Effect of Ni2+ substitution on structural, magnetic, dielectric and optical properties of mixed spinel CoFe2O4 nanoparticles

Nanoparticles of Co1−xNixFe2O4 with x=0.0, 0.10, 0.20, 0.30, 0.40 and 0.50 were synthesized by co-precipitation method. The structural analysis reveals the formation of single phase cubic spinel structure with a narrow size distribution between 13–17 nm. Transmission electron microscope images are in agreement with size of nanoparticles calculated from XRD. The field emission scanning electron microscope images confirmed the presence of nano-sized grains with porous morphology. The X-ray photoelectron spectroscopy analysis confirmed the presence of Fe2+ ions with Fe3+. Room temperature magnetic measurements showed the strong influence of Ni2+ doping on saturation magnetization and coercivity. The saturation magnetization decreases from 91 emu/gm to 44 emu/gm for x=0.0–0.50 samples. Lower magnetic moment of Ni2+ (2 µB) ions in comparison to that of Co2+ (3 µB) ions is responsible for this reduction. Similarly, overall coercivity decreased from 1010 Oe to 832 Oe for x=0.0–0.50 samples and depends on crystallite size. Cation distribution has been proposed from XRD analysis and magnetization data. Electron spin resonance spectra suggested the dominancy of superexchange interactions in Co1−xNixFe2O4 samples. The optical analysis indicates that Co1−xNixFe2O4 is an indirect band gap material and band gap increases with increasing Ni2+ concentration. Dispersion behavior with increasing frequency is observed for both dielectric constant and loss tangent. The conduction process predominantly takes place through grain boundary volume. Grain boundary resistance increases with Ni2+ ion concentration.