  The nature of the often reported room temperature ferromagnetism in
transition metal doped oxides is still a matter of huge debate. Herein we
report on room temperature ferromagnetism in high quality Co-doped ZnO
(Zn1-xCoxO) bulk samples synthesized via standard solid-state reaction route.
Reference paramagnetic Co-doped ZnO samples with low level of structural
defects are subjected to heat treatments in a reductive atmosphere in order to
introduce defects in the samples in a controlled way. A detailed structural
analysis is carried out in order to characterize the induced defects and their
concentration. The magnetometry revealed the coexistence of a paramagnetic and
a ferromagnetic phase at room temperature in straight correlation with the
structural properties. The saturation magnetization is found to increase with
the intensification of the heat treatment, and, therefore, with the increase of
the density of induced defects. The magnetic behavior is fully explained in
terms of the bound magnetic polaron model. Based on the experimental findings,
supported by theoretical calculations, we attribute the origin of the observed
defect-induced-ferromagnetism to the ferromagnetic coupling between the Co ions
mediated by magnetic polarons due to zinc interstitial defects.
