If a recent experiment is confirmed, theorists will be challenged to explain the evidence for weak ferromagnetism in a compound made solely of carbon.
December 2001 page 18
A research team has polymerized carbon-60 at high pressures and temperatures and reported1 that the product exhibits ferromagnetic properties at surprisingly high temperatures: near 500 K. Naturally, such an announcement elicits skepticism, because the constituent molecules have no magnetic moments and also because there have been premature claims of ferromagnetic behavior in other organic polymers.2
The experimenters--who hail from Russia, Sweden, Germany, and Brazil-- were themselves duly cautious about this serendipitous result, discovered while they were looking for superconductivity. After spending two years ruling out spurious effects from possible impurities, the collaborators decided to publish. "They've made a reasonable argument for ferromagnetism," comments Joe D. Thompson of Los Alamos National Laboratory, "and produced compelling evidence that the material should be studied further."
Further studies will be motivated by two factors. One is the intrinsic curiosity about what might cause magnetic behavior in a structure made solely of carbon, an atom that has no unpaired electrons.
The second factor is interest in possible applications. Considerable research is already focused on producing molecule-based magnets, fueled by the ease with which such magnets might be made and the ability to modulate their properties by synthetic chemistry techniques. Some researchers have looked for magnets that have no metallic atoms and that hence might be less dense. However, as team member Tatiana Makarova of the Ioffe Physico-Technical Institute in St. Petersburg, Russia, cautions, "the application of this material is only a dream. The samples are very expensive because we start with a costly commercial fullerene and use nearly the same technology as that for making artificial diamonds."
Magnetism was seen in a metal-free organic compound about ten years ago,3 but it had a very low Curie temperature Tc (the temperature above which magnetization sharply drops). About the same time, a metal-free, fullerene-based molecule was found to exhibit weak ferromagnetism4 with Tc = 16 K. In that material, C60 molecules are intercalated with strong organic donor molecules and receive electrons from them. The newly reported C60 material has attracted attention because it has a high Tc without doping.
Magnetic behavior in fullerenes involves electrons from an unfilled p band. That's rather new, compared with d- and f-band magnetism seen in conventional transition-metal and rare earth magnets.
The behavior of the newly discovered polymerized C60 is similar to that of the inorganic compound calcium hexaboride, which exhibits weak ferromagnetism and a high Curie temperature.5 Both materials have surprisingly low concentrations of spins, which must somehow become strongly coupled. Also, both are close to a metal-insulator transition. Perhaps theorists will find a connecting thread.
Formed under pressure
The reported magnet is a polymerized form of C60 that consists of two-dimensional layers of the "buckyballs" in a highly oriented rhombohedral phase, as shown in figure 1. The polymer samples were formed under high pressure (6 GPa) and high temperatures (around 1000 K) by researchers at the Institute of High Pressure Physics in Troitsk, Russia. The sample properties were measured by experimenters from collaborating institutes: the University of Umeå in Sweden (where Makarova is now a visiting researcher); Ilmenau Technical University and Leipzig University, both in Germany; and the University of Campinas in Brazil. Although formed under extreme conditions, the materials are subsequently handled at normal pressures and temperatures.
The collaborators also looked at samples of similarly produced one-dimensional orthorhombic polymers and two-dimensional tetragonal polymers, but only the rhombohedral C60 manifested ferromagnetic behavior. The temperature dependence of the saturation magnetization, plotted in figure 2, suggests that Tc = 500 K. The magnetization has a hysteresis loop, with the remanent magnetization also plotted in figure 2. The rhombohedral C60 polymers are stable and have shown no degradation during the two years they've been studied.
All samples have the same Tc, Makarova reports. However, the saturation magnetization varied from sample to sample. Throughout preparation, the researchers were extremely careful to avoid impurities, reporting an upper limit of 22 parts per million for the concentration of impurities in their samples. That amount, they estimate, should contribute no more than a few percent of the magnetism they observe. Samples with higher impurity concentrations show smaller saturation magnetism, they say.
The measurements indicate that, on average, only about one in a hundred C60 molecules has an unpaired electron in rhombohedral C60. Any explanation of the magnetic behavior must explain how far-flung spins can become strongly coupled. But the spins may be concentrated in certain domains, and the experimenters report some evidence of a domain structure. In that case, any models of the magnetism must explain why the electrons remain unpaired when carbon atoms with unpaired spins have a strong propensity to form spin-pairing covalent bonds.
Makarova and her colleagues suggest that the material has defects, with one carbon sphere slightly rotated, disrupting the normal bonding between hexagon-hexagon edges of neighboring buckyballs. The resulting bonds might produce a conduction electron that could be the source of itinerant magnetism (so called because the electron is free to travel).
Alternatively, the researchers speculate, the mechanism may stem from self doping caused by destruction of the rhombohedral C60 structure: The magnetic phase appears at temperatures just below the stability limit for the fullerene cages, although Raman and x-ray analyses do not detect any presence of wrecked fullerenes.
If the magnetism does indeed stem from defects in the rhombohedral C60 structure, speculate theorists Marvin Cohen and Steven Louie of the University of California at Berkeley, the effect may not be unique to this material but might show up in other forms of carbon as well.
1. T. L. Makarova, B. Sundqvist, R. Höhne, P. Esquinazi, Y. Kopelevich, P. Scharff, V. A. Davydov, L. S. Kashevarova, A. V. Rakhmanina, Nature 413, 716 (2001).
2. For a cautionary note, see J. S. Miller, Adv. Mater. 4, 435 (1992).
3. M. Tamura et al., Chem. Phys. Lett. 186, 401 (1991).
4. P.-M. Allemand, K. C. Khemani, A. Koch, R. Wudl, K. Holczer, S. Donovan, G. Gruener, J. D. Thompson, Science 253, 301 (1991).
5. D. P. Young, D. Hall, M. E. Torelli, Z. Fisk, J. L. Sarrao, J. D. Thompson, H.-R. Ott, S. B. Oseroff, R. G. Goodrich, R. Zysler, Nature 397, 412 (1999).
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