Physicists and geologists at Trinity College in Dublin have found new evidence for the existence of magnetism in carbon by examining a meteorite that crashed into the Arizona desert some 50,000 years ago. Michael Coey and colleagues examined fragments from the Canyon Diablo meteorite and found that only about two-thirds of the magnetization could be accounted for by the magnetic minerals present in the sample. This means, they say, that the rest of the magnetization is somehow associated with the carbon in the meteoritic graphite nodule (JMD Coey et al. 2002 Nature 420 156)
The magnetic properties of carbon-60 compounds have intrigued physicists since they were first reported in 1991 and researchers have recently discovered weak magnetic behaviour in polymerised rhomohedral carbon-60. Ferromagnetism has previously been observed in other carbon-based ferromagnets, but only at very low temperatures. However, the weakness of the effect makes it difficult to determine the origins of the magnetism – it could be intrinsic or it might be caused by minute concentrations of iron-rich impurities in the samples.
Coey and co-workers characterised the magnetism associated with the ferromagnetic phases in their samples using Mossbauer spectroscopy, chemical analysis and a combination of scanning electron microscopy and X-ray diffraction analysis. From the Mossbauer results, they determined the concentration of the ferromagnetic minerals in each of the graphitic samples and calculated their combined contribution to the magnetization.
The observed magnetization, however, significantly exceeded the magnetization that was due to these magnetic phases. The researchers attribute this difference to the graphite. They calculate the average room temperature magnetization of carbon to be 23.1 Am2 kg-1, which corresponds to 0.05 Bohr magnetons per atom. By comparison the figure for iron is 2.2 Bohr magnetons per atom.
The results raise the question of the origin of the ferromagnetism. It could be that meteoritic graphite differs from its terrestrial counterpart because of the way it was formed or changes it underwent when it landed on Earth. The shock of this impact could produce defects, which are known to increase the magnetic susceptibility of graphite. Another possibility is that the dispersed nanocrystalline ferromagnetic phases induce a magnetic moment in the graphite. The researchers suggest a “magnetic proximity” effect induced at the border between the graphite and the magnetic materials as a possible explanation.
Whatever its origin, the implications of ferromagnetic carbon are likely to be far-reaching. This material could, for example, be used as a high-temperature ferromagnetic semiconductor or in “spintronic” applications.