T. Seshagiri Rao

Dielectric behaviour of mixed Li-Ni ferrites at low frequencies

Abstract The dielectric constants and dielectric loss tangents of mixed Li-Ni ferrites of various compositions were measured at room temperature using a capacitance bridge in the frequency range 10 2 –10 5 Hz. A qualitative explanation for the composition and frequency dependence of the dielectric constant is given. The dielectric constant for these mixed ferrites is roughly inversely proportional to the square root of the resistivity.

Electrical conductivity of Manganese-Magnesium ferrites

Abstract The electrical conductivity of Mn-Mg ferrites of various compositions was studied as a function of composition and of temperature. Based on the Seebeck coefficients observed the mixed ferrites were divided into two groups. The mixed ferrites of group I have a slight excess of cations and show n-type conduction, whilst those of group II have a cation deficiency and show p-type conduction. The mechanism of the electrical conduction is explained in terms of the oxidation of Fe2+ by Mn3+ on octahedral sites. Plots of conductivity (log (σT)) versus temperature ( 10 3 T ) are almost linear; they show a transition near the Curie temperature and a change in slope of the line with further increase in temperature.

Electrical conductivity of lithium-nickel ferrites

The electrical conductivity of Li-Ni ferrites was studied as a function of composition and of temperature. X-ray analysis and Seebeck coefficient studies were also carried out so that the conductivity of these mixed ferrites could be discussed. The dependence of the electrical conductivity of Li-Ni ferrites on composition is explained on the basis of the cation distribution. All the ferrites exhibit a transition near the Curie temperature with a change in slope of the conductivity (log(σT)) versus temperature (103T) curve. It is found that the activation energy in the paramagnetic region is higher than that in the ferrimagnetic region for all the ferrites.

X-ray studies on lithium-nickel and manganese-magnesium mixed ferrites

Abstract The lattice parameters of Li-Ni and Mn-Mg ferrites of various compositions were determined by the X-ray diffraction powder method. The lattice parameter was found to vary linearly with composition in both systems. The mean ionic radius per molecule of the tetrahedral sites was computed for each composition and was found to increase with the lattice parameter. A qualitative explanation for the variation of the lattice parameter is proposed.

Temperature and composition dependence of the elastic moduli of NiZn ferrites

Abstract The elastic moduli of four different compositions of mixed NiZn ferrites in the polycrystalline form have been measured in the temperature range 90 – 580 K using the composite oscillator method. The values of Youngs modulus and the rigidity modulus are corrected to theoretical density using the method of Mackenzie. The data obtained on the mixed ferrites are found to be in reasonable agreement with Van Der Burgts values. It is observed that the longitudinal and shear velocities vary linearly with density as these mixed ferrites have nearly identical mean atomic weights. As in the cases of simple ferrites, the variation of Youngs modulus Y with temperature T (in kelvins) follows Wachtmans equation Y= y 00 − BTe −T 0 T where y00, b and T0 are constants. The rigidity modulus n is found to vary linearly with temperature T according to the equation n = n0 − bT where n0 and B are constants.

DIELECTRIC BEHAVIOUR OF MIXED Mn-Mg FERRITES AT LOW FREQUENCIES

Abstract The dielectric constant and dielectric loss tangent of mixed Mn-Mg ferrites of various compositions were measured at room temperature in the frequency range 10 3 –10 5 Hz using a capacitance bridge. A qualitative explanation is given for the composition and frequency dependence of the dielectric constant. The dielectric constant for these mixed ferrites is approximately inversely proportional to the square root of the resistivity.

Electrical conductivity of CoZn ferrites

Abstract The electrical conductivity of mixed CoZn ferrites was studied as a function of composition and temperature. The existence of the single-phase spinel structure was established by X-ray analysis of the specimens. The Seebeck coefficient for these ferrites was determined in order to discuss the conductivity which is explained in terms of the hopping mechanism. Plots of the logarithm of conductivity versus the reciprocal temperature are almost linear and show a transition near the Curie temperature. The activation energy in the paramagnetic region is higher than that in the ferrimagnetic region.

Effect of magnetic field and temperature on the elastic behaviour of cobalt-zinc ferrites

Abstract A study of the dependence of the elastic behaviour of polycrystalline mixed cobalt-zinc ferrites on temperature and magnetic field has been made using a composite oscillator method. As the temperature is increased the Youngs modulus attains a minimum at a temperature below the Curie point. Thereafter it exhibits a positive temperature coefficient up to the Curie point and decreases with further increase of temperature. This anomalous behaviour has been explained in terms of magnetic anisotropy energy. In the case of cobalt ferrite and Co 0.6 Zn 0.4 Fe 2 O 4 the Youngs modulus is found first to decrease and later to increase with increasing magnetic field, finally becoming constant at the saturation field. In contrast, in the case of Co 0.4 Zn 0.6 Fe 2 O 4 and Co 0.2 Zn 0.8 Fe 2 O 4 the Youngs modulus does not show an initial decrease but increases with increasing magnetic field. The observed ΔE effect in the case of cobalt ferrite has been interpreted in terms of two domain processes, i.e. domain rotation against uniaxial strain anisotropy and the movement of 90° boundary walls. A study of the effect of the simultaneous application of temperature and magnetic field on the elastic behaviour of mixed cobalt-zinc ferrites has also been carried out.

Elastic behaviour of mixed cobalt-zinc ferrites

The composition and temperature dependence of the elastic properties of cobalt-zinc ferrites have been studied using the composite oscillator method originally due to Balamuth and Rose. The values of Youngs modulus (Y) and the rigidity modulus (n) are found to increase with an increase of zinc content. A linear dependence of the longitudinal and shear velocities on the density is also observed, as in the case of the nickel-zinc ferrites. The dependence of elastic behaviour of these mixed ferrites on temperature is similar to that in the case of the simple and mixed ferrites studied earlier. The variation of Youngs modulus with temperature follows Wachtmans equation while a linear equation describes the variation of the rigidity modulus (n) with temperature.

Elastic behaviour of lithium-nickel ferrites

Results are presented of measurements of the elastic properties of polycrystalline Li-Ni mixed ferrites as a function of composition at room temperature.