Structure and magnetic property of (Li, Co)-Codoped ZnO nanocrystals

Zn_0.95-xCo_0.05Li_xO (x=0~0.09) nanocrystals were synthesized by sol-gel method. Detailed characterization by XRD, UV-Vis absorption spectrum, and vibration sample magnetometer (VSM) indicated that Li-Co co-doping did not change the Wurtzite structure of ZnO when xle;7%. With the increase of Li-dopant, the lattice constants a and c decreased owing to the substitution of Zn²⁺ by Li⁺ ions, the ferromagnetism increased gradually. However, the ferromagnetism droped sharply when Li-dopant increased to 9%. At the same time, second-phase clusters of LiCoO₂ were detected in the XRD pattern and the lattice constants almost stopped decreasing. Experimental results suggested that the reaction of Li incorporation could be divided into 3 stages. Li'_Zn deep-level defects and Li_i⁺ interstitials were induced into the material at the former two stages respectively, while the second-phase LiCoO₂ formed at the third stage. These defects and second-phase structures induced by Li-doping played important roles in enhancing the ferromagnetism of the samples, which could be explained by the bound magneton polaron (BMP) model.