Sol-gel synthesis and behaviour of nickel containing ZnO nanoparticles

An attempt has been made to synthesize nickel (up to 5 at%) containing ZnO diluted magnetic semiconductors by sol-gel process using zinc acetate dihydrate, nickel acetate tetrahydrate and oxalic acid as precursors and ethanol as solvent. The process comprises of gel formation, digestion at 80 degrees C for 24 h to obtain oxalate and calcination at 400 degrees C or above for 2 h for decomposition to oxide. The resulting powders correspond to wurtzite hexagonal structure similar to that of pure ZnO and contain nanoparticles of average diameter decreasing with (i) decrease of calcination temperature and (ii) rise in nickel content (range being 13-29 nm). Evidence has been advanced to suggest occupancy of Ni2+ ions in Zn2+ sites (i.e., in the centre of O2- tetrahedra) without any secondary phase. Further, the first nearest neighbor average distance of Ni2+ ions vary inversely as cube root of nickel content ('x'). Optical absorption peaks are attributed to transitions from 3T1 (F) to 3A2(F), 3T1(P) and 3T2(F) states of Ni2+ ion (3d8 configuration). The bifurcation in magnetization versus temperature curves under zero field cooled (ZFC) and field cooled (FC) conditions indicates the presence of magnetic nanoparticles with the blocking temperature (TB) decreasing with increase in nickel content; typical values being 70 and 30 K for 1 and 5 at% of Ni2+ containing ZnO, respectively. The observations of (i) hysteresis loops both at 10 and 298 K and (ii) shift in electron spin resonance (ESR) spectra from the free electron position enunciate ferromagnetic nature of Zn(1-x)Ni(x)O nanoparticles at room temperature.