Single-molecule magnetism in lanthanide organometallics

Molecules that have a magnetic memory are called single-molecule magnets (SMMs). In terms of their size and composition, SMMs have dimensions of a few nanometres and they consist of one or more metal atoms bonded to a group of non-metal atoms called a ligand. The interactions between neighbouring molecules are very weak, meaning that the magnetic properties of an SMM genuinely arise from within individual molecules.

In stark contrast to SMMs, traditional 'bar' magnets used in everyday appliances are purely inorganic materials such as metal oxides or simply magnetic elements. A particularly important example of their application is in computer hard disk drives. In terms of their size and composition, rather than consisting of molecules traditional magnets feature much larger magnetic domains.

Because one of the main differences between SMMs and traditional magnets relates to size, it is possible that SMMs represent the ultimate size limit for magnetic information storage. The properties of SMMs may one day allow them to be developed for use in quantum computers. A problem with SMMs is, however, that their magnetic memories function at temperatures of about -250oC, which can only be reached by cooling with liquid helium and is therefore impractical. Furthermore, the mechanisms by which SMMs relax their magnetization (i.e. the magnetic information is lost or 'wiped') are not clear, but it is likely that gaining an understanding of these processes will lead to enhanced performance at higher temperatures.

We propose a new family of SMMs based on the lanthanide elements (Ln-SMMs). The lanthanides offer considerable potential for developing SMMs because these elements have particularly appealing magnetic properties. Ultimately, our Ln-SMMs will have magnetic memory effects observable above -196oC, which will be a major advance because this temperature can be reached by cooling with liquid nitrogen, a cryogen that is much cheaper than liquid helium, and easier to use.

We will achieve our aims by using a molecular design tool available to us as synthetic chemists: we can make significant changes to the ways in which our ligands interact with our choice of lanthanide. This is important because the non-metal atoms used to interact with the lanthanides, and the symmetry with which the ligands are arranged around the lanthanides, allow us to influence the magnetism.

A unique aspect of our approach to the design of Ln-SMMs is that our synthetic method gives access to an extremely broad range of chemical environments. Existing, conventional Ln-SMMs are almost entirely limited to ligands in which oxygen or nitrogen interacts with the lanthanide, however we can influence the magnetism using carbon, oxygen, sulphur, selenium, tellurium, nitrogen, phosphorus, arsenic, antimony or the halogens.

By understanding the ways in which the different chemical environments influence the molecular magnetism we will be able to identify the optimum conditions for producing Ln-SMMs that function at unprecedentedly high temperatures.

Modern global society cannot function without the ability to create, analyse, store and retrieve large amounts of electronic data. There is a pressing need to develop technology that can process ever-larger amounts of data at faster speeds, and to find new ways of storing that data in more effective, miniaturized ways. Modern computer hard-disk drives use long-range magnetic ordering in bulk materials to carry out their read-write function. The unique magnetic properties of certain lanthanides are used widely in this technology, for example the neodymium-iron-boride magnets and the samarium-cobalt magnets.

Single-molecule magnets (SMMs) are individual molecules with a magnetic memory. The difference between bulk magnets and SMMs is most obviously manifested in their size: bulk magnets consist of particles with dimensions of a few micrometres, whereas SMMs have dimensions on the nanometre scale, and hence as magnetic objects they are more than a thousand times smaller. In contrast to bulk magnets, whose magnetic memories originate from long-range ordering across particles, the magnetic memory in SMMs originates entirely from within individual molecules.

A property of SMMs that introduces potential for economic impact is that their magnetic memory could represent the smallest usable data storage unit. SMMs have been proposed for applications in quantum computers, i.e. computers that can operate at extremely rapid speeds, ultimately reducing the time taken to solve problems from a few years to a few seconds.

Despite the excitement generated by SMMs, a limitation to realizing their applications is that even the 'best' SMMs function at temperatures that can only be reached with liquid helium. The need to develop new SMMs that function at practical temperatures is obvious, and our research is aimed at achieving this goal. Before SMM technology can be mass produced, a considerable amount of fundamental knowledge remains to be developed, and we expect to make a significant contribution to this aspect within the 36-month timeframe of our project.

In achieving our aims, the resulting longer-term economic and societal impact could be considerable. Our project is very likely to lead to new SMM materials with previously unseen properties, thereby leading to new physics. The development of SMMs may well have an important role to play in delivering the next generation of information storage technology, and progress in this area could be revolutionary. If SMM technology becomes viable it is conceivable that new industry could be created around it, resulting in the generation of employment opportunities, wealth creation for the UK, and introducing potential for investment in the UK from overseas.

Our vision for the future of SMMs takes precedent from other transformative materials. Graphene, discovered in 2004, is already shaping the future of the micro-electronics industry, and is considered likely to contribute to the financial well-being of the UK. Superconductivity has attracted enormous interest since the low-temperature phenomenon was discovered in 1911: such materials are used widely, yet superconductors that function at (or above) room temperature are still unknown. However, these century-old materials are still of considerable importance, and the search for room-temperature superconductors is now an EPSRC Physics Grand Challenge. If the Grand Challenge is met, the impact will be revolutionary.

There are clear signs that the SMM field is undergoing a transformation as result of the rise to prominence of lanthanide elements. The introduction of organometallic Ln-SMMs by the Manchester team provided an important new direction in the field; our preliminary studies are already having impact, and investment from EPSRC will allow us to build upon our lead. The remaining challenges in the field are significant, but the potential rewards for surmounting them are immense.