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October 22, 2001
Volume 79, Number 43
CENEAR 79 43 p. 10
ISSN 0009-2347
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Unexpected magnetic behavior is observed in a phase of polymerized C60


The title is as spare and riveting as it is likely to be controversial: "Magnetic Carbon."

A paper with that title was published last week in Nature [413, 716 (2001)]. In it, an international research team reports the serendipitous discovery of ferromagnetic behavior at and above room temperature in a form of polymeric C60.

"If confirmed, this result will represent a breakthrough in the magnetism of metal-free materials," writes Fernando Palacio in an accompanying commentary in Nature.

In his article, Palacio, a physics professor at the Materials Science Institute of Aragón at the University of Zaragoza, in Spain, explains why the claim of a room-temperature carbon magnet is so remarkable. "Although a handful of metal-free magnets have been discovered to date," he notes, "their magnetic properties occur only at very low temperatures." Efforts to detect magnetic ordering at room temperature and above in metal-free organic compounds have been plagued by difficulties in characterizing and reproducing the observed magnetism, he points out.

The Nature paper on magnetic carbon--authored by semiconductor physicist Tatiana L. Makarova of Ioffe Physico-Technical Institute, St. Petersburg, Russia, and her colleagues at five other labs--will likely face skepticism in the materials community. The researchers observed "strong magnetic signals" in rhombohedral C60, a polymer produced by subjecting C60 to high temperatures and pressures. Palacio gives the team credit for excluding, "up to every reasonable limit," the possibility that impurities are causing the observed magnetism. He also believes they have demonstrated the reproducibility of the effect.

However, Palacio describes the phenomenon they observed as "a very weak magnetization--they are probably too optimistic in calling it ferromagnetism." Makarova defends her use of the term ferromagnetism by noting that her team has observed "a finite spontaneous magnetization with the temperature dependence typical for a ferromagnetic transition." The magnitude of the magnetization, in her view, is not so important because it depends on the percentage of magnetic phase in the sample, which they hope to increase.

Makarova and coworkers note that some of their samples have such strong magnetization that they can be lifted off a tabletop using a small metal magnet. Furthermore, they report that the samples remain magnetic up to about 227 °C. The origin of this magnetism, though, remains unclear, and they are continuing to investigate the question.

Scientists are interested in developing metal-free magnets because such magnets "would be electrical insulators (reducing energy losses in some applications) and should be cheaper and lighter than their metallic counterparts," according to Palacio.

Makarova, however, was not seeking a carbon magnet. She and her colleagues, including Pablo Esquinazi, Yakov Kopelevich, and Roland Höhne, were looking for signs of superconductivity in polymerized C60--a reasonable pursuit since certain fullerene derivatives have been shown to be superconducting.

The discovery of unexpected magnetic behavior in an ostensibly pure carbon material leaves some important questions unanswered, Palacio says. It thus has "the right mix of ingredients to be controversial." The "lively discussion" that will ensue, he writes, "will help stimulate research into understanding the behavior of this unusual carbon magnet."


RHOMBOHEDRAL C60 This two-dimensional phase of polymerized C60 resembles highly oriented pyrolytic graphite, except that instead of layers of carbon atoms, there are layers of covalently bonded C60 molecules. The bonding in one layer is shown at left. The relationship between the polymer layers and the original cubic lattice of pristine C60 is shown at right.

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