Submitted to the Council of the College of Science- University of Diyala in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy of Science in Physics

Abstract

Abstract The objective of this thesis was to synthesis and studying the structural, electrical and magnetic properties of magnesium and chromium substituted cobalt ferrites nanoparticles with the stoichiometric formula Co1-xMgxFe2O4 and CoCrxFe2-xO4 (x=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) respectively, were prepared using sol–gel auto-combustion method. The as-burnt powders were calcined at 600, 700, and 800 °C for 3h in order to study the structural and magnetic properties. The samples that calcined at 800 °C pressed into circular pellets and then sintered at 900 °C for 3h for studying the electrical properties. The XRD diffraction analysis for Mg2+ substituted cobalt ferrite nanoparticles showed all the major peaks corresponding to the single spinel structure, while the absence of peaks in Cr3+ substituted cobalt ferrite of the as-burnt indicates the amorphous nature of the samples. The calcined samples at 600 and 700 °C confirm the formation of spinel structure. Moreover, calcined powder samples at 800 °C showed the formation of spinel cubic structure with single phased for all Cr3+ substituted cobalt ferrites. Generally, when the calcination temperature was elevated, the diffraction peaks became sharper, narrower and more intense. Fourier transform infrared (FT-IR) spectra of both series showed two principal absorption bands in the range of (600-400 cm-1), that indicates formation of cubic spinel structure. In Mg2+ substituted cobalt ferrite nanoparticles a shifting in similar trend of the􀀃􀟥􀬵 and 􀀃􀟥􀬶 bands towards the lower frequencies has observed with increasing the calcined temperature and Mg2+ substitutions. While, in Cr3+ substituted, the absence of any absorption peak of the as-burnt sample with increasing the Cr3+ substitution it also indicates the amorphous nature of the samples. However, the calcined samples at 600, 700 and 800 °C exhibited the characteristic FT-IR peaks of the ferrite. A slightly shifting of the 􀀃􀟥􀬵 and XVII 􀀃􀟥􀬶 bands towards higher frequencies has been observed with increase in Cr3+ substitution. The porous nature, nanosized and almost homogeneous distribution of particle size have been showed by field emission scanning electron microscopy (FE-SEM) images. The micrographs of Mg2+ and Cr3+ substituted cobalt ferrite revealed that the microstructure of the ferrites was affected by the substitutions and showed agglomeration with homogenous spherical and polyhedral particles. The optical energy band gap 􁈺􀜧􀯚􁈻 of cobalt ferrite for as-burnt and those calcined at 600, 700 and 800 °C are equals to 1.610, 1.609, 1.605 and 1.598 eV respectively. These energies have been decreased with increasing calcination temperature and in turns increasing the particle size, while showed an increase with increasing the Mg2+ and Cr3+ substitution. The dielectric properties are measured by (LCR) meter in the frequency range of (50Hz-1MHz) at room temperature. The dielectric constant 􁈺􀟝􁇱􁈻, dielectric loss angle 􁈺􀝐􀜽􀝊􀟜􁈻 and dielectric loss factor 􁈺􀟝􁇱􁇱􁈻 for Mg2+ and Cr3+ substituted cobalt ferrites are found to decrease with increasing frequency. This behavior is typical of ferrites as explained by Koop’s model. The dielectric constant was found to increase with an increasing in Mg2+ and Cr3+ substitutions. While, the dielectric loss angle and dielectric loss factor of the cobalt ferrite have been reduced by Mg2+ and Cr3+ substitutions in the present work. The ac conductivity 􁈺􀟪􀯔􀯖􁈻, gradually increased as the frequency increased, and decreased with increasing of Mg2+ and Cr3+ substitutions dependent on the partial substitution and the difference in the grain size. The weakened of magnetic interaction among cations are observed by means of vibrating sample magnetometer (VSM), due to replacement of Fe3+ and Co2+ at B-site (octahedral) by the Cr3+ and Mg2+ ions respectively, which caused decreasing in saturation magnetization 􁈺􀜯􀯦􁈻 and remanance magnetization􁈺􀜯􀯥􁈻. The reduced magnetic anisotropy 􁈺􀜭􁈻 and particle size was found responsible for XVIII decrease in coercivity force 􁈺􀜪􀯖􁈻. In general, the magnetization measurements show a gradual transformations from hard magnetic to softer magnetic characteristics, and that was witnessed with Mg and Cr substitutions as the magnetic parameters; 􀜯􀯦, 􀜯􀯥 and 􀜪􀯖 decreased with substitutions.