Understanding ferromagnetism in graphite

Ferromagnetism usually arises from transition metals rich in 3d and 4f electrons. The occurrence of ferromagnetism in pure carbon, which contains only s and p electrons, is thus surprising—even controversial, given the weakness of the magnetic signal and a Curie temperature well above room temperature. Using magnetic force microscopy and a superconducting quantum interference device to probe the surface and bulk magnetization of graphite, Dutch researchers Jiri Červenka and Kees Flipse (Eindhoven University of Technology) and Mikhail Katsnelson (Radboud University) offer evidence that the ferromagnetism arises from a two-dimensional network of point defects at grain boundaries. The breaking of the lattice’s translational symmetry by the defects leads to localized electron states at the Fermi level. Because of electron-electron interactions, those states become polarized, which, in turn, leads to the formation of local magnetic moments. Grain-boundary defects are more complicated than single vacancies: The figure here shows 2D planes of periodic defects, each an extended zigzag discontinuity that propagates through individual graphene sheets of the bulk crystal. A magnetic moment can be associated with each defect; and the step edge at the surface is a manifestation of the grain boundary buried underneath it. The Curie temperature deduced from experiment is, reassuringly, comparable to the theoretical value based on weak interlayer coupling. (J. Červenka, M. I. Katsnelson, C. F. J. Flipse, Nat. Phys. 5 , 840, 2009 http://dx.doi.org/10.1038/nphys1399 .)