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Background

Background

The study of molecular magnetism is a relatively new addition to the research carried out at Aarhus University, but the field itself has been growing rapidly for the last two decades. It is an excellent example of a global and interdisciplinary field, involving research groups from all continents working on a huge range of topics, including: coordination chemistry; organic and organometallic synthesis; structural characterisation; magnetic measurements; electron paramagnetic resonance; inelastic neutron scattering; and ab initio and DFT calculations. Our current goal is to use our expertise in charge density measurements to find correlations between the magnetic behaviour of these molecules and their electron and/or spin density distributions. Such information could prove crucial in the search for new magnetic molecules with improved properties for technological applications (see below).

Single-molecule magnets (SMMs) are a class of compounds that can retain their magnetisation in the absence of a magnetic field. {Mn12} is a classic example of an SMM (Figure 1). The slow relaxation of magnetisation arises due to a double well in the energy levels arising from its spin states, where the two fully magnetised states (mS = +10 and mS = −10) are the ground states, but where moving between them requires the surmounting of a large energetic barrier (Figure 2). If such a system is magnetised, then it will retain its magnetisation indefinitely. Unfortunately, this effect is currently only observed at very low temperatures (below 15 K). One of the main goals of the field of molecular magnetism is to find new molecules where the barrier is sufficiently high that the effect can be observed at higher temperatures—ideally room temperature. The rational design of such systems will require a much more complete understanding of the underlying physics that cause the effect, and this is something which still remains elusive over twenty years after the first SMM was discovered.

Potential technological applications

One application of SMMs could be their use in ultra-high density information storage, since molecules are typically much smaller than the magnetic domains used in modern hard drives. Currently, the highest areal densities that can be achieved in hard disk drives is around 125 Gbit/cm2. The limiting factor in the density that can be achieved is the size of the domains used to store the bits. If we could build a surface from SMMs, where each domain is a single molecule with its total spin up or down (1 or 0), then the potential storage density would be on the order of 30 Tbit/cm2—around 250 times higher than modern commercial hard drives. Unfortunately, very low temperatures (around 15 K and below) are required for this effect to be observed in even the best current compounds, and quantum tunneling can often bypass the barrier entirely under certain conditions. In order to be able to use these molecules in portable devices we will have to raise the barrier by orders of magnitude, and this will require the isolation of entirely new systems.

A more recently proposed application for molecular magnets is as components in quantum computers. Quantum information processing (QIP) is a computational technique that is fundamentally different to the conventional processing methods used in all modern computers. Rather than logic gates built around the manipulation of binary states, QIP exploits the quantum states of matter, wherein superpositions of states are accessible and can be used to perform algorithms that are not possible with current technology. A well known example is Shor’s algorithm, which allows for the calculation of the prime factors of any given integer. All contemporary encryption techniques rely on the inability of even the most powerful supercomputers to perform such calculations quickly. Since the magnetic states in molecular nanomagnets are quantum in nature, they have been proposed as potential qubits (quantum bits) in the assembly of QIP logic gates.

A third application where magnetic molecules might find use is as magnetocaloric refrigerants. The magnetocaloric effect (MCE) is a property where the difference in entropy between the magnetised and demagnetised states of a material can be exploited to achieve very low temperatures in a cooled sample space. The benefits of these systems compared to conventional compression techniques are that they are more energy efficient and do not rely on the use of environmentally damaging gases. Systems have already been built that utilise gadolinium alloys in this way, but it might be possible to synthesise molecular species that can exhibit vastly improved cooling properties. This is a growing topic within molecular magnetism research.

In all of the above applications, improved compounds will require a full control of the properties of both the individual paramagnetic ions and the interactions between them in large clusters. In the majority of cases, these properties are measured indirectly using powder techniques and are corroborated by theoretical calculations—but much of this information should be directly accessible using charge density techniques. Our approach, therefore, is to perform electron and spin density measurements on compounds where possible correlations have been identified, but where experimental confirmation is lacking. Such studies will hugely strengthen our understanding of these systems.