Nanofoam Exhibits Surprising Magnetic Properties

By Ernie Tretkoff

A new form of carbon exhibits surprising magnetic properties that could make it useful in future spintronics or biomedical applications, researchers reported at the APS March Meeting. The material, called carbon nanofoam for its low density and web-like structure, is the only form of pure carbon known to be ferromagnetic.

Carbon nanofoam is structurally distinct from the other four known forms of carbon—graphite, diamond, fullerenes ( buckyballs), and nanotubes. With a density of about 2 mg/cm³, comparable to that of aerogel, carbon nanofoam is one of the lightest known solid substances.

But what's most remarkable about the material, the researchers said, is that unlike other forms of carbon, the nanofoam is ferromagnetic, like a refrigerator magnet. However, at room temperature, the nanofoam's magnetization disappears a few hours after the material is produced.

A collaboration of researchers from Greece and Australia produced the carbon nanofoam by shooting a high-powered, ultra-fast laser at disordered solid carbon in an argon-filled chamber.

By imaging the material using a high-resolution electron microscope, John Giapintzakis of the University of Crete and colleagues found that the nanofoam has a sponge-like structure, made up of carbon clusters a few nanometers in diameter randomly linked together into a web-like foam.

Because pure carbon is not normally ferromagnetic, the group tested their sample for impurities that might be causing the magnetic behavior. Although they did find traces of iron and nickel, the small amounts of these magnetic elements could not account for all of the ferromagnetism in the nanofoam. The researchers concluded that the magnetic properties come from the complex structure of the nanofoam itself.

David Tománek of Michigan State University, who collaborated with the group on theoretical interpretation, believes that the carbon clusters in the foam are made up of nanotubes joined together into tetrapods. In these four-legged structures, some carbon atoms have a free electron, one that does not form a chemical bond. These unpaired electrons carry a magnetic moment that may lead to the magnetism.

Chemists have long known about such carbon radicals, said Tománek, but until now they have only been found in carbon connected to another element. In this case, the structure is entirely carbon.

The researchers have also done some preliminary studies that suggest that the novel magnetic behavior found in carbon nanofoam could be present in other nano-structured solids of elements that are not normally magnetic, including a compound of boron and nitrogen.

If this behavior turns out to be a general phenomenon, researchers will have to think more about what makes a material magnetic, said Tománek. "We need to revisit our magnetic prejudice."

Giapintzakis suggested that carbon nanofoam could be used in spintronic devices, which are based on a material's magnetic properties. The unique material may also find uses in biomedicine. For instance, the tiny ferromagnetic clusters could be injected into blood vessels to enhance magnetic resonance imaging. The nanofoam could also be implanted in tumors, where it could turn radio waves into a source of heat that would destroy the tumor but leave surrounding tissue unharmed.