Variation of structural and magnetic properties with Co-doping in ZN _1-xCo_xO nanocrystals

The ZN_1-xCo_xO (x=0-0.20) nanocrystals synthesized via sol-gel method have been systematically investigated by using x-ray diffraction, transmission electronic microscopy (TEM), Raman spectrometer (Raman), UV-vis spectrometer (UV-vis), and vibrating sample magnetometer. It is found that the crystal lattice constant (a) and the cell volume of ZN _1-xCo_xO are irrespective of the Co composition x when x<0.05, while it expands gradually as x increasing from 0.05 to 0.2. This result is different from the existent reports, which indicated the lattice constant (a) and the cell volume both increase consecutively with Co-doping. Detailed Raman, UV-vis, and TEM study suggest the expansion of lattice constant when x0.05 is attributed to the formation of theCo₃ O4 phase in ZN_1-xCo_xO due to the self-purification mechanism in nanocrystals growth process. The formation of the antiferromagneticCo ₃ O4 well explains the decrease in ZN_1-xCo_xO ferromagnetism with the increase in Co-doping when x0.05.