Lack of ferromagnetism in n-type cobalt-doped ZnO epitaxial thin films

Epitaxial thin films of cobalt-doped ZnO (Co : ZnO) were deposited by pulsed laser deposition (PLD) on both c-plane and r-plane sapphire (Al2O3). The films exhibited high structural quality with narrow x-ray diffraction (XRD) rocking curve peak widths. X-ray absorption spectroscopy (XANES and EXAFS) confirmed well-ordered Co substitution for Zn in ZnO without the formation of secondary phases. A wide range of n-type conductivities (10(-4)-10(5) Omega cm) was achieved by controlling the deposition conditions, post-annealing in vacuum, and or addition of Al during deposition. Despite the high structural quality of the Co : ZnO thin films, no significant room temperature ferromagnetism was observed under any processing or treatment conditions. The lack of ferromagnetism indicates that itinerant conduction band electrons alone are not sufficient to induce ferromagnetism in Co : ZnO, even when the carrier concentration is a significant fraction of the magnetic dopant concentration. The implications of this observation are discussed.