A study was made of the optical and electrical properties of oxygen-vacancy (VO) related defects in cobalt-doped ZnO materials grown using molecular beam epitaxy. The concentration of VO was probed in samples with various cobalt contents, that were annealed under different conditions, and was correlated with the magnetic properties of the samples. The results suggested that the oxygen vacancy was a major factor in the observed ferromagnetism of Co-doped ZnO.

Ferromagnetism and Point Defects in ZnCoO. Seghier, D., Gislason, H.P.: Physica Scripta, 2012, T148, 014008