  Magneto-optical spectroscopy of a transparent ferromagnetic semiconductor,
anatase TiO2 doped with Co, is carried out at room temperature. A large
magneto-optical response with ferromagnetic field dependence is observed
throughout from ultraviolet to visible range and increases with increasing Co
content or carrier concentration. The magnitude of magnetic circular dichroism
(MCD) per unit thickness has a peak around the absorption edge such a huge
value of ~10400 degree/cm at 3.57 eV for a 10 mol% Co-doped specimen. Although
the results are not sufficient to prove that the ferromagnetism is in the
ordinary framework of diluted magnetic semiconductors, the coexistence of Co
impurity and mobile carrier is shown to transform the band structure of host
TiO2 to generate ferromagnetism.
