Nuclear, magnetic, but not NMR: depleted uranium for more memory

Depleted, but magnetic

A new uranium-containing compound maintains its magnetic behaviour at low temperatures. The discovery could take us a step closer to magnetic memory devices with capacities thousands of times denser than current high-end hard drives.

Stephen Liddle of the University of Nottingham, England, works with molecules containing depleted uranium. Depleted uranium is the form of this otherwise radioactive element left behind after the uranium-235 isotope has been separated from the weakly radioactive uranium-238 during enrichment for nuclear power and weapons applications. Liddle has now synthesised a new compound containing two atoms of depleted uranium linked by a toluene moiety, which he explains retains its magnetism at very low temperatures. Such a "single-molecule magnet" (SMM) could be used to create magnetic data storage media with a potential data density possibly thousands of times greater than conventional magnetic media. Of course, the marketing departments across the computer industry might have difficulty selling a uranium hard drive, but the work by Liddle and colleagues, David Mills, Fabrizio Moro, Jonathan McMaster, Joris van Slageren, William Lewis and Alexander Blake, could be extended to related elements that have none of the infamy of uranium.

"This work is exciting because it suggests a new way of generating SMM behaviour and it shines a light on poorly understood uranium phenomena," says Liddle. SMMs become magnetic below a "blocking" temperature by virtue of the magnetism of the molecule itself rather than long-range ordering as is seen with more common bulk magnetism. Researchers discovered some time ago that certain polymetallic transition metal clusters could exhibit this behaviour. "It could help point the way to making scientific advances with more technologically amenable metals such as the lanthanides. The challenge now is to see if we can build bigger clusters to improve the blocking temperatures and apply this more generally."

Liddle points out that, at this stage, it is too early to say what new discoveries and technology might emerge from this particular research. Single-molecule magnets have studied intensely by teams across the globe because of the aforementioned possibility of instigating a step change in data storage capacity. They also hold potential for the realisation of high-performance computing technology based on spintronic and quantum information processing.

Inherent properties

The researchers explain that uranium has a larger spin-orbit coupling than the lanthanides and relatively diffuse 5f orbitals allowing covalent bonding to occur, something that is near impossible in the 4f lanthanides, all of which could lead to stronger magnetic exchange potential.

"The inherent properties of uranium place it between popularly researched transition and lanthanide metals and this means it has the best of both worlds," Liddle says. "It is therefore an attractive candidate for SMM chemistry, but this has never been realised in polymetallic systems which is necessary to make them work at room temperature." The use of arene bridges, the toluene group, exploits the supramolecular work using these groups for other applications and will allow yet other systems to be constructed based on the known and wide-ranging chemistry of those complexes. The team used X-ray crystallography and spectroscopy to characterise the diuranium complex and to investigate transitions within the structure.

You can see a video about Liddle's latest discovery at http://www.periodicvideos.com/videos/mv_magnetic_uranium.htm

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.