New view on magnetism
For years researchers from the Radboud University Nijmegen have experimented with the limits of what can be achieved with magnetic materials. They continued to discover spectacular new things that were largely not possible according to the prevailing ideas about magnetism. Johan Mentink, a theoretician in the groups of FOM workgroup leaders Prof.dr. Theo Rasing and Prof.dr. Misha Katsnelson, has developed a new theory that can explain the researchers’ experimental observations. The theory predicts how new materials for energy-efficient and far faster hard disks can be designed and it paves the way for new forms of magnetic data storage. Physical Review Letters has just published the results in its latest issue.
Old theory applied to slow magnets
The established theory described magnetic processes from geological timescales down to the nanoseconds in which the majority of modern hard disks record data. At the atomic scale, each magnet is entirely composed of miniscule magnets (spins). At a larger scale, a magnet is magnetic as a result of the coupling between these elementary spins. The established theory (so-called Landau-Lifshitz equation) assumes that this coupling force is infinitely large. Consequently all spins move at the same time and the coupling between the spins remains a hidden entity. However, the established theory does not describe exactly what happens at the time scale in which the coupling itself works: 0.1 picoseconds. (A picosecond is 1 millionth of a millionth of a second.
New ideas for fast magnets
The newly developed theory does, however, describe the magnetic processes that take place at the ultrashort timescale of the coupling between the spins. This reveals the significance of the previously hidden force. And what does this show? The results of the new theory are completely at odds with the expectations of the established theory: the different spins in the magnet were found to move very differently and not at the same time. The behaviour of the spins also depends on the material from which the magnet is made!
A possibly more surprising finding still is that a very short intense disruption – for example with a heat pulse of a thousandth-billionth of a second - can temporarily allow the spins to move against the coupling force. This explains recent experiments in which during the switching of some antiferromagnetic materials (in which the spins in the stable state are in opposite directions - North-South, South-North, - and the material is completely non-magnetic) spins appear to be very briefly magnetic: all spins pointing the same way.
Basis for new applications
Now the researchers know how the coupling force can be used for switching spins, they can design and produce new materials for the fastest storage media for data. Alloys with antiferromagnetic couplings will yield the best result for this. In view of the direct usefulness of the application, a patent has been applied for. However, Mentink is mainly enthusiastic about the really new things that his theory gives rise to. 'We can use the coupling force to develop revolutionary, new and counterintuitive ways of manipulating spins. These new ways include the switching of magnets with unprecedented speed in the order of picoseconds.'
Reference
'Ultrafast Spin Dynamics in Multisublattice Magnets', J.H. Mentink, J. Hellsvik, D.V. Afanasiev, B.A. Ivanov, A. Kirilyuk, A.V. Kimel, O. Eriksson, M.I. Katsnelson and Th. Rasing. Physical Review Letters 2012.