Definitions

# Single-molecule magnet

A single-molecule magnet or 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.

In 2004 it was said that: "Single-molecule magnets (SMMs) are large molecules consisting of several transition metal ions coupled through oxygens and surrounded by various ligands.

As of 2008 there are many discovered types and potential uses. "Single molecule magnets (SMM) are a class of molecules exhibiting magnetic properties similar to those observed in conventional bulk magnets, but of molecular origin. SMMs have been proposed as potential candidates for several technological applications that require highly controlled thin films and patterns. "The ability of a single molecule to behave like a tiny magnet (single molecular magnets, SMMs) has seen a rapid growth in research over the last few years. SMMs represent the smallest possible magnetic devices and are a controllable, bottom-up approach to nanoscale magnetism. Potential applications of SMMs include quantum computing, high-density information storage and magnetic refrigeration.

## Properties and uses

A single molecule magnet is "a molecule that shows slow relaxation of the magnetization of purely molecular origin." "It is a molecule that can be magnetized in a magnetic field, and that will remain magnetized even after switching off the magnetic field. This is a property of the molecule itself. No interaction between the molecules is necessary for this phenomenon to occur.This makes single molecule magnets fundamentally different from traditional bulk magnets. You can dissolve a single molecule magnet in a solvent or put it in some other matrix, like a polymer, and it will still show this property."

The requisites for such a system are:

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.

"These molecules contain a finite number of interacting spin centers (e.g. paramagnetic ions) and thus provide ideal opportunities to study basic concepts of magnetism. Some of them possess magnetic ground states and give rise to hysteresis effects and metastable magnetic phases. They may show quantum tunneling of the magnetization which raises the question of coherent dynamics in such systems. Other types of molecules exhibit pronounced frustration effects, whereas so-called spin crossover substances can switch their magnetic ground state and related properties such as color under irradiation of laser light, pressure or heat. Scientists from various fields - chemistry, physics; theory and experiment - have joined the research on molecular magnetism in order to explore the unprecedented properties of these new compounds.

"Single-molecule magnets (SMMs) have many important advantages over conventional nanoscale magnetic particles composed of metals, metal alloys or metal oxides. These advantages include uniform size, solubility in organic solvents, and readily alterable peripheral ligands, among others.

"'As far as applications go, some academics are working to deposit Mn12 clusters on surfaces, but that too is not very advanced,' Christou says. 'We have been avoiding putting Mn12 on surfaces in our lab because two dimensions might not be the future of information storage,' he notes. 'A lot of us believe the future of SMMs and information storage is going to be three-dimensional. And Mn12 is probably not going to be the future of SMMs either. It's the best at the moment, but we need better compounds.'

"A single molecule magnet is an example of a macroscopic quantum system. [...] If we could detect spin flips in a single atom or molecule, we could use the spin to store information. This would enable us to increase the storage capacity of computer hard disks. [...] A good starting point for trying to detect spin flips is to find a molecule with a spin of several Bohr magnetons. [An electron has an intrinsic magnetic dipole moment of approximately one Bohr magneton.] There is a very well studied molecular magnet, Mn12-acetate, which has a spin S = 10 (Figure 3). This molecule is a disc-shaped organic molecule in which twelve Mn ions are embedded. Eight of these form a ring, each having a charge of +3 and a spin S = 2. The other four form a tetrahedron, each having a charge of +4 and a spin S = 3/2. The exchange interactions within the molecule are such that the spins of the ring align themselves in opposition to the spins of the tetrahedron, giving the molecule a total net spin S = 10.

## Types

The archetype of single-molecule magnets is called "Mn12". It is a polymetallic manganese (Mn) complex having the formula [Mn12O12(OAc)16(H2O)4]. 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.

"Mn4" is another researched type single-molecule magnet. Three of these are:

• [Mn4(hmp)6(NO3)2(MeCN)2](ClO4)2·2MeCN (3),
• [Mn4(hmp)6(NO3)4]·(MeCN) (4), and
• [Mn4(hmp)4(acac)2(MeO)2](ClO4)2·2MeOH (5).

In each of these Mn4 complexes "there is a planar diamond core of MnIII 2MnII 2 ions. An analysis of the variable-temperature and variable-field magnetization data indicate that all three molecules have intramolecular ferromagnetic coupling and a S = 9 ground state. The presence of a frequency-dependent alternating current susceptibility signal indicates a significant energy barrier between the spin-up and spin-down states for each of these three MnIII 2MnII 2."

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.

## History

Although the term "single-molecule magnet" was first employed by David Hendrickson, a chemist at the University of California, San Diego and George Christou (Indiana University) in 1996, the first single-molecule magnet reported dates back to 1991. The European researchers discovered that a Mn12O12(MeCO2)16(H2O)4 complex (Mn12Ac16) first synthesized in 1980 exhibits slow relaxation of the magnetization at low temperatures. This manganese oxide compound is composed 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.

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 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 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.

## Blocking temperature

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. 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.

## 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 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:

$U = S^2|D|,$

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.

Recently, it has been 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 .