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

We report the results of a detailed investigation of sol - gel- synthesized nanoscale Zn1-xCoxO powders processed at 350 degrees 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-xCoxO 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 H-c similar to 350 Oe, 25% remanence M-r, a low saturation magnetization M-s similar to 0.04 emu g- 1 and with a Curie temperature T-C similar to 540 K. The XPS data collected from Zn1-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.