Mini Magnets

Phys. Rev. Focus 12, 20
Bombardment by protons can turn ordinary graphite into an unusually simple artificial magnet.
Figure caption
P. Esquinazi/Univ. of Leipzig
Magnetic Map. A microscope that senses magnetic fields shows magnetic “stripes” in ordinary graphite after it was blasted with protons. This process makes an unusually simple artificial magnetic material.

Permanent magnetism, the familiar property that keeps children’s art and bad poetry stuck to refrigerator doors, occurs naturally in iron and only a few other metals. But in recent years researchers have found a handful of complex metal-free materials that can become magnetic at temperatures near absolute zero. Now a team reports in the 28 November PRL that they have found magnetism at room temperature in ordinary graphite with a pinch of hydrogen added to it. If they can convince skeptics of the effect, the new technique might lead to the tiniest magnets ever made, with possible uses in nanoscale electronics or even cancer treatment.

Permanent magnetism, or ferromagnetism, is a rare property of molecular structures. Atoms with unpaired electrons act like tiny bar magnets, but they ordinarily point in random directions. A material is ferromagnetic only if the arrangement of the atoms in the solid and the configuration of their electrons favors a state in which the bar magnets influence one another to all point in the same direction.

In exploring the possibility of ferromagnetism in new materials, researchers have created complicated organic molecules that act as magnets. Japanese researchers recently reported weak ferromagnetism in a much simpler material–non-crystalline carbon that had been prepared from hydrogen-containing compounds. But many in the scientific community were skeptical of the Japanese work and suspected that metallic contaminants were to blame for the magnetic effects, says Pablo Esquinazi of the University of Leipzig in Germany.

So Esquinazi and his colleagues devised a simple experiment they hoped would avoid the question of contamination. They irradiated ultra-pure graphite with a beam of protons at an energy of 2.25 MeV. These protons became lodged in the graphite, distorting its structure and forming carbon-hydrogen bonds. The irradiated graphite became magnetic in response to an applied field, and retained some of that magnetism when the field was turned off–the classic signature of ferromagnetism. The team found that adding more protons at first made the sample more magnetic, but that at the highest dose they tried, the magnetic response decreased.

Although the evidence is clear, the effect “is not at all well understood theoretically,” Esquinazi admits. To explain his group’s findings, Esquinazi cites recent theories suggesting that hydrogen bonded to the carbon can distort the lattice in just the right way to generate ferromagnetism. The decrease in magnetism at the highest doses might result from damaging the lattice to such an extent that an orderly alignment of atomic bar magnets is less easily attained, he says.

David Tomanek of Michigan State University in East Lansing, who has developed theoretical models explaining magnetism in carbon, “firmly believes” the new results and thinks impurities cannot account for their magnitude. But Joel Miller of the University of Utah in Salt Lake City is more cautious. Though he says Esquinazi’s work is “tantalizing,” he would like to see more direct evidence for the molecular effects the protons supposedly produce in graphite.

The proton irradiation technique can produce ferromagnetic spots in a carbon structure, so might be used to turn carbon nanotubes into the world’s tiniest bar magnets, says Esquinazi. He was surprised to get phone calls from pharmaceutical companies inquiring about such possibilities. There is biomedical interest, he was told, in tiny magnets that could be implanted, for example, in tumors, where they could turn radio waves into a local source of destructive heat.

–David Lindley

David Lindley is a freelance science writer in Alexandria, Virginia.

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