presence of a barrier due to magnetic anisotropy, is called superparamagnetism and is manifested below a temperature called blocking temperature. The prototype of SMMs is the dodecanuclear manganese complex:

[Mn12O12(CH3COO)16(H2O)4]·4H2O·2CH3COOH

(referred to briefly as Mn12Ac), which contains 8 Mn(III) ions and 4 Mn(IV) ions and can be readily prepared by reaction of manganese acetate and potassium permanganate in acetic acid. More recently, nanoclusters containing more manganese atoms have been synthesized. Single-molecule magnets are being intensively researched in the field of molecular electronics because of their characteristics they lend themselves to function as innovative data storage and processing devices.