Defect induced room temperature ferromagnetism and optical properties of (Co, Y) co-doped ZnO nanoparticles

In this work, pristine zinc oxide (ZnO), Co2+ doped zinc oxide (ZCO) and Co-Y co-doped zinc oxides (ZCYO) are synthesized by sol-gel method. To increase the oxygen vacancy, the as-prepared samples are annealed in vacuum furnace. Refinement of the recorded X-ray diffractograms (XRD) shows that all the samples have grown in single hexagonal phase with space group P63mc and the nanocrystallites are grown with average sizes lie in the nanometer regime (10-29 nm). Fourier-transform infrared spectroscopy (FTIR) and photoluminescence (PL) spectroscopy could not detect any secondary phase except the desired one. Interestingly, room temperature ferromagnetism (RTFM) is observed in Co-Y co-doped sample where maximum magnetization is quite high (similar to 0.34 emu/g) compared with undoped ZnO. Absence of any metal cluster in ZCYO by XRD and optical analyses ruled out the ordering due to clustering. Thus origin of RTFM in ZCYO is intrinsic in nature. The defect mediated RTFM observed in ZCYO nanoparticles due to exchange interaction between dopants ions is explained by the oxygen vacancy together with the presence of the increase of surface defects due to co-doping of Co ion and non-magnetic Y ion.