High cluster formation tendency in Co implanted ZnO

ZnO(0001) single crystals have been implant doped with a maximum of 5 at. % of Co at low temperatures. While as-implanted crystals do not show ferromagnetic properties, postimplantation annealing leads to the transformation of the implanted Co ions into small metallic clusters giving rise to a pronounced hysteresis upon magnetization reversal. The dispersed Co ions are in 2+ oxidation state. Positive magnetoresistance could be observed at low temperatures.