Anomalous Hall effect in sputter-deposited Co_xTi_1-xO_2-δ films

Hall effect measurements were performed on epitaxial Co_xTi_1-xO_2-δ thin films grown on (0 0 1) LaAlO₃ by reactive RF magnetron co-sputter deposition. Magnetization measurements reveal ferromagnetic behavior in M-H loop at room temperature for Co_xTi_1-xO_2-δ thin films for which x≥0.02. An anomalous Hall effect was observed for Co_0.10Ti_0.90O_2-δ films grown with the partial pressure of water P(H₂O)=4×10^-4 Torr or less. These films exhibit a positive ordinary Hall coefficient and a positive magnetoresistance. X-ray diffraction on films grown under these conditions shows evidence for Ti_nO_2n-1 phase due to the deficiency of oxygen. In contrast, Hall measurements taken for undoped and Co-doped TiO₂ thin films grown under more oxidizing conditions show only the ordinary Hall effect with a negative Hall coefficient consistent with n-type conduction. For these films, the magnetoresistance was positive and increased monotonically with increasing magnetic field. The results suggest that Co-doped Ti_nO_2n-1 may be a dilute magnetic semiconducting oxide for which the carriers couple to the spin polarization.