  We investigated ZnO(0001) single crystals annealed in high vacuum with
respect to their magnetic properties and cluster formation tendency after
implant-doping with Fe. While metallic Fe cluster formation is suppressed, no
evidence for the relevance of the Fe magnetic moment for the observed
ferromagnetism was found. The latter along with the cluster suppression is
discussed with respect to defects in the ZnO host matrix, since the crystalline
quality of the substrates was lowered due to the preparation as observed by
x-ray diffraction.
