Controlling ferromagnetic domains using light
Aug 21, 2014 3 comments
A variety of magnetic materials can be controlled using only polarized light, according to new work carried out by an international team of researchers. The unexpected and so far unexplained discovery shows that the optical phenomenon, which was previously thought to be possible only in ferrimagnets, is actually much more general. The discovery could potentially have a major impact on data storage, as it could allow magnetic bits to be rapidly switched by optical pulses in state-of-the-art hard drives.
From magnetic tapes to computer hard drives, rewritable data storage has traditionally been achieved using ordering of magnetic domains. Individual bits are stored by setting the magnetization vector of a particular domain to point either up or down. However, as data processing becomes faster in modern computers, data storage needs to speed up too. This presents both practical and theoretical difficulties for magnetic data storage.
The traditional way to flip a bit is to apply a magnetic field. However, speedily setting magnetic domains requires stronger and faster pulsed magnetic fields, and these are difficult to generate in a computer's hard drive. Furthermore, in 2004 researchers using magnetic fields generated by the Stanford Linear Accelerator showed that the extreme fields needed to switch a domain in less than two picoseconds caused a complete breakdown of the magnetic order of a material, apparently placing an ultimate speed limit on magnetic data storage.
In 2006 Theo Rasing and colleagues at Radboud University Nijmegen in the Netherlands showed that the magnetization of domains in ferrimagnets – materials that contain two types of magnetic domain oriented in opposite directions – can be controlled by circularly polarized light. This control could be accomplished by a low-power laser pulse with a duration as short as 40 femtoseconds. But while the rare-Earth ferrimagnets used by Rasing and subsequent researchers were popular in magneto-optical drives, they are not used in computer hard drives because they are easily magnetized and demagnetized. The magnetism of very small domains is therefore quite unstable, thus limiting the density at which the materials can store data.
But now, Stéphane Mangin of the University of Lorraine in France, along with colleagues in the US, Germany and Japan, has demonstrated that this type of optical switching can also be achieved in ferromagnetic films made from materials such as cobalt, platinum, nickel and palladium. These materials are of great interest to hard-drive developers but until now most of the theories explaining optical switching were applicable only to ferrimagnets. "Here we are really showing that is not the case – you can have different kinds of ferromagnetic or other magnetic materials that show this behaviour," says Mangin, but cautions that they still do not know how this occurs.
The researchers tested a selection of ferromagnetic films, varying parameters such as the relative thicknesses, the proportions of different materials and the number of layers, to confirm their finding. Using a standard method, the team viewed each sample under a Faraday microscope, which uses polarized light. A domain polarized in one direction appears black, whereas a domain polarized in the other appears white. The researchers irradiated the samples using 100 femtosecond laser pulses and found that they were able to switch domains as well as introduce polarization to parts with no net polarization.
Both Rasing and Bert Koopmans, a nanomagnetics expert at the Technical University of Eindhoven in the Netherlands, were taken aback. "It gives me a kind of rollercoaster feeling," says Koopmans. "The interpretation of these [optical-switching] experiments has changed throughout the years. I thought that I understood everything and this experiment has completely frazzled my mind." Both researchers, however, insist that more needs to be done to demonstrate that the work can be useful in hard drives. "In this paper, there are no results from single pulses – only from accumulated pulses," says Rasing. "So, it is a bit difficult to tell whether it is really very fast optical switching or heating by the laser – although the helicity is clearly important here – that has been demonstrated." Mangin and his collaborators are currently working on both a theoretical explanation and practical development of the technique.
The research is published in Science.
About the author
Tim Wogan is a science writer based in the UK