21 - McConnel Model for the Magnetism of C60-based Polymers

Abstract

This chapter presents the McConnel model for the magnetism of C60-based polymers. The s–p magnetism observed in carbon-based materials as well as in other organic and nontraditional inorganic materials is a consequence of strong e–e correlation effects. The latter lead to ground states that can be described as a superposition of states of various spin-multiplicities and manifest themselves as FM-configurations when the high-spin-multiplicity states are dominant. In the ferromagnetic charge transfer salts, described by the McConnell model, such a possibility arises when either the acceptor or donor molecules exhibit a triplet ground state. The donor and/or the acceptor sites can be considered as the main sources that induce lone pair electron orbitals and provide unpaired spins. The delocalization may be global, or it may extend over a small number of either trivalent or tetravalent C-atoms. The latter type of delocalization is analogous to that appearing in alternant hydrocarbons in which the unpaired electrons are distributed over only the active carbon atoms. It is found that the generalized McConnell ferromagnetism is facilitated by the presence of structural and topological defects, which develop the necessary charge transfers and, associated with them, exchange interaction pathways.