We have investigated the magnetic properties of Co-doped ZnO films on glass substrates prepared by sputtering. Annealing and chemical doping are performed to study the correlation between magnetism, conductivity, and defects. While as-deposited Co-doped ZnO film shows a paramagnetic behavior, the room-temperature ferromagnetism is induced by annealing in a ${\text{H}}_2$ atmosphere and is suppressed by annealing in air, indicating oxygen vacancy has strong relevance to the observed ferromagnetism. The ferromagnetism is also changed by impurity doping. The $s\left(p\right)\text{−}d$ exchange interaction has been investigated by magnetotransport and magnetic circular dichroism (MCD) measurements. Large positive magnetoresistance at low temperatures and linear responses of both Hall effect and MCD to magnetic field suggest that the $s\text{−}d$ exchange interaction gives rise to paramagnetic influence on electronic properties. Aggregation of a Co rich ZnO phase and a nanoscaled Co metal may give rise to the ferromagnetic behavior.

• Received 28 November 2007

DOI:https://doi.org/10.1103/PhysRevB.78.104417