Light boosts plastic magnet

By Chhavi Sachdev, Technology Research News

Storing reams of data on the small metal platters that make up computer disk drives depends on the ability to change the magnetism of microscopic pieces, or domains, of the metal. Altering magnetic domains in order to record the ones and zeros of digital information is usually accomplished by applying an electric field.

Using light instead of electricity, however, could lead to faster and cheaper data storage devices. Magnets that respond to light rather than electricity do exist, but function only at extremely low temperatures.

Researchers at Ohio State University have increased the temperature range of light-sensitive magnets by making them out of plastic.

The plastic magnet's functional temperature range is still considerably below freezing, but in solid state physics, coldness is relative. At minus 198 degrees Celsius, which is 62 degrees warmer than the operating range of previous light magnets, the magnet can be made more or less powerful by exposing it to one of two different colors of light, according to Arthur Epstein, a professor of physics and chemistry at Ohio State University.

The material is made from a mixture of tetracyanoethylene polymers and manganese ions. Ions are electrically-charged atoms, and polymers are macromolecules made up of many smaller molecules.

The researchers used two different colors of light to increase the magnetic pull of a piece of the material, then decrease the magnetic pull. When they shone blue light on the plastic, it changed shape slightly, which increased the magnetic strength about 50 percent, said Epstein. Green light partially reversed the change, decreasing it by about 37 percent, he said.

The shape change can also be reversed by heating the system to near room temperature, said Epstein. The increased thermal motion jiggles the magnets' molecules into resuming their initial shapes, he said.

The shape change makes the material more magnetic because it leads to better alignment of the spin state of individual magnetic domains, according to Epstein. "Think of spin state as arrows associated with individual electrons. They can point up or down. If the spins are random in direction the material is not a magnet. If the spins all point the same direction we have a ferromagnet."

Atoms naturally fall into arrangements that are energetically efficient. Initially, the material is in its ground state, "which has the lowest possible energy and is thus stable," said Dusan Pejakovic, an Ohio State physics researcher. When illuminated, the photons kick the material's electrons into a higher-energy, excited state, making it less stable, he said. In most materials, the electrons in the excited state quickly drop back into the ground state.

In the researchers' plastic magnets, however, the change leads to a slight atomic rearrangement. "The ions can find an alternative configuration to… the ground state," said Pejakovic. In this alternative configuration, neighboring spins are better aligned and the material's magnetism is increased, he said.

The material will stay in this more magnetic, alternative state as long as the temperature remains sufficiently low. It's electrons "will need an additional kick -- by heat or by green light -- to drop back to the ground state," he said.

This is similar to throwing a bowling ball up a slope, said Pejakovic. "Normally, we expect the ball to roll back to the bottom of the hill because it seeks the minimum of potential energy, similar to electrons dropping into the ground state," he said. If the slope were not too steep and the surface deformable enough, however, the ball might sink into the surface and get stuck on the slope. "The ball will then need an additional push downhill in order to reach the bottom, much like electrons need to be pushed to the ground state by heat or by green light," said Pejakovic.

In a film of the plastic magnetic material, areas that are more or less magnetic could represent the ones and zeros of digital information, Pejakovic said. "Imagine a film of magnet. Initially, the film has some level of magnetization that is equal at every point," Pejakovic said. A laser could used to change some areas in order to write data to the film. "Wherever you have a laser spot, you have an increased level of magnetization; that's a 1. Wherever you had the initial value of magnetization, which is a lower, it's a zero," he said.

In existing metal light-sensitive magnets, writing data requires temperatures at least as cold as minus 250 degrees Celsius, or 20 degrees above absolute zero. In these magnets ones and zeros are recorded when light causes magnetic ions to align in one direction or another.

The Ohio researchers are working to further increase the temperature range of their light-sensitive plastic magnet. Their ultimate goal is to make the magnet viable at room temperature, said Epstein.

The work is novel and important, said Tatiana Makarova, an associate professor of solid state physics at Umea University in Sweden. "The research in this area is very intensive: people search for the material that switches its properties responding to an external signal," she said.

The researchers have produced a reversible magnet by combining existing technologies, Makarova said. "It is a ready-made system for writing and erasing data," she said.

The technology has the potential to be used in practical media, she added. "From the point of view of a solid state physicist [minus 198 degrees Celsius] is a sauna. If they managed to make a magnet working at [minus 198], they will surely succeed in making a room-temperature magnet," she said.

Practical products made from the material are likely to take at least five years, said Epstein.

Epstein and Pejakovic's colleagues were Joel S. Miller at the University of Utah, and Chitoshi Kitamura at the Himeji Institute of Technology, Japan. They published the research in the February 4, 2001 issue of the journal Physical Review Letters. The research was funded by the Air Force Office of Scientific Research and the U.S. Department of Energy.

Timeline:  5 years<
Funding:   Government
TRN Categories:   Materials Science and Engineering; Data Storage Technology
Story Type:   News
Related Elements:  Technical paper, "Photoinduced Magnetization in the Organic-Based Magnet Mn(TCNE)x," Physical Review Letters


May 1/8, 2002

Page One

Team spins mirror fibers

Light flashes fire up nanotubes

Quantum force powers microslide

Light boosts plastic magnet

Metal crystals cover glass


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