Single-molecule magnets (SMM) are a class of metalorganic compounds, that show superparamagnetic behavior below a certain blocking temperature at the molecular scale. In this temperature range, SMMs exhibit magnetic hysteresis of purely molecular origin. Contrary to conventional bulk magnets and molecule-based magnets, collective long-range magnetic ordering of magnetic moments is not necessary.
where is the coupling constant between spin i (operator ) and spin j (operator ). For positive J the coupling is called ferromagnetic (parallel alignment of spins) and for negative J the coupling is called antiferromagnetic (antiparallel alignment of spins).
- a high spin ground state,
- a high zero-field-splitting (due to high magnetic anisotropy), and
- negligible magnetic interaction between molecules.
Measurements take 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. 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.
The archetype of single-molecule magnets is called "Mn12". It is a polymetallic manganese (Mn) complex having the formula [Mn12O12(OAc)16(H2O)4], where OAc stands for acetate. It has the remarkable property of showing an extremely slow relaxation of their magnetization below a blocking temperature. [Mn12O12(OAc)16(H2O)4]·4H2O·2AcOH which is called "Mn12-acetate" is a common form of this used in research. 
Single-molecule magnets are also based on iron clusters 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.
The ferrous cube complex Fe4C40H52N4O12 (commonly called [Fe4(sae)4(MeOH)4]) was the first example of a single-molecule magnet involving an Fe(II) cluster, and the core of this complex is a slightly distorted cube with Fe and O atoms on alternating corners. Remarkably, this single molecule magnet exhibits non-collinear magnetism in which the atomic spin moments of the four Fe atoms point in opposite directions along two nearly perpendicular axes. Theoretical computations showed approximately two magnetic electrons are localized on each Fe atom with the other atoms being nearly nonmagnetic, and the spin-orbit coupling potential energy surface has three local energy minima with a magnetic anisotropy barrier just below 3 meV.
Although the term "single-molecule magnet" was first employed in 1996, the first single-molecule magnet was reported in 1991. The complex Mn12O12(MeCO2)16(H2O)4 complex (Mn12Ac16), first described in 1980, exhibits slow relaxation of the magnetization at low temperatures. This manganese oxide compound features a central Mn(IV)4O4 cube surrounded by a ring of 8 Mn(III) units connected through bridging oxo ligands.
It was known in 2006 that the "deliberate structural distortion of a Mn6 compound via the use of a bulky salicylaldoxime derivative switches the intra-triangular magnetic exchange from antiferromagnetic to ferromagnetic resulting in an S = 12 ground state.
A record magnetization was reported in 2007 for [Mn(III) 6O2(sao)6(O2CPh)2(EtOH)4], with S = 12, D = -0.43 cm−1 and hence U = 62 cm−1 or 86 K at a blocking temperature of 4.3 K. This was accomplished by replacing acetate ligands (OAc) 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.
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-beds 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.
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 manifests itself as an energy barrier that spins must 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 "Mn(OAc)12" 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 K).
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.
Recently, it has been reported that the energy barrier, U, is slightly dependent on Mn12 crystal size/morphology, as well as the magnetization relaxation times, which varies as function of particle size and size distributions .
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