Nonferromagnetic nanocrystalline ZnO:Co thin films doped with Zn interstitials

ZnCoO thin films were synthesized via a wet chemical route and subsequently annealed in Zn vapor to increase the conductivity by introducing Zn interstitials. All samples show small hysteresis loops close to the detection limit of the magnetometer. Thus the samples were thoroughly investigated to obtain evidence for further ferromagneticlike behavior. Optical and magneto-optical experiments show the crystal field transitions of $Co2+$ in the near infrared and visible spectral range. At energies above 2.8 eV a charge transfer transition of $Co2+$ is observed. The results of magnetotransport measurements are explained by the formation of an impurity band situated below the conduction band. No further evidence for ferromagnetism is obtained.