Magnetic Nanocarbon


T. L. Makarova Institute of Physics, Umeå University

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Almost without exception, carbon modifications—graphite, diamond, fullerenes, and carbon nanotubes—are diamagnetic.

Diamagnetism of diamond is described in a simple model based on magnetic contributions of localized electrons: two Langevin diamagnetic terms, χ c and χ v, arising from the core and valence electrons, respectively, and a Van Vleck paramagnetic susceptibility χ vv. The Van Vleck term results from virtual magnetic dipole transitions between the valence band and the conduction band. The absence of Landau diamagnetism, χ L, and Pauli paramagnetism, χ P, is due to the absence of the states at the Fermi level. The magnetic moment of diamond is temperature independent.

Graphite has an anisotropic diamagnetic susceptibility. If the magnetic field is perpendicular to the basal plane, the graphite susceptibility is larger than for any other substance, excluding superconductors. The in-plane susceptibility is very small as it is close to the diamagnetic susceptibility of a carbon atom. For graphite, only one contribution to χ, namely the large and anisotropic orbital contribution of valence electrons χ v, is usually considered in theoretical approaches. Pauli paramagnetism in ideal graphite should be absent.

The electronic properties of graphite planes of finite size (graphenes) differ radically from those of bulk graphite. Calculating magnetic susceptibility of graphenes, one must take into account not only the sum of χ c, χ v and χ vv, but also the Pauli paramagnetism χ P, Landau diamagnetism χ Lof conduction electrons and the Curie paramagnetism χ C. The diamagnetic susceptibility of graphene planes of a finite size is primarily controlled by the concentration of charge carriers, and this concentration in turn depends on structural defects. Three types of defects in graphite can be recognized provisionally: edges of the planes, porosity, and so-called topological defects (Gaussian curvature).