Particle size dependence of the magnetic, dielectric and gas sensing properties of Co substituted NiFe^sub 2^O^sub 4^ nanoparticles

Changes in the magnetic, dielectric and gas sensing properties of Ni0.8Co0.2Fe2O4 nanoparticles are reported. These polycrystalline nanoparticles (NPs) were prepared by evaporation method in the presence of egg-white as a bio-template. X-ray diffraction (XRD)studies of the samples showed single phase ferrite cubic structure without any secondary phases, with the crystallite size D = 10.5 nm, 16.4 nm and 21.9 nm for the samples heat-treated at 600 °C, 750 °C and 900 °C respectively. TEM micrograph shows nearly spherical shaped particles with particle size consistent with the XRD results. The magnetic hysteresis loops measured at ambient show that with increase in D, saturation magnetization MS increases but coercivity HC decreases. This size dependence of MS and HC is interpreted in terms of the core-shell model with spins in the shell of thickness d = 0.4 nm not contributing to MS becauseof disorder resulting from the lower symmetry at the surface.The frequency dependence of the dielectric properties of the samples although typical of that reported in other ferrites show significant particle size dependence. The sample with the smallest D = 10.5 nm shows the best performance as a sensor for detecting different gases (LPG, H2, NH3, and CO) at the operating temperature of 250 °C using changes in the electrical resistivity on controlled exposure to gases as the criterion.