We report systematic investigations of structural, vibrational, resonance and magnetic properties of nanoscale NiO powders prepared by ball milling process under different milling speeds for 30 hours of milling. Structural properties revealed that both pure NiO and as-milled NiO powders exhibit face centered cubic structure, but average crystallite size decreases to around 11 nm along with significant increase in strain with increasing milling speed. Vibrational properties show the enhancement in the intensity of one-phonon longitudinal optical (LO) band and disappearance of two-magnon band due to size reduction. In addition, two-phonon LO band exhibits red shift due to size-induced phonon confinement effect and surface relaxation. Pure NiO powder exhibit antiferromagnetic nature, which transforms into induced ferromagnetic after size reduction. The average magnetization at room temperature increases with decreasing the crystallite size and a maximum moment of 0.016 μB/f.u. at 12 kOe applied field and coercivity of 170 Oe were obtained for 30 hours milled NiO powders at 600 rotation per minute milling speed. The change in the magnetic properties is also supported by the vibrational properties. Thermomagnetization measurements at high temperature reveal a well-defined magnetic phase transition at high temperature (TC) around 780 K due to induced ferromagnetic phase. Electron paramagnetic resonance (EPR) studies reveal a good agreement between the EPR results and magnetic properties. The observed results are described on the basis of crystallite size variation, defect density, large strain, oxidation/reduction of Ni and interaction between uncompensated surfaces and particle core with lattice expansion. The obtained results suggest that nanoscale NiO powders with high TC and moderate magnetic moment at room temperature with cubic structure would be useful to expedite for spintronic devices.
ZnO thin films were deposited onto glass substrates using sol-gel spin coating technique from starting solutions having different precursor concentrations (0?1, 0?3 and 0?5M). The effects of precursor concentration and annealing temperature on the physical properties of the films were investigated. The X-ray diffraction studies confirm that all the films have preferential orientation along the (002) plane with hexagonal wurtzite structure. The optical transparency gradually decreases (from 95 to 80%) as the precursor concentration increases. The optical energy gap is in the range of 3?18-3?32 eV. The systematic study shows that the post-annealing process has significant impact on the quality of the films. The SEM images depict that the grain size decreases as the precursor concentration increases, and the AFM images show that the film annealed at 550uC has well defined uniform grains. The lowest dislocation density is observed for 0?3M annealed at 550uC.
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