Single-Molecule Magnets
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Single-Molecule Magnets
George Christou, Dante Gatteschi, David N. Hendrickson, and Roberta Sessoli Introduction 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. An important point is that the properties of magnetic particles scale exponentially, and therefore either it must be possible to address individual particles, or ensembles of absolutely identical particles must be available. This has presented a formidable challenge, but an attractive potential solution is provided by the recent realization that molecules containing several transition-metal ions can exhibit properties similar to nanoscale magnetic particles (nanomagnets). For this reason, such polynuclear metal complexes exhibiting superparamagnetic-like properties have been called single-molecule magnets (SMMs).1 An overview of ongoing SMM research is presented in this review, with the goal of describing the factors determining their magnetic properties and giving some indication of possible applications.
Origin of Single-Molecule Magnetism In 1993, the first SMM, the compound [Mn 1 2 O 1 2 (O 2 CCH 3 ) 1 6 (H 2 O) 4 ] · 4H 2 O · 2CH3CO2H (Complex 1), was discovered.2–4 A drawing of the Mn12O12 core of Complex 1 is shown in Figure 1. Variablefield magnetization and high-frequency electron paramagnetic resonance (HFEPR)
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data indicate that 1 has an S 10 ground state. The large spin ground state arises from antiferromagnetic interactions between the S 3/2 spins of MnIV ions and the S 2 spins of MnIII ions, which do not compensate. An axial zero-field splitting is present, and this leads to a splitting of the S 10 state into 21 levels, each characterized by a spin projection quantum number, ms , where S ms S. Each level has an energy given as E(ms) ms2D, where for 1 it has been found that the axial zerofield splitting parameter D 0.50 cm1 ( 0.70 K). The negative sign of D leads to a potential-energy barrier between the “spin-up” (ms 10) and “spin-down” (ms 10) orientations of the magnetic moment of an individual Mn12 molecule (Figure 2). In other words, in order to flip the spin of a Mn12 molecule from along the z axis (Figure 1) to along the z axis of the disc-like Mn12O12 core, it takes some energy (the barrier in Figure 2) to reorient the spin
Figure 1. Drawing of the [Mn IV4Mn III8(-O)12 ] 16 core of [Mn12O12(O2CCH3)16(H2O)4 ]·4H2O· 2CH3CO2H (C
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