الفهرس 
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Abstract
In this study, the physical, magnetic and electrical properties of four systems of mixed ferrites are investigated. The chemical compositions of these systems are:
1) Li0.3-0.5xZn0.4CaxFe2.3-0.5xO4 (x=0.0, 0.01, 0.02, 0.03 and 0.05), referred as (Ca-system).
2) Li0.3Zn0.4BixFe2.3-xO4 (x= 0.0, 0.02, 0.05 and 0.075), referred as (Bi-system).
3) Li0.3+0.5xZn0.4ZrxFe2.3-1.5xO4 (x=0.0, 0.01, 0.02, 0.03 and 0.05), referred as (Zr-system).
4) Li0.3+xZn0.4NbxFe2.3-2xO4 (x=0.0, 0.01, 0.02, 0.03 and 0.05), referred as (Nb-system).
According to the obtained results, the conclusions could be divided into general and special conclusions:
I- General conclusions:
1- All the investigated samples are formed in a single phase.
2- All systems improved the magnetization, the coercivity and the squareness (except Nb-system) which are important properties in memory applications.
3- All studied systems improved the initial permeability and the homogeneity (except Ca-system) which are important properties in transformer core applications.
4- The thermoelectric power α measurements indicated that the majority of charge carriers are electrons for all systems.
5- The temperature dependence of resistivity and thermoelectric power showed that the electron hopping mechanism is the appropriate one to account for the conduction mechanism in the ferrimagnetic region.
II- Special conclusions:
For Ca-system:
It has the maximum improvement in coercivity (55%) and squareness (160%) while it decreased both the homogeneity and initial permeability. It decreased also the resistivity (except for x=0.05) and consequently increased the dielectric constant.
For Bi-system:
It has the maximum improvement of initial permeability (82%) and homogeneity (4 times). It increased the resistivity and decreased Curie temperature.
For Zr-system:
It has the maximum improvement in magnetization (14.1%) and resistivity. It has the minimum loss (25% of unsubstituted sample).
For Nb-system:
It has the minimum porosity (4.5%). It decreased the coercivity, the squareness and Curie temperature. It has the maximum improvement of the dielectric constant (75%).
Based on the above statements, it is concluded that the substitution with different valent ions generally improves the magnetic and electrical properties of Li-Zn ferrite which are promising results for different applications.