  Possibility of a ferromagnetic semiconductor single wall carbon nanotube
(SWCNT), where ferromagnetism is due to coupling between doped magnetic
impurity on a zigzag SWCNT and electrons spin, is investigate. We found, in the
weak impurity-spin couplings, at low impurity concentrations the spin up
electrons density of states remain semiconductor while the spin down electrons
density of states shows a metallic behavior. By increasing impurity
concentrations the semiconducting gap of spin up electrons in the density of
states is closed, hence a semiconductor to metallic phase transition is take
place. In contrast, for the case of strong coupling, spin up electrons density
of states remain semiconductor and spin down electron has metallic behavior.
Also by increasing impurity spin magnitude, the semiconducting gap of spin up
electrons is increased.
