Single-molecule magnet

A single-molecule magnet (SMM) is an object that is composed of molecules each of which behaves as an individual superparamagnet. This is distinct from a molecule-based magnet, in which a group of molecules behave collectively as a magnet.

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The requisites for such a system are:

• a high spin ground state
• a high zero-field-splitting (due to high magnetic anisotropy)
• have negligible magnetic interaction between molecules.

The combination of these properties can lead to an energy barrier, so that, at low temperatures, the system can be trapped in one of the high-spin energy wells.

Molecular magnets exhibit an increasing product (magnetic susceptibility times temperature) with decreasing temperature, and can be characterized by a shift both in position and intensity of the a.c. magnetic susceptibility.

Single-molecule magnets represent a molecular approach to nanomagnets (nanoscale magnetic particles). In addition, single-molecule magnets have provided physicists with useful test-bed for the study of quantum mechanics. Macroscopic quantum tunneling of the magnetization was first observed in Mn₁₂O₁₂ characterized by evenly spaced steps in the hysteresis curve. The periodic quenching of this tunneling rate in the compound Fe8 has been observed and explained with geometric phases.

Although the term single-molecule magnet was first employed by David Hendrickson, a chemist at the University of California, San Diego and coworkers in 1996 ^[1], the first single-molecule magnet reported dates back to 1991. ^[2] The European researchers discovered that a Mn₁₂O₁₂(MeCO₂)₁₆(H₂O)₄ complex (MnAc12) first synthesized in 1980 ^[3] exhibits slow relaxation of the magnetization at low temperatures. This manganese oxide compound is comprised of a central Mn(IV)₄O₄ cube surrounded by a ring of 8 Mn(III) units connected through bridging oxo ligands. In addition, it has 16 acetate and 4 water ligands ^[4].

Due to the typically large, bi-stable spin anisotropy, single-molecule magnets promise the realization of perhaps the smallest practical unit for magnetic memory, and thus are possible building blocks for a quantum computer. Consequently, many groups have devoted great efforts into synthesis of additional single molecule magnets; however, the Mn₁₂O₁₂ complex and analogous complexes remain the canonical single molecule magnet with a 50 cm^-1 spin anisotropy.

The spin anisotropy is manifests itself as an energy barrier that spins have to overcome when they switch from parallel alignment to antiparallel alignment. This barrier (U) is defined as:

where S is the dimensionless total spin state and D the zero-field splitting parameter (in cm^-1). D can be negative but only its absolute value is considered in the equation. The barrier U is generally reported in cm^-1 units or in units of Kelvin (see: electronvolt). The higher the barrier the longer a material remains magnetized and a high barrier is obtained when the molecule contains many unpaired electrons and when its zero field splitting value is large. For example the MnAc12 cluster the spin state is 10 (involving 20 unpaired electrons) and D = -0.5 cm^-1 resulting in a barrier of 50 cm^-1 (equivalent to 60 Kelvin).

The effect is also observed by Hysteresis experienced when magnetization is measured in a magnetic field sweep: on lowering the magnetic field again after reaching the maximum magnetization the magnetization remains at high levels and it requires a reversed field to bring magnetization back to zero.

Measurements takes place at very low temperatures. The so-called blocking temperature is defined as the temperature below which the relaxation of the magnetisation becomes slow compared to the time scale of a particular investigation technique ^[5]. A molecule magnetised at 2 K will keep 40% of its magnetisation after 2 months and by lowering the temperature to 1.5 K this will take 40 years ^[5].

Single-molecule magnets are also based on iron clusters ^[5] because they potentially have large spin states. In addition the biomolecule ferritin is also considered a nanomagnet. In the cluster Fe8Br the cation Fe8 stands for [Fe₈O₂(OH)₁₂(tacn)₆]⁸⁺ with tacn representing 1,4,7-triazacyclononane.

A record magnetization was reported in 2007 for a compound related to MnAc12 ([Mn(III) ₆O₂(sao)₆(O2CPh)₂(EtOH)₄]) with S = 12, D = -0.43cm^-1 and hence U = 62 cm^-1 or 86 K ^[6] at a blocking temperature of 4.3 K. This was accomplished by replacing acetate ligands by the bulkier salicylaldoxime thus distorting the manganese ligand sphere. It is prepared by mixing the perchlorate of manganese, the sodium salt of benzoic acid, a salicylaldoxime derivate and tetramethylammonium hydroxide in water and collecting the filtrate.

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