Optical data storage does not require expensive magnetic materials as synthetic alternatives work just as well. This is the finding of an international team from York, Berlin and Nijmegen, published Thursday February 27 in Applied Physics Letters. The team’s discovery brings the much cheaper method for storing data using light a step closer. It was Professor Rasing, physicist at Radboud University Nijmegen and FOM workgroup leader, who came up with the new synthetic material.
When you store a file on your laptop or PC, the computer creates a code consisting of zeros and ones. These are actually tiny magnetic poles (spins) that can point in one of two directions: the ‘zero’ state or the ‘one’ state. Switching these spins using a magnetic field is a relatively slow, energy-intensive process. An alternative is to switch them using light, which was first achieved by Radboud researchers six years ago. They have been searching for suitable materials ever since. Theo Rasing: ‘Optical switching is only possible in special magnets, called ferrimagnets. However, these magnets are made of expensive rare earth metals, which are also difficult to produce at the nano-scale. Now, we have shown for the first time that it is also possible to switch synthetic ferrimagnets optically.’
When you store a file on your laptop or PC, the computer creates a code consisting of zeros and ones. These are actually tiny magnetic poles (spins) that can point in one of two directions: the ‘zero’ state or the ‘one’ state. Switching these spins using a magnetic field is a relatively slow, energy-intensive process. An alternative is to switch them using light, which was first achieved by Radboud researchers six years ago. They have been searching for suitable materials ever since. Theo Rasing: ‘Optical switching is only possible in special magnets, called ferrimagnets. However, these magnets are made of expensive rare earth metals, which are also difficult to produce at the nano-scale. Now, we have shown for the first time that it is also possible to switch synthetic ferrimagnets optically.’
Ferrimagnets
Ferrimagnets have the unusual property that the spins are not all of the same magnitude. ‘They are similar to anti-ferromagnets, in which the spins are found in pairs with opposite directions. However, because the magnetic poles have different magnitudes, ferrimagnets have a net magnetic moment,’ explains Rasing. This can be simulated by anti-ferromagnetically coupling thin layers of iron with a spacer layer. ‘The iron is ferromagnetic – all the spins have the same magnitude and direction. It is therefore possible to create a net magnetic moment by combining two layers of different thicknesses and opposing magnetisation directions, for example. Coupling the spins works in a very similar manner, in the same two-step process that we previously developed for the normal ferrimagnets.’
Figure 1. Left: in a ferrimagnet, there are an equal number of spins in the up and in the down directions, but the magnitudes of the spins vary. The result is net magnetisation. Right: the synthetic ferrimagnet is made of two very thin layers of iron with a connecting layer in between. In the iron layers, all the spins have the same direction and magnitude. By combining layers with different magnetisation directions and different thicknesses, it is nevertheless possible to create net magnetisation.
Other than with normal ferrimagnets, the production of synthetic ferrimagnets does not require the use of rare earth metals. This makes them cheaper and better for the environment, and therefore more suitable for use in computers. Rasing: ‘I really believe that this is the start of a fundamentally new form of data storage, and possibly data processing too.’
Hot topic
When Rasing came up with his concept for the synthetic magnet, he immediately contacted a group in York that could model the switching process. Rasing: ‘Their model showed that it really did work, and so we applied for a joint patent. It immediately became a hot topic, and there are already groups in San Diego, France and Germany working on actually producing and testing the synthetic ferrimagnets. International cooperation is therefore essential, and I expect the combination of theory, modelling and experimentation in the various groups to bear much more fruit in the coming years.’
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