Carbon Nanofoam is the First Pure-Carbon Magnet

Discovered a few years ago, carbon nanofoam is the fifth known allotrope of carbon, the others being graphite, diamond, fullerene (e.g., C-60 molecules), and carbon nanotubes. The foam is, along with aerogel, one of the lightest known solid substances (with a density of ~2 mg/cm³).

But at this week's APS March Meeting in Montreal, physicists announced an even more interesting property: though made entirely from carbon atoms that are normally considered nonmagnetic, the foam nevertheless can act like a ferromagnet.

Blasting a high-power laser at disordered solid carbon, a Greece-Australia-Russia research collaboration (John Giapintzakis, University of Crete/IESL-FORTH, giapintz@iesl.forth.gr and Andrei Rode, Australian National University, avr111@rspysse.anu.edu.au) creates a gossamer web made of carbon-atom clusters (with an average diameter of 6-9 nanometers) randomly interconnected.

The foam has other interesting properties: it also is a semiconductor, making it attractive for device applications. The most salient property of carbon nanofoam, however, is its magnetism.

Unlike other forms of carbon, such as graphite and diamond, freshly produced carbon nanofoam is ferromagnetic; that is, it is initially attracted strongly to a permanent magnet at room temperature.

Although the room-temperature ferromagnetic behavior disappears after a few hours, it persists at lower temperatures. Consequently this "ferromagnetic semiconductor" might have very useful applications for spintronics, the emerging field of devices based on a material's magnetic properties.

Addressing the initial skepticism about pure carbon having ferromagnetic properties, the researchers acknowledged that they found traces of iron and nickel impurities in their foam, but calculated that the small amounts of these magnetic materials could only account for 20% of the strength of the ferromagnetic fields in the foam.

Researchers have concluded that the observed novel magnetic behavior is an intrinsic property of the carbon nanofoam and can be traced to its complex microstructure. Namely, carbon atoms in the foam forms heptagon structures, 7-corner, 7-edge polygons that have an unpaired electron, one that does not form a chemical bond and has a magnetic moment which may lead to the magnetism. The researchers also have preliminary indications that the novel magnetic behavior also occurs in another nano-compound made of boron and nitrogen, two other elements that are ordinarily non-magnetic.

Speaking at an APS news conference, theoretical collaborator David Tomanek of Michigan State (tomanek@pa.msu.edu) said that he hoped that the carbon nanofoam and similar compounds would remove what he termed a "magnetic prejudice," the idea than an element should be stereotyped as either magnetic or nonmagnetic.

One possible application of the carbon nanofoam is in biomedicine, as tiny ferromagnetic clusters that could be injected in blood vessels may significantly increase the quality of magnetic resonance imaging pictures. (Paper A17.005)