Synthesis And Magnetic Properties Of Half-metal CrO₂ Films

Theoretical calculations predicted that chromium dioxide (CrO₂) is a half- metal ferromagnet. And its half metallicity has been confirmed experimentally. Due to its high spin polarization, CrO₂ thin films have drawn a great deal of interest. On the other hand, chromium sesquioxide (Cr₂O₃), the thermodynamically stable oxide of chromium has also been widely investigated because of its optical and antiferromagnetic properties. In this paper, the fabrication and physical properties of these two metallic oxide thin films were studied.Epitaxial and polycrystalline CrO₂ thin films have been grown on single crystal Al2O3 substrates and alumina films separately by the atmospheric pressure chemical vapor deposition (CVD) method. The effects of the growing conditions on the thin films deposition and the magnetic properties of the CrO₂ films were investigated. Epitaxial Cr₂O₃ thin films were also achieved on single crystal Al2O3 substrates by CVD method, and its optical properties were studied by UV-visible transmission spectra.The effects of the growing conditions on the deposition of CrO₂ thin films on single crystal Al2O3 substrates were investigated. It's found that the substrate temperature, oxygen flux and substrate orientation could influence the fabric and quality of the thin films. Firstly, high quality and epitaxial CrO₂ thin films could be obtained at the substrate temperature of about 380 Celsius degrees. Below or above this temperature a complex mixture film of chromium oxide or epitaxial Cr₂O₃ thin films will be obtained. Secondly, the oxygen flux was also important for the thin films deposition. It's found that an oxygen flux of 100ml/min is suitable for the formation of high quality and epitaxial CrO₂ thin films. Below this Cr₂O₃ thin films will be formed. While above this the quality of the CrO₂ thin films will be bad. Finally, under the proper substrate temperature and oxygen flux, the influence of the substrate orientation on the growth of the CrO₂ thin films was studied. (101) and (200) oriented CrO₂ thin films were formed on (012)and (006) oriented single crystal Al2O3 substrates separately.The morphology of the CrO₂ thin films was investigated by Atomic Force Microscopy (AFM) and Scanning Electron Microscope (SEM). It's found that the films are quite uniform and the surface is smooth. The root-mean-square (RMS) surface roughness of the films was about 2.6nm. This indicates that high quality epitaxial CrO₂ thin films have been prepared on single crystal Al2O3 substrates by the atmospheric pressure chemical vapor deposition. The investigation also showed that the deposition rate would slow down after long time deposition.The magnetic properties of CrO₂ thin films were investigated by the Vibrating Sample Magnetometer (VSM). The CrO₂ thin films show in-plane anisotropy. When the magnetic field was parallel to the surface of the films, the coercive force of (101) orientation film was 40Oe, while it was 105Oe for (200) orientation film. The coercive force will decrease with the increase in thickness of the films.The growth of polycrystalline CrO₂ thin films on alumina films was also studied. The alumina film has ordered nano dents on the surface. Polycrystalline CrO₂ thin films can be easily deposited on this substrate by same CVD method. The structure, morphology and magnetic properties of the film were investigated too.High quality epitaxial Cr₂O₃ thin films were also synthesized and studied. AFM micrographs indicated that the surfaces of the films were quite smooth, and the root-mean-square (RMS) surface roughness of the film was about 1.9nm. UV-visible transmission spectra indicated that the transmission of the film was about 70% in the range of visible light. The calculated value of the optical bandgap energy for the Cr₂O₃ thin film was about 3.14 eV. It's in accord with other's results.