  We report on the structural and magnetic properties of a cobalt-implanted ZnO
film grown on a sapphire substrate. X-ray diffraction and transmission electron
microscopy reveal the presence of a (10-10)-oriented hexagonal Co phase in the
Al2O3 sapphire substrate, but not in the ZnO film. Co clusters, with a diameter
of is about 5-6 nm, form a Co rich layer in the substrate close to the
ZnO/Al2O3 interface. Magnetization measurements indicate that there exist two
different magnetic phases in the implanted region. One originates from the Co
clusters in Al2O3, the other one belongs to a homogeneous ferromagnetic phase
with a ferromagnetic Curie temperature far above room temperature and can be
attributed to Co substitution on Zn sites in the ZnO layer. We have observed
magnetic dichroism at the Co L2,3 and O K edges at room temperature as well as
the multiplet structure in x-ray absorption spectra around the Co L3 edge,
supporting the intrinsic nature of the observed ferromagnetism in Co-implanted
ZnO film. The magnetic moment per substituted cobalt is found about 2.81 Bohr
magneton which is very close to the theoretical expected value of 3 Bohr
magneton per Co atom for Co 2+ in its high spin state.
