الفهرس 
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Abstract
The samples were prepared by two different methods standard ceramic technique and coprecipitation method. In the first method weighing metal oxides in stoichimetric ratios then the mixture are grinded together for 3 hours. The powder was pressed using uniaxial press of pressure 1.9 × 108 Nm-2, presintered at 900 oC for 10 hours using heating rate of 4 oC/min and final sintering at 1200 oC for 7 hours at the same rate. X-rays and FTIR were carried out to assure the formation of the samples in single phase spinel structure. Different transmission bands were obtained indicating the presence of different cations in the different lattice sites. The electrical properties of NixMn0.8- xMg0.2Fe2O4; 0 ≤x ≤ 0.40, were studied as a function of temperature, frequency. In the first group the effect of Ni2+ ions substitution with different content on the structural and electrical properties of NixMn0.8- xMg0.2Fe2O4; 0 ≤x ≤ 0.40 was studied. Compositional curve were studied for the investigated samples and it was found that, the dielectric constant ( ε/ ) , dielectric loss (tanδ) and conductivity with Ni content all of them increase up to x = 0.20 and then decrease after this concentration. Though the concentration with x = 0.20 is considered as critical concentration due to anomalous behavior in crystal size and dielectric constant ( ε/ ) , dielectric loss (tanδ) and conductivity .In the second group the effect of laser irradiation on the structural, electrical properties of NixMn0.8- xMg0.2Fe2O4; 0 ≤x ≤ 0.40 was also studied. The dielectric constant ( ε/ ) and dielectric loss (tanδ) for the samples of the two groups discussed clearly. The data showed that the dielectric constant increases with temperature and decreases with frequency. The increase of the conductivity with temperature in the first region is attributed to the valance exchange interaction between iron ions Fe2+↔ Fe3+ + e- while in the second region of the temperature (550 ≤ T ≤ 750K) the increase in conductivity is due to the valence exchange between (Ni2+, Ni3+) and (Mn2+, Mn3+) in cooperation with the well known Verway conduction mechanism. The conductivity and angular frequency as a function of temperature were also studied and it was found that, the conductivity increases with increasing frequency and temperature. The conduction mechanism was discussed in view of correlated barrier hopping model. For the second group and after laser irradiation it was found that the samples still save the spinel phase structure where no deformation has been occurred and Laser irradiation increases the porosity of the samples and the crystal size increases after laser irradiation. An inflection point has been occurred depending on Ni content in the first temperature region. The dielectric constant ( ε/ ) and dielectric loss (tanδ) for the samples are higher than before laser irradiation and after a certain temperature the dielectric constant ( ε/ ) and dielectric loss (tanδ) for the samples are lower than those before laser irradiation. It was found that the resistivity of the samples increases after laser which make the sample are more will be applicable in micro processor and many other applications. In the third group, the samples NixMn0.8-xMg0.2Fe2O4; 0 ≤ x ≤ 0.40 were prepared by co- precipitation method using X-ray diffractograms are characteristics of cubic spinel structure as compared and with ICDD card number 2-1034 . Also for the third group it was found that the dielectric constant ε/ decrease in case of coprecipitation method except at x = 0.20 at higher temperature. In the first group the crystal size were 60-200 nm but in co-precipitation method were 14-34nm. In the co –precipitation method it was found that as Ni content increases lattice parameter increase which is a different trend for the first group.