Effect of Calcination on Structural and Magnetic Properties of Co-Doped ZnO Nanostructures

Diluted magnetic semiconductors have gained much interest in spintronics due to the involvement of both spin and charge. Among various diluted magnetic semiconductors, ZnO lightly doped with transition metals (Mn, Fe, Co, Ni, In, V, and Cr) has gained interest due to the presence of room-temperature ferromagnetism. Among various dopants, cobalt is a potential candidate because its ionic radius (Co²⁺ = 0.58 Å) is extremely close to zinc (Zn²⁺ = 0.6 Å). Zinc acetate dihydrate and cobalt nitrate are used as precursor materials. Nanostructures are synthesized at a low temperature of 70 °C. The dopant concentration is varied by 1, 3, 5, 7, and 9 wt.%. XRD results indicate the formation of hexagonal wurtzite structure even under as-synthesized conditions. Crystallite size decreased from 22.7 to 17.8 nm as dopant concentration was increased in the range 0-7 wt.%. XRD of the samples with 7 wt.% Co concentration is calcined in the temperature range of 100 °C-500 °C. Scanning electron microscope images show nanostructures with a grain size less than 50 nm. Saturation magnetization increases from 5.07 to 6.4 emu/g as dopant concentration was increased to 7 wt.% under as synthesized conditions. Nanostructures calcined at 400 °C show the highest saturation magnetization of 9.9 emu/g. Magnetic hysteresis equivalent to that of multilayered structure is obtained after calcination of Co-doped ZnO nanoparticles at 300 °C. A single layer of these synthesized Co-doped ZnO can be used instead of complex structures in spintronic devices.