Influence of reducing anneal on the ferromagnetism in single crystalline Co-doped ZnO thin films

This paper reports that the high-quality Co-doped ZnOsingle crystalline films have been grown on $a$-plane sapphiresubstrates by using molecular-beam epitaxy. The as-grown films showhigh resistivity and non-ferromagnetism at room temperature, whilethey become good conductive and ferromagnetic after annealing in thereducing atmosphere either in the presence or absence of Zn vapour.The x-ray absorption studies indicate that all Co ions in thesesamples actually substituted into the ZnO lattice without formattingany detectable secondary phase. Compared with weak ferromagnetism(0.16~$mu _{rm B}$/Co$^{2 + })$ in the Zn$_{0.95}$Co$_{0.05}$Osingle crystalline film with reducing annealing in the absence of Znvapour, the films annealed in the reducing atmosphere with Zn vapourare found to have much stronger ferromagnetism (0.65~$mu _{rmB}$/Co$^{2 + })$ at room temperature. This experimental studiesclearly indicate that Zn interstitials are more effective thanoxygen vacancies to activate the high-temperature ferromagnetism inCo-doped ZnO films, and the corresponding ferromagnetic mechanism isdiscussed.