Influence of working pressure on the crystal structural and photo-magneto-electric properties of TiO₂ thin films

Intrinsic TiO₂ films were prepared by RF-magnetron sputtering at different working pressures (0.2 Pa-4.0 Pa). All as-deposited intrinsic TiO₂ films show hysteresis behavior from 2 K to 340 K, and the largest room temperature (RT) saturation magnetization (M_s) is 1.96 emu/cm³ for the amorphous TiO₂ film deposited at 2.0 Pa. From the X-ray diffraction (XRD) patterns of TiO₂ films, we observe that the rutile and anatase crystal structures coexist in the TiO₂ films at 0.2 Pa. With the working pressure increasing, the transformation between the rutile crystalline and anatase crystalline appears. When the working pressure is increased to 2.0 Pa, the crystalline state of TiO₂ film changes from polycrystalline to amorphous. Using the high resolution transmission electron microscopy (HRTEM), we can observe the crystalline transformation by the electron diffraction pattern and HRTEM image clearly. The transmission spectra of TiO₂ films are measured by the spectrophotometer, the visible (VIS) transmittance of TiO₂ films is more than 85%, the absorption edge shifts to the longer wavelength with the amorphous transformation, and the E_g decreases to 3.14 eV, so the increasing structural defects play an important role in the ferromagnetism of intrinsic TiO₂ films. In addition, all of as-deposited samples are high-resistance state and the resistivity is higher than 10⁷ Ω·cm. We can rule out the effect of the carrier-mediated RKKY coupling on the system ferromagnetism.