18 – Ferromagnetic and Superconducting Instabilities in Graphite

This chapter outlines the ferromagnetic and superconducting instabilities in graphite. A structural disorder, topological defects, or adsorbed foreign atoms can be responsible for the occurrence of both ferromagnetic and superconducting patches in graphitic structures. It is suggested that the ferromagnetism in microporous carbon and nominal rh-C 60 samples has a common origin, possibly associated with fullerene-like fragments. There exists both experimental and theoretical evidence that fullerene-like fragments, with positive and/or negative curvature, should be a common feature of microporous carbons. Such fragments should naturally appear in rh-C6o samples with partially destroyed C6o molecules. It is found that an adsorption followed by a desorption of oxygen gas at the graphite surface is responsible for the appearance and vanishing of the ferromagnetism, respectively. It is observed that as the applied field increases, the normal state orbital diamagnetism of graphite overcomes the paramagnetic contribution to the magnetization resulting in a negative total magnetization. The results indicate that the superconducting state is highly anisotropic and is associated with the graphite planes.