Carbon’s mysterious magnetism

From Science News, June 2nd, 2007; Vol.171 #22 (p. 350)

An X-ray experiment has produced the most conclusive evidence yet that carbon can be made into a permanent magnet.

Only a few elements are magnetic at room temperature. They are metals whose atoms have a magnetic moment arising from the spin of an unpaired electron. Pairs of electrons with opposite spin produce no net magnetic moment. Since carbon tends to form covalent bonds, which contain paired electrons, it seems an unlikely candidate to be magnetized.

But several experiments have suggested that under certain conditions, forms of bulk carbon such as graphite can acquire a feeble permanent magnetization. Most physicists regard these results with skepticism, however, since even trace contamination by a metal such as iron could make a sample slightly magnetic.

Now, a team led by Hendrik Ohldag of the Stanford (Calif.) Synchrotron Radiation Laboratory has found magnetism in a sample of graphite that had been irradiated with protons. The researchers detected a magnetic moment through the effect it had on the absorption of polarized X rays. To rule out contamination, they tuned the energy of the X rays so that they interacted with carbon atoms but not with iron atoms. The results appear in the May 4 Physical Review Letters.

Ohldag says that the proton bombardment could have permanently deformed the hexagonal lattice of carbon atoms in graphite, creating some noncovalent bonds between atoms.

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