Structural stability and magnetic properties of Co-doped or adsorbed polar-ZnO surface

The structural stability and magnetic properties of Co-doped or -adsorbed polar-ZnO surface systems are investigated using first-principles calculations. It is found that Co ions energetically prefer to substitute for Zn in the outermost layer of both Zn- and O-terminated surfaces. Substitutional Co ions are antiferromagnetically coupled at ideal ZnO surfaces, which indicates additional defects or strain played a critical role in the reported ferromagnetism of ZnO : Co system. In addition, it is found that the adsorbed Co ions are preferred to ferromagnetic coupling, which may provide a new approach for spin injection of a ZnO film.