  Pulsed laser deposited films of Co doped anatase TiO2 are examined for Co
substitutionality, ferromagnetism, transport, magnetotransport and optical
properties. Our results show limited solubility (up to ~ 2 %) of Co in the
as-grown films and formation of Co clusters thereafter. For Ti0.93Co0.07O2-d
sample, which exhibits a Curie temperature (Tc) over 1180 K, we find the
presence of 20-50 nm Co clusters as well as a small concentration of Co
incorporated into the remaining matrix. After being subjected to the high
temperature anneal during the first magnetization measurement, the very same
sample shows a Tc ~ 650 K and almost full matrix incorporation of Co. This Tc
is close to that of as-grown Ti0.99Co0.01O2-d sample (~ 700 K). The transport,
magnetotransport and optical studies also reveal interesting effects of the
matrix incorporation of Co. These results are indicative of an intrinsic
Ti1-xCoxO2-d diluted magnetic semiconductor with Tc of about 650-700 K.
