Magnetic susceptibility of carbon-experiment and theory

Carbon exists in many forms, crystalline and amorphous, with different electronic structures. These polymorphs are usually diamagnetic in the pure, defect-free state but their susceptibility spans more than two orders of magnitude. Large diamagnetic susceptibilities, found in nanotubes, and in graphite with the field applied perpendicular to the graphene sheets, are temperature-dependent and exhibit quantum oscillations at low temperatures, which are related to the distribution of conduction electrons over the Landau levels. In the case of nanotubes, the curvature of the tube acts like an effective magnetic field, limiting the thermal population to just the two lowest Landau levels at room temperature, which explains the field and temperature-dependence of the diamagnetism. Reports of ferromagnetism in some forms of carbon are briefly discussed.