There has been considerable debate regarding the origin of ferromagnetism in the ferromagnetic semiconductor anatase phase TiO2 doped with Co, ranging from carrier induced ferromagnetism in a framework of diluted magnetic semiconductor to merely the precipitation of ferromagnetic and metallic Co precipitates in the TiO2 matrix. This paper reports on a systematic investigation of the structures, together with the electronic and magneto-optical properties, of thin films of anatase phase TiO2, doped with Co. Films of anatase Ti1-xCoxO2 (x = 0-0.10) were grown epitaxially on LaSrAlO4 (001) substrates, using pulsed laser deposition method. In order to control n-type carrier concentration, the oxygen pressure during the growth was systematically varied, yielding films with insulative, semiconductive (carrier concentration n∼1×1017 cm-3), and metallic (n∼2×1019 cm-3) properties. Reflection high energy electron diffraction patterns, atomic force microscopy, and transmission electron microscopy show no evidence of Co segregation in these films. Magneto-optical response was examined by measuring magneto-optical circular dichroism (MCD) spectra at room temperature. A large MCD peak appears at a photon energy close to the band gap of TiO2. The magnetic field dependence of MCD signal indicates ferromagnetic behavior, which is similar to that of the magnetization. The MCD intensity increases systematically with the increase of n or x. Since MCD response originates from charge carriers at the band edge, one can conclude that interaction between the charge carriers and Co impurities is essential to realize ferromagnetism.
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