  Unexpected ferromagnetism has been observed in carbon doped ZnO films grown
by pulsed laser deposition [Phys. Rev. Lett. 99, 127201 (2007)]. In this
letter, we introduce carbon into ZnO films by ion implantation. Room
temperature ferromagnetism has been observed. Our analysis demonstrates that
(1) C-doped ferromagnetic ZnO can be achieved by an alternative method, i.e.
ion implantation, and (2) the chemical involvement of carbon in the
ferromagnetism is indirectly proven.
