Preparation of the CrO 2 thin films using a Cr 8 O 21 precursor
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Preparation of the CrO2 thin films using a Cr8O21 precursor Y. Muraoka1,2, K. Iwai1, S. Yoshida1, T. Wakita1, M. Hirai1,2, T. Yokoya1,2, Y. Kato3, T. Muro3, and Y. Tamenori3 1
Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan 2 Faculty of Science, Research Laboratory for Surface Science, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan 3 Japan Synchrotron Radiation Research Institute (JASRI)/SPring-8, 1-1-1 Kouto, Sayo, Hyogo, 679-5148, Japan
ABSTRACT We have prepared a CrO2 thin film by chemical vapor deposition from a Cr8O21 precursor and studied the bulk and surface physical properties. The CrO2 thin film is grown on TiO2(100) substrate by heating precursor and TiO2 (100) substrate together in a sealed quartz tube. The prepared film is found from x-ray diffraction analysis to be an (100)-oriented single phase. The magnetization and resistivity measurements indicate that the film is a ferromagnetic metal with a Curie temperature of about 400 K. Cr 3s core-level and valence band photoelectron spectroscopy spectra reveal the presence of a metallic CrO2 in the surface region of the film. Our work indicates that preparation from a Cr8O21 precursor in a closed system is promising for obtaining a CrO2 thin film with the metallic surface. INTRODUCTION Chromium dioxide (CrO2) has attracted considerable attention because it is ferromagnetic at room temperature (TC = 393 K) and a half-metallic band structure fully spin-polarized at the Fermi level [1]. Several classes of materials are predicted to have this half-metallic ferromagnetic property, including the Heusler alloys, magnetites, Fe3O4. Among them, CrO2 is presently the only material with experimentally proven spin polarization close to 100 % as measured Andreev reflection and Meservey-Tedrow tunneling [2,3]. These results are motivation to use this material in spintronics devices such as magnetic field sensing and information storage. It is known that CrO2 can be produced in the form of thin film by thermal decomposition of CrO3 (extremely hygroscopic and toxic) using chemical vapor deposition (CVD). High quality CrO2 films have been deposited on single-crystal rutile (TiO2) and sapphire (Al2O3) substrates. However, there is a serious problem about a surface of the films. Insulating layer like Cr2O3, which is the most stable phase in Chromium oxide binary system, is easily formed on a surface of CrO2 thin film [4]. Such an insulating layer prevents us to develop the spintronics devices using half-metallic nature for applications. Thus it is highly required to prepare CrO2 thin films without any insulating layer up to the surface of the film.
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Recently, Ivanov et al. reported that epitaxial CrO2 thin films are successfully prepared from Cr8O21 precursor [5]. Since Cr8O21 is less hygroscopic and less toxic compared with CrO3, it would be a more appropriate precursor. In addition, they found from low-energy electron diffraction observations that epitxial growth of
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