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XAFS and CEMS study of dilute magneto-optical semiconductor, Fe doped TiO2 films Kiyoshi Nomura a,⁎, Hiromi Eba b, Kenji Sakurai b, Alexandre Rykov a,c, Tetsuya Hasegawa d a School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b National Institute for Material Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan c Siberian Synchrotron Radiation Center, Lavrent'eva 11, Novosibirsk, 630090, Russia d School of Science, The University of Tokyo,7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Available online 6 April 2007 Abstract TiO2 films doped with 6% Fe were prepared by pulsed laser deposition (PLD) under different oxygen pressures, and characterized by X-ray absorption fine spectra (XAFS) and conversion electron Mössbauer spectra (CEMS). The edge energy and spectrum profiles of Fe– and Ti K X-ray absorption showed only Fe3+ and Ti4+ states for rutile TiO2 films prepared under 10−1 Torr, the metallic Fe and Ti4+ for rutile TiO2 films prepared in 10− 6 Torr, and the metallic Fe and the average valance of less than “4+” for Ti in TinO2n−x films prepared by the PLD under 10−8 Torr. The metallic Fe clusters are also found in the TEM images of TinO2n−x film. Magnetic property of Fe doped TiO2 films prepared by PLD at high vacuum (10− 6 and 10−8 Torr) is considered to originate mainly from the magnetic metal iron clusters. © 2007 Elsevier B.V. All rights reserved. Keywords: Fe doped TiO2 films; Dilute magneto-optical semiconductor (DMS); Ti K X-ray; Fe K X-ray; EXAFS; XANES; Mössbauer spectra 1. Introduction It has been found recently that TiO2 films doped with a small amount of Co show dilute magneto-optical semiconductor (DMS) properties at room temperature [1]. DMS material are prospective to develop new spintronics devices such as spin field effect transistor. The ferromagnetic properties of Ti1−x 57 FexO2 increase with the decrease of doping 57Fe content [2]. Fe doped rutile TiO2−δ films show the ferromagnetism at room temperature. The resistivity shows nearly metallic behavior at room temperature but semiconducting at low temperature [3]. Fe doped TiO2 films prepared on γ-Al2O3 substrate at the temperatures from 600 to 675 °C by pulsed laser deposition (PLD) under oxygen pressure of PO2=10−6 Torr also show the ferromagnetism and Kerr effect strongly [4]. Magnetic domain structures were observed in Fe doped TiO2 films prepared in 10−6–10−8 Torr, suggesting the presence of long range ordering of magnetic moment induced by Fe doping in these thin films. These films were characterized by 57Fe conversion electron Mössbauer spectroscopy (CEMS) in order to clarify the chemical states of iron doped into TiO2 films [5]. We found three kinds of iron species in the TiO2 films. A doublet of paramagnetic Fe3+ (isomer shift, δ=0.37(2) mm/s and quadrupole splitting, Δ=0.9(2) mm/s) was observed for transparent TiO2 films prepared under the low-vacuum condition of 10− 1 Torr. Two sextets with the larger magnetic field (Bhf=33.0 T) and the smaller magnetic field (Bhf=29.5 T) were observed for TiO2 films prepared under high-vacuum pressure of 10−6 and 10−8 Torr. The purpose of this paper is to clarify more the chemical states of Fe-doping species and host Ti ions in these oxides. Therefore, we measured Fe- and Ti- K X-ray absorption fine structure (XANES and EXAFS) of these films as prepared and annealed, using synchrotron radiation. Fe clusters in these oxides were also observed by transmisson electron microscopes (TEM). These observations consist with the results of CEMS. 2. Experimental details Samples No. 1, No. 2, and No.3 were as prepared on γ-Al2O3 substrate heated at 650 °C by PLD (KrF laser wave length: 248 nm, output power: 5 J/cm2 pulse, pulse interval:6 ns, pulse Thin Solid Films 515 (2007) 8649–8652 www.elsevier.com/locate/tsf ⁎ Corresponding author. Fax: +81 3 5841 6017. E-mail address: k-nomura@t-adm.t.u-tokyo.ac.jp (K. Nomura). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.03.177 |
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