Comprehensive microstructures of 7% cobalt-doped rutile TiO2 thin films grown on c-plane sapphire by pulsed laser deposition were characterized using transmission electron microscopy (TEM). The effects of oxygen pressure during growth on the Co distribution inside the films were investigated, and the detailed growth mechanism of both TiO2 and TiO2 + Co was discussed. The similar oxygen sublattices and low mismatch between (100) rutile and c-plane sapphire favors the rutile phase. However, the three-fold symmetry of the substrate surface resulted in three rutile domain orientation variants, and they grow adjacent to each other. Cobalt was found to precipitate out as nanocrystals inside the TiO2 matrix as the growth pressure of oxygen was decreased. At 0.05 mTorr oxygen pressure, almost all of the Co segregates into crystallographically aligned nanocrystals with a particle size of 4.4 +/- 0.15 nm. All the samples have magnetic coercivity at room temperature. The magnetic moment per Co atom increased with decreased oxygen pressure, suggesting that the Co that replaced the Ti2+ in the TiO2 lattice does not have a large magnetic moment. (c) 2006 Elsevier B.V. All rights reserved.