The roles of co-doping ions (Li, Ga and Cu) and defects (oxygen vacancy and hydrogen impurity) in magnetic interactions in ZnO:Co systems were studied systematically using an ab initio method with density functional theory and the standard molecular field model. The results showed that where defects were not included in ZnO's lattice carrier mediated magnetism was only achievable in shallow p-type co-doping, such as ZnO:Co + Cu. However, in deep p-type co-doping (ZnO:Co + Li) and deep n-type co-doping (ZnO:Co + Ga), the carriers generally did not induce spontaneous magnetism. It was also found that the oxygen vacancy, due to its deep donor nature, had a minor favouring effect on ferromagnetic ordering among Co ions. The observed ferromagnetism in such systems could be attributed to the interaction of Co ions with unintentional hydrogen contamination rather than co-dopants or oxygen vacancies.

Predominant Role of Defects in Magnetic Interactions in Codoped ZnO:Co. M.H.N.Assadi, Y.B.Zhang, S.Li: Journal of Physics - Condensed Matter, 2010, 22[29], 296004