Evidence of structural defect enhanced room-temperature ferromagnetism in Co-doped ZnO.

We report experimentally that room-temperature ferromagnetism of Co-doped ZnO is strongly correlated with structural defects, and that carriers involved in carrier-mediated exchange are by nature by-products of the creation of the defects. On the other hand, the carrier concentration has 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 are observed. Furthermore, high-temperature annealing results in evolution of the local Co structure from Co(2+) replacing Zn(2+) to a Co(3)O(4)-based phase, which most likely accounts for the transition from ferromagnetism to paramagnetism in the Co:ZnO films.