It was found experimentally that the room-temperature ferromagnetism of Co-doped ZnO was strongly related with structural defects, and that the carriers involved in carrier-mediated exchange were natural by-products of the creation of the defects. On the other hand, the carrier concentration had pronounced effects upon the absorption edge in the optical transmission spectra of epitaxial Co:ZnO films, in which both a blue-shift and a red-shift were observed. High-temperature annealing resulted in an evolution of the local Co structure, from Co2+ replacing Zn2+, to a Co3O4-based phase. This was thought to account for the transition from ferromagnetism to paramagnetism in Co:ZnO films.

Evidence of Structural Defect Enhanced Room-Temperature Ferromagnetism in Co-Doped ZnO. C.Song, S.N.Pan, X.J.Liu, X.W.Li, F.Zeng, W.S.Yan, B.He, F.Pan: Journal of Physics - Condensed Matter, 2007, 19[17], 176229 (8pp)