Magnetic circular dichroism (MCD) measurements of cobalt-doped titanium dioxide films*

Cobalt-doped TiO$_2$ has generated interest as a dilute magnetic oxide displaying room-temperature ferromagnetism with $T_c \ge 650\,$K for low-doped materials. However, controversy surrounding the mechanism for such a high $T_c$ and the observation of Co clusters cast doubts on this system as an intrinsic ferromagnetic oxide. A recent study\footnote{S. R. Shinde \textit{et al.}, Phys. Rev. B \textbf{67}, 115211 (2003).} reporting the importance of growth conditions on Co solubility confirms the existence of ferromagnetism in films showing no direct evidence of clustering. MCD offers promise as a technique to characterize the intrinsic nature of magnetism and probe the band structure. A sensitive heterodyne technique using a photoelastic modulator measures MCD in the visible frequency range at magnetic fields up to 1.5\,T. We report a comparison of MCD measurements on thin films of well-oxygenated anatase Ti$_{1-x}$Co$_x$O$_{2-\delta}$ ($x\le 0.07$) exhibiting no clustering with those on clustered films. Additionally, we compare MCD results to optical absorption measurements,\footnote{J. R. Simpson \textit{et al.}, Phys. Rev. B \textbf{69}, 193205 (2004).} which reveal a shift of the band edge upon cobalt doping and an absence of mid gap cobalt impurity states.