P. Venugopal Reddy

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.

Far-infrared spectral studies of some lithium-nickel mixed ferrites

Abstract The far-infrared spectra of Li-Ni mixed ferrites reveal four absorption bands (two at high frequencies and two at low frequencies) in the frequency range 700-200 cm -1 . The high-frequency band ( v 1 ) is assigned to the tetrahedral metal complexes, while the absorption bands v 2 and v 3 are attributed to the octahedral metal complexes. On the other hand, the lowest-frequency band ( v 4 ) is attributed to the lattice vibrations of tetrahedral metal ions. It has been observed that all the principal bands of both the unsubstituted and 0.06 mol of nickel-substituted lithium ferrite samples are found to exhibit a shoulder; the phenomenon is explained as being due to the splitting of the main bands, which itself is attributed to the presence of Fe 2+ ion content on octahedral sites.

Elastic behavior of a Y-Ba-Cu-O sample prepared by the MPMG method

Abstract Ultrasonic velocity measurements have been undertaken by the pulse transmission technique using YBaCuO samples prepared by the melt-powder-melt-growth (MPMG) process over a temperature range of 80–300 K. It has been observed that the sample exhibits a very high value of Youngs and rigidity moduli, thereby indicating that the material is an excellent one from the point of view of its mechanical strength. Further, the Youngs modulus apart from exhibiting a hump at around 120 K, is found to increase continuously with decreasing temperature. In the superconducting phase, however, the modulus is found to remain almost constant. It has been concluded that the observed elastic behavior, to a large extent, is analogous to that of normal oxide materials. In order to understand how far the experimentally observed behavior deviates at 120 K, the elastic moduli of the sample were also estimated theoretically and compared with the empirical values.

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 transport properties of manganese-magnesium mixed ferrites

Abstract Thermoelectric power studies are made of MnMg mixed ferrites over the temperature range 300–700 K by the differential method. The Seebeck coefficient S is found to decrease while the carrier concentration n and the charge carrier mobility μ are found to increase with increasing temperature. On the basis of these results an explanation for the conduction mechanism in MnMg mixed ferrites is suggested.

Magnetoelectric behavior of sodium doped lanthanum manganites

Nanocrystalline samples of sodium doped manganites with compositional formula La1−xNaxMnO3 (0.025⩽x⩽0.25) were prepared by polyvinyl alcohol assisted precursor method. After characterizing the samples by x-ray diffraction and transmission electron microscopy a systematic investigation of electrical, magnetic, and thermopower properties has been undertaken. The resistivity data were analyzed using effective medium approximation. From the analysis it has been found that the metallic fraction is increasing up to x=0.10 and remains constant with further doping. A close examination of the resistivity data clearly indicates that the sodium doped samples are slowly transformed from colossal magnetoresistance behavior to charge ordering behavior. Thermoelectric power data at low temperatures were analyzed by considering the magnon drag concept, while the high temperature data were explained by small polaron conduction mechanism.

Charge transport in Li‐Ni ferrospinels

The electrical conductivity (σ) and thermopower (Q) of Li‐Ni mixed ferrites have been studied as a function of composition and temperature. The conduction mechanism in these ferrites has been explained with the help of carrier concentration (n) and charge carrier mobility (μ) calculated from the measured experimental values. The composition and frequency dependence of dielectric constant (e’) and dielectric loss tangent (tan δ) have also been studied at low frequencies. A relationship between dielectric constant (e’) and electrical resistivity (ρ) has been arrived at. A qualitative explanation for observing a maximum in the plots of dielectric loss tangent and frequency has also been given.