[meteorite-list] Magnetic Meteorites

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:15 2004
Message-ID: <200211141714.JAA20632_at_zagami.jpl.nasa.gov>


Magnetic meteorites
Belle Dume
13 November 2002

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.
Received on Thu 14 Nov 2002 12:14:53 PM PST

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