D. Ravinder

Dielectric properties of polycrystalline mixed nickel-zinc ferrites

Dielectric properties such as dielectric constant (e′), dielectric loss tangent (tanδ) and complex dielectric constant (e″) has been investigated in the frequency range 100 kHz–1 MHz. The variation of these parameters with composition, frequency and temperature is explained qualitatively. The dielectric constant for these ferrites is approximately inversely proportional to the square root of the resistivity. An attempt is made to explain the possible mechanism.

Dielectric behaviour of erbium substituted Mn-Zn ferrites

Dielectric properties such as dielectric constant (ε′) and dielectric loss tangent (tan□δ) of mixed Mn-Zn-Er ferrites having the compositional formula Mn0.58Zn0.37Fe2.05−xErx04 (where itx = 0.2, 0.4, 0.6, 0.8 and 1.0) were measured at room temperature in the frequency range 1–13 MHz using a HP 4192A impedance analyser. Plots of dielectric constant (ε′) vs frequency show a normal dielectric behaviour of spinel ferrites. The frequency dependence of dielectric loss tangent (tan δ) was found to be abnormal, giving a peak at certain frequency for all mixed Mn-Zn-Er ferrites. A qualitative explanation is given for the composition and frequency dependence of the dielectric constant and dielectric loss tangent. Plots of dielectric constant vs temperature have shown a transition near the Curie temperature for all the samples of Mn-Zn-Er ferrites. However, Mn0.58Zn0.37Er1.0Fe1.05O4 does not show a transition. On the basis of these results an explanation for the dielectric mechanism in Mn-Zn-Er ferrites is suggested.

Synthesis and magnetic properties of NiFe2−xAlxO4 nanoparticles

Nanocrystalline Al-doped nickel ferrite powders have been synthesized by sol-gel auto-ignition method and the effect of non-magnetic aluminum content on the structural and magnetic properties has been studied. The X-ray diffraction (XRD) revealed that the powders obtained are single phase with inverse spinel structure. The calculated grain sizes from XRD data have been verified using transmission electron microscopy (TEM). TEM photographs show that the powders consist of nanometer-sized grains. It was observed that the characteristic grain size decreases from 29 to 6 nm as the non-magnetic Al content increases, which was attributed to the influence of non-magnetic Al concentration on the grain size. Magnetic hysteresis loops were measured at room temperature with a maximum applied magnetic field of 1T. As aluminum content increases, the measured magnetic hysteresis curves become more and more narrow and the saturation magnetization and remanent magnetization both decreased. The reduction of agnetization compared to bulk is a consequence of spin non-collinearity. Further reduction of magnetization with increase of aluminum content is caused by non-magnetic Al^{3+} ions and weakened interaction between sublattices. This, as well as the decrease in hysteresis was understood in terms of the decrease in particle size.

High-frequency dielectric behaviour of polycrystalline zinc substituted cobalt ferrites

The dielectric constant (ɛ′) and complex dielectric constant (ɛ′′) of zinc substituted cobalt ferrites have been measured at room temperature in the high frequency range 100 kHz to 1 MHz. The values of dielectric loss tangent (tan δ) have been computed from ɛ′ and ɛ′′. Plots of dielectric constant (ɛ′) versus frequency show a normal dielectric behaviour of the spinel ferrites. The frequency dependence of dielectric loss tangent (tan δ) is found to be abnormal, giving a peak at certain frequency for all the ferrites under investigation. A qualitative explanation is given for the composition and frequency dependence of the dielectric constant and dielectric loss tangent. The dielectric constant for these mixed ferrites is approximately inversely proportional to the square root of the resistivity. A plot of dielectric constant versus temperature shows a transition near the Curie temperature. An attempt is made to explain the possible mechanism for this observation.

Far-infrared spectral studies of mixed lithium–zinc ferrites

Abstract The far-infrared spectra of Li–Zn mixed ferrites have been studied in the frequency range 700–200 cm −1 . Four absorption bands are observed. The high frequency band ( ν 1 ) is assigned to the tetrahedral complexes, while the absorption bands ν 2 and ν 3 are attributed to the octahedral complexes. On the other hand, the lowest frequency band ( ν 4 ) is attributed to the lattice vibrations of tetrahedral metal ions. The observed results can be explained on the basis of Fe 2+ concentration.

Dielectric behaviour of mixed Mg-Zn ferrites at low frequencies

Abstract The dielectric constant ( e ′), dielectric loss tangent (tan δ ) and complex dielectric constant ( e ″) of mixed Mg–Zn ferrites of various compositions have been studied in the frequency range of 1–100 kHz using a capacitance bridge. Plots of dielectric constant vs. frequency show a normal dielectric behaviour of spinel ferrites. The frequency dependence of a dielectric loss tangent (tan δ ) is found to be abnormal, giving a peak at a certain frequency for all the compositions. A qualitative explanation is given for the dielectric behaviour of Mg–Zn ferrites. The dielectric constant for these mixed ferrites is approximately inversely proportional to the square root of the resistivity. The observed results can be explained on the basis of an electron exchange between Fe 2+ and Fe 3+ ions.

High-frequency dielectric behaviour of Li-Mg ferrites

Abstract Dielectric properties such as dielectric constant ( e ′) and dielectric loss tangent (tan δ ) of mixed Li–Mg ferrites have been measured at room temperature in the frequency range 1 to 13 MHz using an HP 4192A impedance analyser. Plots of dielectric constant ( e ′) vs. frequency show a normal dielectric behaviour of spinel ferrites. The frequency dependence of dielectric loss tangent (tan δ ) is found to be abnormal, giving a peak at certain frequency for all mixed Li–Mg ferrites. A qualitative explanation is given for the composition and frequency dependence of the dielectric constant and dielectric loss tangent.

Abnormal dielectric behaviour in polycrystalline zinc-substituted manganese ferrites at high frequencies

Abstract Dielectric constant ( e ′), dielectric loss tangent (tan δ ) and complex dielectric constant ( e ″) have been measured for the samples of the type Mn 1− x Zn x Fe 2 O 4 ( x =0.3, 0.5, 0.7 and 0.9). The measurements were carried out in the high frequency range of 100 kHz–1 MHz. The dielectric behaviour is found to be abnormal, giving a peak at a certain frequency. The peak shifts to lower frequency due to increasing zinc concentration. The variation of dielectric behaviour with composition, frequency and temperature is explained qualitatively. The dielectric constant for these mixed ferrites is approximately inversely proportional to the square root of the resistivity. An attempt is made to explain the possible mechanism.

Electrical transport properties of cadmium substituted copper ferrites

Abstract Electrical transport properties such as electrical conductivity (σ) and thermoelectric power (S) of cadmium substituted copper ferrites, forming the chemical formula Cu1−xCdxFe2O4, where x=0.2, 0.4, 0.6, 0.8 and 1.0 have been investigated from room temperature to well beyond the Curie temperature. Plots of log(σT) vs. 103/T are linear and show a transition near the Curie temperature except in the case of cadmium ferrite. Based on the Seebeck coefficient (S), the ferrites under investigation have been classified as n-type semiconductors. The values of charge carrier concentration and mobility have been computed from experimental values of Seebeck coefficient and electrical conductivity. The activation energy in the ferrimagnetic region is in general less than that in the paramagnetic region. An attempt is made to explain the conduction mechanism in these ferrites. The properties of cadmium substituted copper ferrites have been correlated with these of zinc substituted copper ferrites, cadmium and zinc being two non-magnetic divalent ions occupying essentially tetrahedral A sites when substituted in ferrites.

Electrical conductivity of Ni-Zn-Gd ferrites

Abstract The electrical conductivity of polycrystalline gadolinium substituted mixed nickel–zinc ferrite was investigated from room temperature to well beyond the Curie temperature. Plots of log(σT) vs. 103/T show a transition near Curie temperature. The activation energy in the ferromagnetic region is, in general, less than that in the paramagnetic region. An attempt is made to explain the conduction mechanism in these ferrites.