Enhanced electron-mediated ferromagnetism in Co-doped ZnO nanowires

We perform density functional calculations to investigate the magnetic coupling properties of Co-doped ZnO nanowires (NWs). The ferromagnetism of NWs is strongly affected by the position of the minority Co t(a) levels and their population that is controlled by additional electron doping. While the antiferromagnetic state is energetically more favorable than the ferromagnetic state in carrier-free NWs, electron doping greatly enhances the stability of ferromagnetism. Compared with bulk ZnO, the minority t(a) levels relative to the conduction band edge have a tendency to decrease with decreasing of the wire diameter, indicating that electron concentrations to achieve the ferromagnetism are much reduced. The short-range nature of the magnetic coupling between two Co ions suggests that sufficiently high doping levels of the Co ions are needed to yield ferromagnetic NWs. (C) 2010 American Institute of Physics. [doi:10.1063/1.3463412]