الفهرس 
Introduction and Literature ReviewOnly 14 pages are availabe for public view
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Abstract
Several series of spinel ferrites prepared using two different methods: dry (solid state) and wet chemical (citrate, sol-gel). Structural, micro-structural, magnetic properties and cations distribution were studied for nano-size nickel ferrites samples prepared by citrate (CA), sol-gel (SG) and conventional solid state (SR) methods. The crystallite sizes are found to be critically depends on the preparation methods and the annealing temperatures at which they were prepared. The lattice parameter, oxygen positional parameter and crystallite size are increased by increasing the annealing temperature, whereas the strains are decreased. The tetrahedral bond lengths are increased for CA samples and on the other hand they remain constant for SG samples by increasing the annealing temperatures. On the contrary the octahedral bond lengths and angles decreased by increasing the annealing temperatures. Mössbauer spectra at room temperature shows superparamagnetic doublet, which considered an indicator for the presence of nano-particles size. Cations distributions determined by Mӧssbauer and the modified Bertaut method are constituents. Significantly, cations distribution is affects the magnetic properties of the samples under investigation. The superparamagnetic is greater for nano-particles with smaller size and accordingly the average internal magnetic field.
Spinel ferrite series Ni0.7Zn0.3Fe2–yGayO4 with y=1 and 0.5 were synthesized in the bulk and nano-structured forms. Refinement of atomic structure through precise analysis of neutron diffraction patterns measured with high resolution TOF-diffractometer was done. Obtained results confirm that at room temperature NiFe2O4 parent compound is a fully inverted spinel (δ=0 ± 0.01) with a cubic symmetry of the lattice. The analysis of Zn/Ga doped NiFe2O4 shows that the cubic symmetry is conserved; all Zn ions are in A-sites, while Ga ions are equally distributed between A- and B-sites of spinel structure. No structural or magnetic phase transitions were found down to 10 K. The level of microstrains considered very small, while the size of coherently scattered domains is large enough. Crystallographic study were performed for NiFe2O4, NiGd0.02Fe1.98O4 and Ni1−xZnxGd0.02Ga0.2Fe1.78O4, (0.0≤x≤ 0.5) samples prepared by ceramic and citrate methods. It is observed that as gadolinium introduced into the system, the deviation from single phase and existence of additional peaks attributed that indexed as gadolinium silicate oxide phase, which is present in all the ceramic samples due to interaction of gadolinium with crucible. The lattice parameter and crystallite size increase by substitution of Gd3+ and by increasing Zn2+ content, which can be explained on the basis of ionic radii. The increase in lattice parameter and crystallite size induces a lattice strain in the crystal structure, and the best cations distribution between the tetrahedral and octahedral site is obtained by Bertaut method. For the same samples prepared in chemical method, the broad diffraction peaks indicate that the materials are crystallized in the nano spinel single phase without any other phases or extra peaks unlike ceramic method. The values of the lattice constant were found to be decrease by introducing Gd and Ga, and it gradually increases by increasing Zn content (0.1≤x≤0.5), which induces a lattice strain in the crystal. The cations distribution performed using Bertaut method showed that Zn cation preferred to occupy A-site, while Gd cations preferred to occupy B-site, whereas Fe, Ga and Ni ions partially distributed between A and B sites. The vibrational frequencies values for FT-IR bands in tetrahedral (ʋA) and octahedral (ʋB) sites are highly confirmed with estimated cations distributions. Mössbauer Effect spectra at RT for NiFe2O4, NiGd0.02Fe1.98O4 and Ni1-xZnxGa0.2Gd0.02Fe1.78O4 (0.0 ≤ x ≤ 0.5) sampls prepared by the standard ceramic method were fitted using two sextets, corresponding to Fe3+ ions in A and B sites, which indicates that the bulk samples are magnetically ordered. On the other hand; the spectra of all the nano samples prepared by citrate method have the signature of superparamagnetism, where pronounced central doublets were detected for all samples. Due to the effect of grain size, the values of saturation magnetization, observed and calculated magnetic moments obtained from hysteresis loops for the bulk samples are found to be more than that for the samples prepared by citrate method. Ac electrical conductivity for the bulk and nano ferrite samples Ni0.7Zn0.3GaFeO4, Ni0.7Zn0.3Ga0.5 Fe1.5 O4 and Ni0.7Zn0.3Gd0.02Ga0.2Fe1.78O4 explained that the conductivity increases with increasing temperature in a behavior similar to the most semi-conducting materials. At low temperature the Ac conductivity is frequency dependence, and by increasing temperature it becomes frequency independence. The dielectric loss tangent (tanᵟ ) and dielectic constant (ε’) of the prepared samples were measured in wide range of frequency and temperature using two probe method .the results were explained in the light of cation- anion –cation and cation –cation interactions over the two sides .