Ferromagnetism of Fe implanted ZnO – a phenomenon related to defects?

ZnO(0001) single crystals annealed in high vacuum have been investigated with respect to their magnetic properties. Annealing at mild temperatures up to 773 K leads to ferromagnetic properties without any transition metal doping. Fe ion implantation at low temperatures leads to similar results. However, no evidence for the relevance of the Fe magnetic moment for the generation of ferromagnetism was observed. Nevertheless, the degradation of the magnetization with time is much less pronounced for the implanted crystals with respect to the purely annealed samples.