Magnetic and optical properties of Co-doped ZnO nanocrystalline particles

We carried out a systematic study on the effect of Co doping on the structural, magnetic and optical properties of ZnO nanocrystalline particles, using x-ray diffraction, x-ray photoelectron spectroscopy (XPS), Quantum Design PPMS-9 magnetometry, and Ultra Violet-Visible spectroscopy. The Zn1-xCoxO nanoparticles with 0≤x≤0.1 were successfully prepared by the formal solid-state reaction method. The XPS results and the XRD analysis with full structural Rietveld refinement reveals that all Zn1-xCoxO nanocrystalline particles in this Co-doping range have hexagonal wurtzite structure. Co atoms have been successfully incorporated in tetrahedral sites and no secondary phases or Co rich clusters were detected. For all Co-doped ZnO nanoparticles under investigation, the field dependence of the magnetization curves exhibits ferromagnetic behavior with relatively small coercive fields at room temperature. The coercive fields increased with Co-doping and decreased with temperature, as expected. In addition, we found a signature for antiferromagnetic ordering between the Co ions. We also observed a strong correlation between the magnetic and optical behavior of the Co-doped ZnO nanoparticles. Optical diffuse reflectance and absorption spectra exhibit a red shift at room temperature in the absorption band edge with increasing Co-doping. The red shift is attributed to the sp-d exchange interaction between free charge carriers in ZnO band and the localized magnetic moments.