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. (a) Magnetization hysteresis curves of the Zn1-xCoxO sample with x = 0.05 measured at 300 K and 150 K. The inset shows the low field region of the magnetization hysteresis curve for the same sample with x = 0.05 measured at 300 K. (b) ZFC and FC magnetizations of Zn1-xCoxO with x = 0.05 as a function of temperature measured at 100 Oe. (c) The Tauc plots, (F(R)*hυ)2 versus hυ, of Zn1-xCoxO nanoparticles with different Co doping. (d) The energy gap as a function of Co concentration (x). [Display omitted] •A systematic study was done on the effect of Co-doping on ZnO physical properties.•The magnetic properties were studied in a wide range of temperatures and fields.•We observed a ferromagnetic behavior at room temperature in all Co-doped ZnO.•We found a strong correlation between the magnetic and the optical behavior.•A clear explanation of the magnetic and optical behaviors was introduced.