It was shown experimentally that the room-temperature ferromagnetism of Co-doped ZnO was strongly correlated with structural defects, and that carriers involved in carrier-mediated exchange were by nature by-products of the creation of the defects. On the other hand, the carrier concentration had pronounced influences on the absorption edge in optical transmission spectra of epitaxial Co:ZnO films, in which both a blue-shift and a red-shift were observed. Furthermore, high-temperature annealing results in evolution of the local Co structure from Co2+ replacing Zn2+ to a Co3O4-based phase, which most likely accounts for the transition from ferromagnetism to paramagnetism in the Co:ZnO films.

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