Molecule-based magnets

Molecule-based magnets are a class of materials capable of displaying ferromagnetism. This class expands the materials properties typically associated with magnets to include low density, transparency, electrical insulation, and low-temperature fabrication, as well as combine magnetic ordering with other properties such as photoresponsiveness. Essentially all of the common magnetic phenomena associated with conventional transition-metal and rare-earth-based magnets can be found in molecule-based magnets.^[1]

Background

Molecule-based magnets comprise a class of materials which differ from conventional magnets in one of several ways. Most traditional magnetic materials are comprised purely of metals (Fe, Co, Ni) or metal oxides (CrO₂) in which the unpaired electrons spins that contribute to the net magnetic moment reside only on metal atoms in d- or f-type orbitals.

In molecule-based magnets, the structural building blocks are molecular in nature. These building blocks are either purely organic molecules, coordination compounds or a combination of both. In this case, the unpaired electrons may reside in d or f orbitals on isolated metal atoms, but may also reside in highly localized s and p orbitals as well on the purely organic species. Like conventional magnets, they may be classified as hard or soft, depending on the magnitude of the coercive field.

Another distinguishing feature is that molecule-based magnets are prepared via low-temperature solution-based techniques, versus high-temperature metallurgical processing or electroplating (in the case of magnetic thin films). This enables a chemical tailoring of the molecular building blocks to tune the magnetic properties.

Specific materials include purely organic magnets made of organic radicals for example p-nitrophenyl nitronyl nitroxides ^[2], decamethylferrocenium tetracyanoethenide^[3], mixed coordination compounds with bridging organic radicals ^[4], Prussian blue related compounds ^[5], and charge transfer complexes ^[6].

Molecule-based magnets derive their net moment from the cooperative effect of the spin-bearing molecular entities, and can display bulk ferromagnetic and ferrimagnetic behavior with a true critical temperature. In this regard, they are contrasted with single-molecule magnets, which are essentially superparamagnets (displaying a blocking temperature versus a true critical temperature). This critical temperature represents the point at which the materials switches from a simple paramagnet to a bulk magnet, and can be detected by ac susceptibility and specific heat measurements.

History

The first synthesis and characterization of molecule-based magnets was accomplished by Wickman and co-workers. This was a diethyldithiocarbamate-Fe(III) chloride compound.^[7][8]

Theory

The mechanism by which molecule-based magnets stabilize and display a net magnetic moment is quite different than that present in traditional metal- and ceramic-based magnets. For metallic magnets, the unpaired electrons align through quantum mechanical effects (termed exchange) by virtue of the way in which the electrons fill the orbitals of the conductive band. For most oxide-based ceramic magnets, the unpaired electrons on the metal centers align via the intervening diamagnetic bridging oxide (termed superexchange). The magnetic moment in molecule-based magnets is typically stabilized by one or more of three main mechanisms:

• Through space or dipolar coupling
• Exchange between orthogonal (non-overlapping) orbitals in the same spatial region
• Net moment via antiferromagnetic coupling of non-equal spin centers (ferrimagnetism)

In general, molecule-based magnets tend to be of low dimensionality. Classic magnetic alloys based on iron and other ferromangetic materials feature metallic bonding, with all atoms essentially bonded to all nearest neighbors in the crystal lattice. Thus, critical temperatures at which point these classical magnets cross over to the ordered magnetic state tend to be high, since interactions between spin centers is strong. Molecule-based magnets, however, have spin bearing units on molecular entities, often with highly directional bonding. In some cases, chemical bonding is restricted to one dimension (chains). Thus, interactions between spin centers are also limited to one-dimension, and ordering temperatures are much lower than metal/alloy-type magnets. Also, large parts of the magnetic material are essentially diamagnetic, and contribute nothing to the net magnetic moment.

These aspects of molecule-based magnets present significant challenges toward reaching the ultimate goal of "room temperature" molecule-based magnets. Low-dimensional materials, however, can provide valuable experimental data for validating physics models of magnetism (which are often of low dimension, to simplify calculations).

Applications

Molecule-based magnets currently remain laboratory curiosities with no real world applications. As indicated, this is largely due to the very low critical temperature at which these materials become magnetic. This is related to the magnitude of the magnetic coupling, which is very weak in these materials. In this regard, they are similar to superconductors, which require cooling for use.

References

1. ^ Molecule-Based Magnets Materials Research Society Retrieved on 20 December 2007
2. ^ Bulk ferromagnetism in the β-phase crystal of the p-nitrophenyl nitronyl nitroxide radical Chemical Physics Letters, Volume 186, Issues 4-5, 15 November 1991, Pages 401-404 Masafumi Tamura, Yasuhiro Nakazawa, Daisuke Shiomi, Kiyokazu Nozawa, Yuko Hosokoshi, Masayasu Ishikawa, Minuro Takahashi, Minoru Kinoshita doi:10.1016/0009-2614(91)90198-I
3. ^ Sailesh Chittipeddi K. R. Cromack Joel S. Miller A. J. Epstein Phys. Rev. Lett. 58, 2695–2698 (1987) Ferromagnetism in molecular decamethylferrocenium tetracyanoethenide (DMeFc TCNE)
4. ^ Caneschi A., et al. Acc. Chem. Res. 22, 392 (1989)
5. ^ S. Ferlay, et al. Nature 378, 701 (1995)
6. ^ Miller J.S., et al. Chem. Rev. 88, 201 (1988)
7. ^ Wickman, H.H., et al. Phys. Rev. 155, 563 (1967).
8. ^ Wickman, H.H., et al. Phys. Rev. 163, 526 (1967).