Ferromagnetism in inhomogeneous Zn1-xCoxO thin films

We report on a systematic study of structural, optical, electrical, and magnetic properties of Zn1-xCoxO (x=0.05-0.29) thin films codoped with Al (< 0.1%). Both codoped (in which Co is cosputtered with other elements) and delta-doped (in which Co is doped digitally in the host matrix) samples have been prepared and studied. Prior to doping of Co, growth conditions were optimized to produce ZnO:Al films with a resistivity of about 1.3 m Omega cm. Although all the films with x in the range of 0.05-0.29 showed clear hysteresis at room temperature in magnetometry measurement and absorption peaks associated with the d-d transitions of Co2+ ions, only the most heavily doped samples have shown clear anomalous Hall effect. The latter also showed strong, but photon energy dependent, magnetic circular dichroism and negative magnetoresistance at room temperature. These results in combination with detailed structural analysis by transmission electron microscope and x-ray diffraction study revealed that the ferromagnetic properties of Zn1-xCoxO were mostly originated from secondary phases and Co precipitates. The influence of inhomogeneity on the interpretation of various measurement results is also discussed. (c) 2006 American Institute of Physics.