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Magnets see the light

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18.02.2002

Light-tunable magnets would work only in the coolest computers.
© Getty Images

Light-sensitive ’plastic’ magnets could replace your hard drive.

A ’plastic’ magnet that responds to light could lead to new ways of storing and reading large amounts of computer data. Light would be used to store information in cheap, fast and high-capacity ’magneto-optic’ memories.

The light-switchable magnet is the first to be made from organic (carbon-based) molecules. This means its discoverers, Arthur Epstein of Ohio State University in Columbus and Joel Miller of the University of Utah in Salt Lake City, should be able to use clever chemical tricks to fine-tune the properties of the material¹.

Their first goal is to raise the temperature at which the magnetic switching operates. At present, the material works only when cooled to below -198 oC.

Although this sounds impractically low, it is within spitting distance of the temperature at which nitrogen liquefies:
-196 oC. A relatively cheap refrigerant, liquid nitrogen could quite feasibly be used to cool commercial devices incorporating a modified light-sensitive magnet, Epstein and colleagues hope.

Magnetic memories store information in tiny ’magnetized domains’, where magnetic field lines points either ’up’ or ’down’. This allows magnetic media to store the binary (zeros and ones) data of the digital world. In conventional magnetic memories, the direction of the magnetic field is switched electronically; magneto-optic systems do the switching with light, usually from lasers.

There is nothing new about magneto-optic memories. Some commercial hard-disk drives already exist that use light to read and write information stored in magnetic films. But in these systems the laser switches the magnetic medium by warming it.

Epstein and colleagues’ material contains manganese atoms mixed with small organic molecules, and becomes more magnetic when it absorbs blue light. The light alters the shape of the organic molecules, changing their magnetic properties. The researchers can reverse the effect either by shining green light on the material or by heating it above
-23oC.

In a future memory device, information could be encoded in the material as regions of ’stronger’ or ’weaker’ magnetism, which could be written and erased using tightly focused lasers. This could lead to information storage at very high densities.

Epstein admits that applications of this effect are still a long way off - the organic magnet needs a lot of improvement before it has the properties demanded of commercial devices.

References

1. Pejakovic, D. A., Kitamura, C., Miller, J. S. & Epstein, A. J. Photoinduced magnetization in the organic-based magnet Mn(TCNE)x.y(CH2Cl2). Physical Review Letters, 88, 057202, (2002).

PHILIP BALL | Source: © Nature News Service

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