Effect of Co doping on the structural, optical and magnetic properties of ZnO nanoparticles

J. Hays, K. M. Reddy, N. Y. Graces, M. H. Engelhard, V. Shutthanandan, M. Luo, C. Xu, N. C. Giles, C. Wang, S. Thevuthasan, A. Punnoose

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    Abstract

    We report the results of a detailed investigation of sol-gel-synthesized nanoscale Zn1-xCoxO powders processed at 350 °C with 0≤x≤0.12 to understand how the structural, morphological, optical and magnetic properties of ZnO are modified by Co doping, in addition to searching for the theoretically predicted ferromagnetism. With x increasing to 0.03, both lattice parameters a and c of the hexagonal ZnO decreased, suggesting substitutional doping of Co at the tetrahedral Zn2+ sites. For x>0.03, these trends reversed and the lattice showed a gradual expansion as x approached 0.12, probably due to additional interstitial incorporation of Co. Raman spectroscopy measurements showed a rapid change in the ZnO peak positions for x>0.03, suggesting significant disorder and changes in the ZnO structure, in support of additional interstitial Co doping possibility. Combined x-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance spectroscopy, photoluminescence spectroscopy and diffuse reflectance spectroscopy showed clear evidence for tetrahedrally coordinated high-spin Co2+ ions occupying the lattice sites of ZnO host system, which became saturated for x>0.03. Magnetic measurements showed a paramagnetic behaviour in Zn1-xCo xO with increasing antiferromagnetic interactions as x increased to 0.10. Surprisingly, a weak ferromagnetic behaviour was observed for the sample with x ≤ 0.12 with a characteristic hysteresis loop showing a coercivity Hc∼350Oe, 25% remanence Mr, a low saturation magnetization Ms∼0.04emug-1 and with a Curie temperature Tc∼540K. The XPS data collected from Zn 1-xCoxO samples showed a gradual increase in the oxygen concentration, changing the oxygen-deficient undoped ZnO to an excess oxygen state for x ≤ 0.12. This indicates that such high Co concentrations and appropriate oxygen stoichiometry may be needed to achieve adequate ferromagnetic exchange coupling between the incorporated Co2+ ions.

    Original languageEnglish
    Article number266203
    JournalJournal of Physics Condensed Matter
    Volume19
    Issue number26
    DOIs
    Publication statusPublished - 4 Jul 2007

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    ASJC Scopus subject areas

    • Materials Science(all)
    • Condensed Matter Physics

    Cite this

    Hays, J., Reddy, K. M., Graces, N. Y., Engelhard, M. H., Shutthanandan, V., Luo, M., Xu, C., Giles, N. C., Wang, C., Thevuthasan, S., & Punnoose, A. (2007). Effect of Co doping on the structural, optical and magnetic properties of ZnO nanoparticles. Journal of Physics Condensed Matter, 19(26), [266203]. https://doi.org/10.1088/0953-8984/19/26/266203