Carbon Coaxed Into Magnetic Foam

MONTREAL--Researchers reported at the American Physical Society meeting here today that they've discovered a novel structure of carbon that's strongly magnetic at room temperature, a first for an all-carbon material. The magnetism, alas, disappears just hours after it's synthesized. But if the researchers can make the magnetic behavior stick around, the material could have uses ranging from medical imaging to exotic electronic circuitry.

The new form of carbon was first discovered 5 years ago by a group led by physicist Andrei Rode at the Australian National University. While working to come up with a new technique for synthesizing straw-shaped carbon molecules called nanotubes, Rode got a bit of a frothy surprise. He and his colleagues fired a high-power, fast-pulsed laser at a target of amorphous carbon enveloped in argon gas, and they created an extremely lightweight foam of nanosized carbon clusters. After spotting initial hints of magnetic behavior, Rode teamed up with magnetic specialists led by John Giapintzakis at the University of Crete in Greece.

Giapintzakis's group confirmed that the carbon foam is initially magnetic but this behavior disappears after a few hours at room temperature. Giapintzakis says extensive tests on the nanofoam show that impurities--which have spoiled earlier claims of magnetic carbon--could only account for up to 20% of the magnetism present. "We are sure we do not have an impurity effect," he says.

Instead, Giapintzakis argues that the effect occurs when carbon atoms condense out of the superhot plasma generated by the laser into tiny tetrapod-shaped structures. At the foci of these tetrapods, several carbon atoms are forced to bind to just three neighbors rather than the usual four, leaving them with free electrons that are magnetically active, he says. Over time, these structures likely break down, reducing the material's ferromagnetism.

In addition to its magnetic behavior, the new carbon nanofoam is also a semiconductor. That means the material has the potential to manipulate both an electron's charge and spin, a property related to its magnetic behavior. That in turn could make the materials attractive building blocks for spintronic devices, which compute by manipulating electron spins.

"It's a very intriguing material," says Mildred Dresselhaus, a physicist at the Massachusetts Institute of Technology in Cambridge. David Tomanek, a physicist at Michigan State University, East Lansing, who collaborated with Giapintzakis on the magnetic theory of the nanofoam, adds that the work underscores the ability of nanotechnology to change long-held understandings of what materials can be magnetic.

Related site
Abstract of a paper on the foam by Giapintzakis's team