Organic magnetism hots up
Oct 18, 2001
Physicists have shown for the first time that pure carbon can be magnetic at room temperature. Tatiana Makarova of the Ioffe Physico-Technical Institute in St Petersburg and co-workers accidentally stumbled across the magnetic behaviour while searching for signs of superconductivity in polymerized carbon-60. The discovery could eventually lead to metal-free magnets that are cheaper and lighter than their metallic counterparts (T Makarova et al 2001 Nature 413 716).
The magnetic properties of electron-doped carbon-60 compounds have intrigued physicists since they were first reported in 1991. Ferromagnetism has previously been observed in a handful of other organic materials, but only at very low temperatures. Indeed, the highest temperature previously reported for an organic magnet is 65 kelvin for a sulphur-based compound under a pressure of 16 kilobar.
Pure carbon-60 exists in a crystalline state in which the isolated molecules are bound together by weak van der Waals forces. But under high pressures, the material can transform into polymers where the molecules are held together tightly by covalent bonds. The exact crystal structure of these polymers depends on the pressure and temperature applied.
Makarova and co-workers from Russia, Sweden, Germany and Brazil prepared a series of samples under different conditions and found that only the two-dimensional "rhombohedral" phase is ferromagnetic. In this phase, the covalently bonded carbon-60 molecules are arranged in highly oriented layers like graphite. The magnetic characteristics persisted up to 500 kelvin ? well above room temperature. The team even demonstrated that the magnetization of the organic magnets is strong enough for a small magnet to lift them off a surface.
Makarova and colleagues were careful to protect the carbon-60 crystals from magnetic impurities, such as iron, nickel and cobalt, and believe that the results can only be explained by the intrinsic properties of the carbon-60 polymer. Although the origin of the ferromagnetism remains a mystery, the team speculates that structural defects or the creation of unpaired electrons during the polymerization process might be the answer. The group now plans to investigate samples prepared under different conditions in greater detail to determine the precise cause of the magnetic behaviour.
About the author
Valerie Jamieson is Features Editor of Physics World