Defect and dopant induced room temperature ferromagnetism in Ni doped ZnO nanoparticles

ZnO nanoparticles were doped with Ni by simple chemical solution deposition method. All the synthesized nanoparticles crystallize in the wurtzite ZnO structure. The UV-Visible absorption measurement shows broad absorption band in the visible range around 440 nm, 618 nm and 660 nm which are d-d transitions characteristic of Ni2+ within the tetrahedral symmetry. Compared to higher doping concentration, 1% Ni doped sample shows higher magnetic moment at room temperature with a saturation magnetization of 0.0123 emu/g. The coercivity of 365 Oe for the 1% doped sample reduces to 217 Oe for 5% Ni doped sample. Fourier Transform Infrared Spectroscopy (FTIR) measurement shows the consistent formation of the characteristic E2 mode in ZnO even after Ni doping and is supported by the Raman measurements. The incorporation of Ni dopant in the ZnO host lattice is further confirmed by X-ray Photoelectron Spectroscopy (XPS) measurement. The prominent visible luminescence in the undoped sample is quenched due to doping as observed from luminescence measurements. Magnetic measurements were also done in the doped samples after annealing. Systematic studies on structural, optical & magnetic measurements reveal that the nature of defects as well as Ni2+ ions are responsible for the magnetic moment obtained at room temperature.