Ferromagnetic semiconductor single-wall carbon nanotubes

The possibility of a ferromagnetic semiconductor single-wall carbon nanotube (SWCNT), where ferromagnetism is due to coupling between a doped magnetic impurity on a zigzag SWCNT and electron spin, is investigated. We found in the weak impurity-spin couplings, at low impurity concentrations, the spin up electron density of states remains semiconducting while the spin down electron density of states shows a 2D semi-metallic graphene behaviour. By increasing impurity concentrations the semiconducting gap of spin up electrons in the density of states is closed, hence a semiconductor to semi-metallic disordered graphene phase transition takes place. In contrast, for the case of strong coupling, the spin up electron density of states remains semiconducting and that for spin down electrons has a semi-metallic 2D graphene behaviour. Also by increasing the magnitude of the spin impurity, the semiconducting gap of spin up electrons is increased.