Available online 24 April 2008.
The crystal structure of reactive sputtered MnxTi1−xO2 films turns from anatase to rutile as x increases. All the films are ferromagnetic, with a Curie temperature above 340 K. Vacuum annealing enhances the ferromagnetism of the films, but O2 annealing weakens it, indicating that the ferromagnetism is related to the oxygen-vacancy defects created by Mn+2 dopants at Ti+4 cations. The average room-temperature moment per Mn decreases from 0.482 μB at x = 0.026 to 0.078 μB at x = 0.375. Meanwhile, the optical band gaps value decreases linearly from 3.35 eV at x = 0 to 1.73 eV at x = 0.375, suggesting that Mn ions substitute for Ti ions uniformly and the ferromagnetism is not from magnetic Mn oxide impurities. The high-temperature ferromagnetism makes the MnxTi1−xO2 films useful for the applications in spintronic devices.
Keywords: Compound semiconductors; Magnetic thin films; Magnetic properties; Vacancies; Optical transmission
Fig. 1. XRD patterns of as-deposited MnxTi1−xO2 films with different x: (a) 0, (b) 0.026, (c) 0.048, (d) 0.066 and (e) 0.176. The inset is the XRD pattern of Mn0.066Ti0.934O2 films annealed in vacuum at 500 °C for 2 h.
Fig. 2. M–H curves of as-deposited MnxTi1−xO2 films with x = 0.066 at different temperatures. The insets are the ZFC and FC curves of the films with x = 0.066 at 500-Oe field (bottom right) and the variation of moment per Mn with x (top left).
Fig. 3. Typical Mn2p XPS spectra for as-deposited MnxTi1−xO2 films with different x.
Fig. 4. Room-temperature M–H curves of as-deposited and 500 °C annealed MnxTi1−xO2 films with x = 0.066.
Fig. 5. Transmission spectra of MnxTi1−xO2 films with different x.
Fig. 6. Dependence of on photon energy for MnxTi1−xO2 films with different x. The inset gives the derived optical band gap values.
Lattice parameters calculated from the XRD patterns of anatase and rutile phases in the MnxTi1−xO2 films