  Here we present a study of magnetism in \CTO\ anatase films grown by pulsed
laser deposition under a variety of oxygen partial pressures and deposition
rates. Energy-dispersive spectrometry and transition electron microscopy
analyses indicate that a high deposition rate leads to a homogeneous
microstructure, while very low rate or postannealing results in cobalt
clustering. Depth resolved low-energy muon spin rotation experiments show that
films grown at a low oxygen partial pressure ($\approx 10^{-6}$ torr) with a
uniform structure are fully magnetic, indicating intrinsic ferromagnetism.
First principles calculations identify the beneficial role of low oxygen
partial pressure in the realization of uniform carrier-mediated ferromagnetism.
This work demonstrates that Co:TiO$_2$ is an intrinsic diluted magnetic
semiconductor.
