The hypothesis that high-Curie-temperature ferromagnetism in cobalt-doped ZnO (${\mathrm{Co}}^{2+}\mathrm{\text{:}}\mathrm{ZnO}$) is mediated by charge carriers was tested by controlled introduction and removal of the shallow donor interstitial zinc. Using oriented epitaxial ${\mathrm{Co}}^{2+}\mathrm{\text{:}}\mathrm{ZnO}$ films grown by chemical vapor deposition, kinetics measurements demonstrate a direct correlation between the oxidative quenching of ferromagnetism and the diffusion and oxidation of interstitial zinc. These results demonstrate controlled systematic variation of a key parameter involved in the ferromagnetism of ${\mathrm{Co}}^{2+}\mathrm{\text{:}}\mathrm{ZnO}$ and, in the process, unambiguously reveal this ferromagnetism to be dependent upon charge carriers. The distinction between defect-bound and free carriers in ${\mathrm{Co}}^{2+}\mathrm{\text{:}}\mathrm{ZnO}$ is discussed.

• Received 18 May 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.037203