  We have studied the structural, magnetic and electronic properties of
Co-implanted ZnO (0001) films grown on Al2O3 (1120) substrates for different
implantation doses and over a wide temperature range. Strong room temperature
ferromagnetism is observed with magnetic parameters depending on the cobalt
implantation dose. A detailed analysis of the structural and magnetic
properties indicates that there are two magnetic phases in Co-implanted ZnO
films. One is a ferromagnetic phase due to the formation of long range
ferromagnetic ordering between implanted magnetic cobalt ions in the ZnO layer,
the second one is a superparamagnetic phase, which occurs due to the formation
of metallic cobalt clusters in the Al2O3 substrate. Using x-ray resonant
magnetic scattering, the element specific magnetization of cobalt, oxygen and
Zn was investigated. Magnetic dichroism was observed at the Co L2,3 edges as
well as at the O K edge. In addition, the anomalous Hall effect is also
observed, supporting the intrinsic nature of ferromagnetism in Co-implanted ZnO
films.
