Ferromagnetism in Graphene Stacks

Because the density of states at the Fermi level of neutral graphene layers is proportional to external magnetic field, the ground state in strong fields is expected to have a broken symmetry - most likely ferromagnetism. In systems with stacked graphene layers this tendency competes with inter-layer hopping which favors paramagnetic ground states or perhaps other types of broken symmetries. We present a criterion for the stablility of the ferromagnetic state and discuss its application to single-layer graphene, to weakly coupled epitaxial graphene layers on SiC or other substrates, and to bulk graphite. We use the Slonczewksi-Weiss-McClure model to explain why the dominant inter-layer hopping process in Bernal (AB) stacked graphite does not compete with ferromagnetism.