Single-molecule magnet

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.^[1] Contrary to conventional bulk magnets and molecule-based magnets, collective long-range magnetic ordering of magnetic moments is not necessary.^[1]

Intramolecular coupling

The magnetic coupling between the spins of the metal ions is mediated by superexchange interactions and can be described by the following isotropic Heisenberg Hamiltonian:

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.

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.^{[1] [2] [3] [4] [5]}

Blocking temperature

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.^[6] 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.^[6]

Future applications

As of 2008, there are many discovered types and potential uses.^{[7] [8]}

One possible use of SMMs is superior magnetic thin films to coat hard disks.

Types

Ferritin

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.^[9] [Mn₁₂O₁₂(OAc)₁₆(H₂O)₄]·4H₂O·2AcOH, which is called "Mn₁₂-acetate" is a common form of this used in research.^[10]

Single-molecule magnets are also based on iron clusters^[6] 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.^[11] 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.^[12] Theoretical computations showed that 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.^[13]

History

Although the term "single-molecule magnet" was first employed in 1996,^[14] the first single-molecule magnet was reported in 1991.^[15] The complex Mn₁₂O₁₂(MeCO₂)₁₆(H₂O)₄ complex (Mn₁₂Ac₁₆), first described in 1980,^[16] 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.^[17]

It was known in 2006 that the deliberate structural distortion of a Mn₆ compound by the use of a bulky salicylaldoxime derivative switches the intra-triangular magnetic exchange from antiferromagnetic to ferromagnetic, resulting in an S = 12 ground state.^[18]

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^[19] 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.

In 2011 it was reported by the University of Nottingham that a dinuclear complex of depleted uranium could be mistaken for a single-molecule magnet by chemist Stephen Liddle.^[20]

Detailed behavior

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 AC 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 or in kelvins. 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, in 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).^[21]

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 maximal 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 .^[22]

See also

• Ferromagnetism
• Single-molecule experiment
• Magnetic anisotropy

References

External links

• Molecular Magnetism Web, Jürgen Schnack