Co-metal clustering as the origin of ferromagnetism in Co-doped ZnO thin films

The origin of ferromagnetism in ZnO-based systems was investigated using Co-doped ZnO thin films as prototypical examples of II-VI-based diluted magnetic semiconductors. In spite of the atomic-scale dissolution of Co ions in wurtzite ZnO, both the magnetization-temperature curve and the magnetization-field curve demonstrated that Zn1-xCoxO thin films were paramagnetic for xless than or equal to0.12. On the other hand, Zn1-xCoxO films with x greater than 0.12 were characterized by the Co-metal clustering and apparently showed room-temperature ferromagnetism. The discrepancy between the zero-field cooling and the field cooling curves further indicates that Co-doped ZnO films (for x>0.12) are superparamagnetic and the observed ferromagnetism originates from the nanometer-sized Co clusters. (C) 2004 American Institute of Physics.