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).
Read more about this topic: Molecule-based Magnets
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