Optical studies of pure and (Cu, Co) doped nickel zinc ferrite films deposited on quartz substrate

In this work, the optical properties of pure and doped films were investigated as a function of annealing temperature. Films with compositions Ni_0.5Zn_0.5Fe₂O₄, Ni_0.35Cu_0.2Zn_0.45Fe₂O₄, and Ni_0.35Co_0.2Zn_0.45Fe₂O₄ were deposited on quartz substrate using the sol–gel method. The grown films were annealed at 500 and 800 °C in a rapid thermal annealing furnace. The single-phase spinel structure of these films was confirmed by x-ray diffraction (XRD) results. The average crystallite size calculated from the XRD data was observed to increase with the annealing temperature and decrease for films doped with Cu and Co. The lattice constant was observed to decrease with the annealing temperature and increase for films doped with Cu and Co. The cross-sectional images obtained from field emission scanning electron microscope were used to calculate the thickness of these films. Ultraviolet-visible spectroscopy was used to obtain the absorbance spectra as a function of wavelength in the range of 200–800 nm. The bandgap obtained from the absorbance spectra was seen to decrease for films annealed at higher temperatures for pure and doped films. Furthermore, the bandgap of doped films was seen to decrease in comparison to that of pure films. Optical parameters such as refractive index, extinction coefficient, optical conductivity, and real and imaginary parts of the dielectric constant were observed to increase with the reduction in the bandgap.