Pulsed laser deposited films of Co-doped anatase ${\mathrm{TiO}}_2$ 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 the ${\mathrm{Ti}}_{0.93}{\mathrm{Co}}_{0.07}{\mathrm{O}}_{2-\delta }$ sample, which exhibits a Curie temperature ${(T}_C)$ 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 $T_C\sim 650\mathrm{K}$ and almost full matrix incorporation of Co. This $T_C$ is close to that of as-grown ${\mathrm{Ti}}_{0.99}{\mathrm{Co}}_{0.01}{\mathrm{O}}_{2-\delta }$ 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 ${\mathrm{Ti}}_{1-x}{\mathrm{Co}}_x{\mathrm{O}}_{2-\delta }$ diluted magnetic semiconductor with $T_C$ of about 650–700 K.

• Received 3 December 2002

DOI:https://doi.org/10.1103/PhysRevB.67.115211