Role of Co clusters in wurtzite Co:ZnO dilute magnetic semiconductor thin films

The magnetic nature of Zn1-xCoxO dilute magnetic semiconductor (DMS) thin films grown by pulsed laser deposition is investigated by x-ray absorption fine structure spectroscopy and x-ray diffraction. We show that a single phase of the substitutional Co atoms occupied Zn sites in the ZnO matrix exists in the Zn0.98Co0.02O DMS thin film while a secondary phase of the Co clusters is formed in Zn0.95Co0.05O and Zn0.90Co0.10O thin films. Despite the formation of Co clusters, the average magnetic moment M-S per Co atom is sharply decreased with increasing Co concentration, which suggests that the small Co clusters are superparamagnetic. For the Zn0.98Co0.02O DMS thin film, the local structural distortion around the substitutional Co atoms is interpreted as the origin of intrinsic weak room-temperature ferromagnetism.