الفهرس 
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Abstract
In the present thesis, the impact of the nonstoichiometric substitution on the structural and magnetic properties of Co-Ni nanoferrites Co0.5Ni0.5+xFe2xO4 and vanadium-substituted Co-Zn nanoferrites Co0.5Zn0.5VxFe2–xO4 has been investigated. The nonstoichiometric substitution has been proved to be quite effective in tailoring and understanding the structural and magnetic properties of the prepared nanoferrites. Samples of both systems were synthesized via the citric precursors autocombustion method, and the sintering process was conducted at 600 °C for two hours. The nonstoichiometric substitution has been confirmed to be compensated by the appearance of higher and/or lower valance states of the involved cations to recover the charge neutrality. The structural and magnetic parameters have successfully been correlated to both the sample’s composition and the cation distribution among the interstitial sites. The structural characteristics were analyzed using the X-ray diffraction (XRD) patterns. The X-ray diffraction patterns were used to confirm the cubic phase formation and its purity. The structural parameters, including the lattice parameter, the microstrain, and the crystallite size, were deduced through the recorded patterns. The crystallite size was obtained using both the Williamson-Hall method and the Scherrer formula. The surface morphology of the investigated samples was verified through field-emission scanning electron microscopy (FE-SEM) equipped with energy-dispersive X-ray (EDAX). The elemental Abstract iii composition of the investigated samples and the concentration of the involved cations have been examined through the EDAX spectra. Images of selected samples obtained from transmission electron microscopy (TEM) have confirmed the shape, morphology, and size obtained from XRD calculations. Moreover, FTIR spectra were used to confirm the cubic spinel structure of the investigated samples and to detect the vibrational frequencies corresponding to the different valence states of the involved cations. The magnetic characteristics, including saturation magnetization, remnant field, and coercivity, were obtained from M-H hysteresis loops traced at room temperature using vibrating sample magnetometry (VSM). The magnetic parameters have been strongly correlated with the structural parameters and the cation distribution. For both systems, hysteresis loops revealed a reduction of magnetization with increasing the substitution level. Finally, a cation distribution has been proposed for each singlephase sample based on the experimental data of XRD, FTIR, and VSM. The suggested cation distributions have successfully explained the variation of the lattice parameter, crystallite size, IR frequencies, magnetization, and coercivity with the substitution level. Besides the experimental data, the cation distribution supports the compensation of the nonstoichiometric substitution by the appearance of higher and/or lower valance states of the involved cations. For both systems, the coercivity has remarkably correlated with the combined effects of the crystallite size and the cation distribution.