This evolution is largely due to developments with thin magnetic films. The ISIS facility has played an important role in this progress, helping scientists to understand exactly what is going on inside these materials.
Over 90% of the data we create is stored magnetically, and it is now cheaper to record data on to a disc than to keep it on paper. iPods, laptops and personal video recorders contain vast amounts of data storage. So just where does this data go?
The answer is usually a thin magnetic film. Magnetic layers that are just a few atoms thick straddle a nonmagneticlayer, making a special ‘sandwich’. The resistance of this sandwich depends on whether the magnetism in the layers points the same way. By understanding the detailed magnetic properties of each layer scientists are able to exploit the data storage capabilities to their maximum.
Using ISIS, Chris Marrows from Leeds University and his colleagues have been studying the behaviour of these materials. They use a technique called neutron reflectometry in which a pencilthin beam of neutrons is reflected from the magnetic layers. The resulting neutron diffraction pattern depends upon the magnetic state of the layers. “We can work out how big the magnetic field is and in which direction it is pointing,” explains Marrows.
In particular Marrows and his colleagues are interested in the exchange bias – the communication between the bottom two magnetic layers – which helps to stabilise the system. By building an artificial version of an exchange bias system they have been able to visualise its detailed arrangement. It has turned out to have a spiral structure, with the magnetic particles all winding upwards, like a spring.
By understanding the exact nature of these thin magnetic layers manufacturers are able to cram more data into ever-smaller spaces. Neutron diffraction has turned out to be an ideal tool for this kind of visualisation because it is non-destructive and can be used to study layers that are only a few nanometres in thickness.
Chris Marrows, Mannan Ali, Paul Steadman, Bryan Hickey (Leeds), Sean Langridge, Tim Charlton (ISIS)
Research date: December 2002
Exchange bias in spin-engineered double superlattices, P Steadman et al, Phys Rev Lett 89 (2002) 077201
Mapping domain disorder in exchange-biased magnetic multilayers, CH Marrows et al, Phys Rev B 66 (2002) 024437
Controlled magnetic roughness in a multilayer that has been patterned using a nanosphere array, S Langridge et al, Phys Rev B 74 (2006) 014417