New Phenomena and Opportunities in Molecule-Based Magnets
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Opportunities in Molecule-Based Magnets Arthur J. Epstein
Introduction Molecule-based magnets display the full range of phenomena observed in conventional transition-metal and rareearth-based magnets, as well as unusual phenomena unique to themselves. These materials form in a wide range of one-, two-, and three-dimensional network structures, with disorder often playing a substantial role. The first molecule-based magnets with spins in p orbitals had ordering temperatures of less than 5 K.1 Now, systems that order at temperatures above 350 K are known.2 As outlined in the introductory article in this issue, molecule-based magnets include materials with spins only in organic moieties (in p orbitals), materials with spins both on metal ions and organic moieties (in p orbitals), and those materials with spins on metal ions with the exchange pathway provided by organic moieties that do not contain spin. The introductory article also outlines the three origins of ferromagnetic exchange, orthogonality of orbitals, configuration interaction, and dipole–dipole interaction.
Ferromagnetism in MoleculeBased Magnets: Consequences of Dimensionality and Disorder The development of magnetic order depends upon the strength of magnetic coupling between the adjacent spincontaining groups. The structural bonding motif and the shape of the molecular orbitals in which the unpaired spins reside determine the dimensionality of the magnetic interactions. This article begins with two examples of ferromagnetic order that illustrate the role of structural and bonding motifs in determining the magnetic
MRS BULLETIN/NOVEMBER 2000
ordering temperatures and the means by which the spins develop local order as temperature is reduced toward the ordering, or critical, temperature Tc . Spins that are coupled only to nearest neighbors along a chain (a common structural motif in molecule-based magnets) cannot obtain long-range order at finite temperatures because the increase in entropy at finite temperatures introduces disorder in the alignment of spins along an isolated chain. The weak magnetic coupling between chains is essential for the development of long-range magnetic order at finite temperatures. A signature of the role of dimensionality is the magnetic specific heat, which has a maximum at the magnetic ordering temperature. For materials whose exchange J is nearly the same in all three directions, magnetic order is only short-range until the material is cooled relatively close to Tc . The magnetic order develops in all three directions at the same rate (if J is isotropic), and there is a large specific-heat peak with a clear maximum at Tc with little entropy of ordering above Tc. This contrasts with quasi1D materials, which develop relatively long-range magnetic order along the chains above Tc , with little magnetic order between chains until very close to Tc . The specific heat has only a very weak maximum at Tc for quasi-1D materials. Twodimensional systems (those that have magnetic interactions only within a plane) are intermediate in
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