  The origin of room temperature (RT) ferromagnetism (FM) in Ni doped ZnO
samples are systematically investigated through physical, optical, and magnetic
properties of nanostructure, prepared by simple low-temperature wet chemical
method. Reitveld refinement of X-ray diffraction pattern displays an increase
in lattice parameters with strain relaxation and contraction in Zn/O occupancy
ratio by means of Ni-doping. Similarly scanning electron microscope
demonstrates modification in the morphology from nanorods to nanoflakes with Ni
doping, suggests incorporation of Ni ions in ZnO. More interestingly, XANES
(X-ray absorption near edge spectroscopy) measurements confirm that Ni is being
incorporated in ZnO as Ni2+. EXAFS (Extended X-ray Absorption Fine Structure)
analysis reveals that structural disorders near the Zn sites in the ZnO samples
upsurges with increasing Ni concentration. Raman spectroscopy exhibits
additional defect driven vibrational mode at 275 cm-1, appeared with Ni-doped
sample only and the shift with broadening in 580 cm-1 peak, which manifests the
presence of the oxygen vacancy (VO) related defects. Moreover, in
photoluminescence (PL) spectra we observed peak appears at 524 nm, indicates
the presence of singly ionized VO+, which may activate bound magnetic polarons
(BMPs) in dilute magnetic semiconductors (DMSs). Magnetization measurements
indicate weak ferromagnetism at RT, which rises with increasing Ni
consolidation. It is therefore proposed that effect of the Ni-ions as well as
the inherent exchange interactions rising from VO+ assist to produce BMPs,
which are accountable for the RT-FM in Zn1-xNixO (0<x<0.125) system.

Materials Science