Effect of High Pressure on Single Molecule Magnets

Single molecule magnets (SMMs) are molecules, which, like bulk magnets, are able to retain magnetisation following exposure to a magnetic field. The magnetic properties of SMMs occur at low-temperature, and there is great interest in making new SMMs which work at higher temperatures. In order to design new SMMs we need to understand how structure affects a molecule’s magnetic properties. SMMs can be twisted and distorted by applying high pressure, something what can be measured using X-ray crystallography. Parallel magnetic measurements allow us to determine what affect these distortions have on magnetic properties. These experiments provide the most direct link between structure and magnetism.

Single molecule magnets (SMMs) consist of a core containing metal ions connected by oxygen and nitrogen atoms derived from organic ligand molecules. SMMs are nano-magnets, much smaller than the oxide particles currently used in magnetic tapes and hard-disks. They could enable the highest possible density of information storage. SMMs are of intense scientific interest, but the property of greatest importance is the energy needed to reorient the molecular magnetic moment, conceptually the energy cost of swapping the north and south poles of a bar magnet. This energy depends on the distances between the metals, and the angles subtended at the atoms which link the metals together.
Our idea was simple: could we use pressure to alter magnetic properties of SMMs by changing the geometry around the metal ions? We envisaged that we could push the metals closer together, or change the angles at bridging groups. It gives us great satisfaction to report that this exactly what we have been able to do.
We have studied SMMs up to 10 GPa (100 000 atm), but it was clear early-on in the project that effects can be seen at pressures much less than this. For example the largest energy barrier of any SMM (in an Mn6 cluster in which the Mn-atoms are linked through N and O atoms) was manipulated using pressure of only 1.5 GPa. In Mn12-acetate derivatives (the most famous class of SMM) pressure can alter the orientations of long Mn-ligand bonds, with a profound effect on the magnetism.
The results of this project are significant in ways that go beyond the area of single molecule magnetism. The effect of pressure on the Mn6 complexes can be explained by changes the Mn-N-O-Mn torsion angles. Study of numerous different Mn6 derivatives had pointed to the importance of this torsion angle, but the high-pressure experiments enabled it to be tuned within the same complex without the complicating issues of chemical modification. This shows that that high-pressure can be used to probe the relationship of magnetism and structure, a conclusion relevant to all magnetic materials, not just SMMs.
Our results are also significant because they have begun to explore metal chemistry at giga-pascal pressures. In one example a unique step-wise polymerisation was observed in a copper citrate complex, where in the second step (at 4 GPa) the number of atoms bound to the copper atoms (the coordination number) decreases. This observation appears to contradict the general trend for increased coordination numbers at high pressures, though it can be understood in terms of the behaviour of other Cu-O bonds in the complex.
We have also made great progress in high-pressure technology. Cells for measuring magnetism at up to 3 GPa and at temperatures near absolute zero have been designed, built and used in this project; experience in this area enabled us to develop a unique pressure cell for diffraction which can be used with an open-flow low-temperature device. Our crystal structures are the largest for which high-pressure, atomic-resolution structural data have ever been obtained, and we made frequent use of Station 9.8 at the Daresbury Synchrotron Source. Over the course of this grant this station became the best anywhere in the World for high-pressure chemical crystallography. This was achieved by developing new pressure cells and data collection and processing strategies to yield data as good and as rapidly as conventional experiments at ambient pressure.
Advances made on Station 9.8 have been transferred to I19 at Diamond. Software for data processing developed for I19 has also been found to be useful for ambient pressure data-sets and is being used by the synchrotron arm of the EPSRC National Crystallography Service, demonstrating again that addressing the challenges of high-pressure bring tangible benefits beyond the immediate area of extreme conditions research.