R. Suryanarayanan

Raman spectroscopic studies of oxygen defects in Co-doped ZnO films exhibiting room-temperature ferromagnetism

The optical properties, Raman spectra, and magnetization of Zn1−xCoxOxa0(0≤x<0.1) thin films are reported. Optical transmission spectra confirm the substitution of Co2+ cations for Zn2+ ions at the tetrahedral sites of ZnO. Raman spectra of films annealed in air show an additional broad vibrational mode, attributed to disorderedxa0–Zn–O–Co–xa0local vibrations. These modes disappear on vacuum annealing, suggesting the formation of oxygen vacancies close to the dopant sites (–Zn––Co–). Magnetization measurements show that air-annealed films lack significant moment, whereas vacuum-annealed samples with intermediate Co concentrations (0.012<x<0.05) exhibit room-temperature ferromagnetism. Using the framework of the bound magnetic polaron model, the polaron size is estimated to be roughly 7.8xa0Axa0for this system.

Room temperature ferromagnetism in Cr-doped In2O3 on high vacuum annealing of thin films and bulk samples

We report on the observation of room temperature ferromagnetism in Cr-doped In2O3 bulk samples and spin-coated thin films. The samples showed a clear ferromagnetism above 300K with magnetic moments of 0.008 and 0.22μB∕Cr at 300K for the bulk and thin film, respectively, only after high vacuum (HV) annealing at 600°C. The vacuum annealed Cr-doped In2O3 thin films showed a typical semiconducting behavior with a room temperature resistivity of 0.73Ωcm, while bulk samples were more conducting (23mΩcm). We present systematic investigations on the influence of HV annealing on the carrier concentrations, resistivity, and magnetic properties of the samples.

Room temperature ferromagnetism in spin-coated anatase- and rutile-Ti0.95Fe0.05O2 films

Thin films of Ti1?xFexO2?(x = 0 and 0.05) have been prepared on sapphire substrates by the spin-on technique starting from metal?organic precursors. When heat treated in air at 550 and 700??C respectively, these films present pure anatase and rutile structures as shown both by x-ray diffraction and Raman spectroscopy. Optical absorption indicates a high degree of transparency in the visible region. Such films show a very small magnetic moment at 300?K. However, when the anatase and the rutile films are annealed in a vacuum of 1 ? 10?5?Torr at 500 and 600??C respectively, the magnetic moment, at 300?K, is strongly enhanced, reaching 0.46??B/Fe for the anatase sample and 0.48??B/Fe for the rutile one. The ferromagnetic Curie temperature of these samples is above 350?K. When the ferromagnetic rutile sample is reheated in air, the magnetic moment reduces strongly. The data seem to indicate that oxygen defects created as a result of vacuum annealing may be responsible for the observed ferromagnetism in our samples.Thin films of Ti1-xFexO2 (x=0 and 0.05) have been prepared on sapphire substrates by spin-on technique starting from metal organic precursors. When heat treated in air at 550 and 700 °C respectively, these films present pure anatase and rutile structures as shown both by X-ray diffraction and Raman spectroscopy. Optical absorption indicate a high degree of transparency in the visible region. Such films show a very small magnetic moment at 300 K. However, when the anatase and the rutile films are annealed in a vacuum of 1x10-5 Torr at 500 °C and 600 °C respectively, the magnetic moment, at 300 K, is strongly enhanced reaching 0.46 µB/Fe for the anatase sample and 0.48 µB/Fe for the rutile one. The ferromagnetic Curie temperature of these samples is above 350 K.

Surface ferromagnetism and exchange bias in vacuum annealed Co3−yZnyO4 films

We report on the structural and the magnetic properties of spin coated Co3−yZnyO4 thin films. The microstructure of high vacuum annealed films exhibit a crystalline Zn:CoO core with a disordered oxygen deficient amorphous Co:ZnO surface. Weak ferromagnetism (m∼15mμB∕Co) and an exchange bias (HE∼500Oe) are observed at 10K despite the absence of any Co metal clusters. HE is smaller for samples with smaller Zn fractions (for Co3O4-HV, HE∼250Oe). Adding Zn significantly enhances HE. The exchange anisotropy in an antiferromagnetic material due to surface disordered weak ferromagnetism and its overall implications to dilute magnetic semiconducting oxides are discussed.

Magnetism in Zn1−xCoxO (0⩽x<0.1) and Co3−yZnyO4 (y=0, 0.25, and 1) thin films

We have investigated the properties of homogeneous spinel Co3−yZnyO4 (y=0, 0.25, and 1) and wurtzite CoxZn1−xO (0<x<0.1) films. Air annealed Co3−yZnyO4 films were found to develop only a small magnetic moment (∼10−3μB∕Co). Conversely, these samples exhibited greatly enhanced magnetic moments after vacuum annealing. The vacuum annealed films reveal the presence of Zn doped CoO, but no detectable Co metal clusters. When field cooled to 10K, the hysteresis curves of the vacuum annealed samples are displaced along the magnetization axis, which we attribute to uncompensated surface spins.

Structural, magnetic, and electrical properties of spin coated ilmenite-pseudobrookite xFeTiO3-(1-x)Fe2O3 thin films

We report on the structural, magnetic, optical, and electrical properties of iron titanate thin films prepared using a spin-coating technique having nominal compositions of FeTiO3 and Fe1.4Ti0.6O3. X-ray diffraction measurements show clear evidence for the presence of both ilmenite and pseudobrookite crystal structures, which is confirmed using Raman spectroscopy. X-ray photoemission spectroscopy studies indicate that Fe is present mainly in the 2+u2009valence state, with some Fe3+ present on the more heavily oxidized surface of the films. The as-prepared samples exhibit weak ferromagnetism at room temperature. While the valence state of Fe is not significantly affected by vacuum annealing, the saturation magnetization is increased dramatically, reaching nearly 220u2009emu mole−1 for the Fe-rich film. The optical band gap was found to be roughly 2.0u2009eV for all samples, with negligible changes on vacuum annealing.We report on the structural, magnetic, optical, and electrical properties of iron titanate thin films prepared using a spin-coating technique having nominal compositions of FeTiO3 and Fe1.4Ti0.6O3. X-ray diffraction measurements show clear evidence for the presence of both ilmenite and pseudobrookite crystal structures, which is confirmed using Raman spectroscopy. X-ray photoemission spectroscopy studies indicate that Fe is present mainly in the 2+u2009valence state, with some Fe3+ present on the more heavily oxidized surface of the films. The as-prepared samples exhibit weak ferromagnetism at room temperature. While the valence state of Fe is not significantly affected by vacuum annealing, the saturation magnetization is increased dramatically, reaching nearly 220u2009emu mole−1 for the Fe-rich film. The optical band gap was found to be roughly 2.0u2009eV for all samples, with negligible changes on vacuum annealing.