Studies on Electric and Magnetic Properties of Cobalt Ferrite and its Modified Systems

Abstract

Electric and magnetic properties of bulk and nano cobalt ferrite and modified with Bi3+, Cr3+ and K2CrO4 in nano level have been investigated. Bulk cobalt ferrite (CoFe2O4) system was prepared by conventional solid state route and nano CoFe2O4 system was prepared by auto combustion method of the chemical route. The contribution of microstructures (intrinsic grain and extrinsic grain boundary, sample surface-electrode contact) to the conduction mechanism was investigated by complex impedance spectroscopic analysis. Both the intrinsic and extrinsic conductions were observed in the bulk cobalt ferrite system whereas intrinsic grain conduction is found absent in its nano system. It is observed that though the room temperature resistivity of the nano system is higher than bulk one but the rate of decrease in resistance with elevated temperature is higher in former case. Perhaps, this may be the reason behind the early surface-conduction in nano system. The high resistance of nano system at room temperature (RT) is due to the increase in density of high resistive grain boundaries. The high dielectric loss in nano system may be due to the early conduction of charge carriers in high resistive regions in comparison to bulk system. It is observed that the saturation magnetization of bulk cobalt ferrite was found to be higher than nano system. However, coercivity is higher in nano-cobalt ferrite which can be related to the surface spin effects. Bismuth substituted nano cobalt ferrite (CoFe2-xBixO4, x= 0, 0.05, 0.1, 0.15) samples were prepared by auto combustion technique. The single phase XRD pattern confirmed the successful substitution of the larger cation. Surface morphology from FESEM image indicated the control average particle growth (50 nm – 160 nm) as an effect of bismuth substitution. The increased particle size has effectively modified the electrical properties of the system in three major ways: (a) increase in resistivity, (b) evolution of grain relaxation and (c) reduction in dielectric loss and surface conduction. Additionally, magnetic behavior is also affected due to control particle growth. Magnetic hysteresis study at room temperature confirmed the rise in saturation magnetization (MS = 74.5 to 86.5 emu/g.) and reduction in coercivity (HC = 1633 to 1524 Oe).
The samples of CoFe2-xCrxO4 (x = 0, 0.15, 0.3) series were prepared by auto combustion route. X-ray diffraction technique was used to confirm the phase formation and structure analysis. The surface morphology of samples was imaged by the field emission scanning electron microscope. The average particle size was found to be ~55, ~43 and ~35 nm for x= 0, 0.15, 0.3 respectively. The substitution of Cr3+ in the parent systems caused a significant reduction in particle size. The samples were subjected to magnetic characterization and also studied with Mossbauer spectroscopy at room temperature. Analysis of extracted parameters from Mossbauer spectroscopy concluded that Cr3+ has replaced the Fe3+ at B-site (octahedral). The decrease in magnetization at B-site was responsible for the observed reduced saturation magnetization and coercivity. The Cole-Cole plots of impedance showed the growth of semicircle describing the effect of grain boundary and the suppression of semicircle describing the electrode-sample surface conduction effect. The increasing radius of Cole-Cole semicircles indicated the enhancement of the material resistivity which was also confirmed by the dc resistivity measurement. All these results were explained on the basis of occupancy of Cr3+ at B-site, surface anisotropy potential and reduced particle size. The parent cobalt ferrite system further modified with K2CrO4 in which chromium exists in its highest oxidation state (6+). The samples were synthesized by the auto-combustion method with different dopant concentration. The modified systems contain all the characteristic XRD peaks of cobalt ferrite and no peaks related to secondary phases are observed. The effect of dopant further reduces the particle size and at its higher percentage, the particle size is reduced to ~15 nm. An interesting result of metallic to semiconducting transition behavior is observed in the modified cobalt ferrite system of higher dopant concentration. From the impedance spectroscopic analysis it is revealed that grain conduction was active in the temperature belt of metallic region. The variations of magnetic moment along with coercivity with addition of K2CrO4 were explained on the basis of particle size.