  Cobalt and silver co-doping has been undertaken in ZnO thin films grown by
pulsed laser deposition in order to investigate the ferromagnetic properties in
ZnO-based diluted magnetic materials and to understand the eventual relation
between ferromagnetism and charge carriers. Hall transport measurements reveal
that Ag doping up to 5% leads to a progressive compensation of the native
n-type carriers. The magnetization curves show ferromagnetic contributions for
all samples at both 5 K and room temperature, decreasing with increasing the Ag
concentration. First principles modeling of the possible configurations of
Co-Ag defects suggest the formation of nano-clusters around interstitial Co
impurity as the origin of the ferromagnetism. The Ag co-doping results in a
decrease of the total spin of these clusters and of the Curie temperature.
