Role of cobalt in room-temperature ferromagnetic Co-doped ZnO thin films

A series of Co-doped ZnO thin films were prepared under various deposition conditions using the pulsed laser deposition method. X-ray photoelectron spectroscopy (XPS) and XPS depth profiling were used to detect the elemental valence states of Zn, Co and O. It was found that the films deposited under low temperature and high oxygen pressure exhibited intrinsic ferromagnetic properties due to oxidation of Co (Co2+) from the material. However, when the films were deposited under high temperature and low oxygen pressure, metallic cobalt (Co0) appeared and the ferromagnetism was greatly enhanced.