The effect of annealing time and gas flow rate on optical and magnetic properties of (Al,Co) co-doped ZnO nanoparticles by varying Al concentration

The effect of Al, a trivalent substitution was investigated in Co doped ZnO with regards to the role of additional carriers in n-type magnetic semiconductors. Nanoparticle samples of Zn1?xCo0.04AlxO with different Al compositions (x = 0, 0.005, 0.010, 0.015 and 0.020) were annealed at 600 oC for different time (5 hours and 8 hours) and different gas flow rates (25 sccm and 50 sccm). All nanoparticles attained Wurtzite hexagonal structure in a stable form after annealing in forming gas. The effect of annealing time and gas pressure were investigated. Magnetization measurements revealed that air annealed samples displayed paramagnetic properties. Nevertheless, after annealing in forming gas, magnetic moments are stabilized and follow a monotonic trend. Diffuse reflectivity measurements revealed the incorporation of Co2+ ions into the Wurtzite hexagonal lattice. The optical band-gap was found to be increasing with increasing Al concentration of up to 0.01, followed by a decrease with further increase in Al concentration