K. V. Ramesh

Effect on structural and magnetic properties of aluminum substituted Ni–Zn nanoferrite system prepared via citrate-gel route

Nickel zinc aluminum nanoferrites with general formula Ni0.5Zn0.5Fe2-xAlxO4(x = 0.0, 0.05, 0.1, 0.15, 0.2 and 0.25) were prepared by citrate-gel autocombustion method and heat treated in air at 1100°C for 4 h. The crystallography, surface morphology and magnetic properties were studied by using X-ray diffraction (XRD), scanning electron microscope (SEM) and vibrating sample magnetometer (VSM), respectively. XRD analysis confirms that the system exhibits polycrystalline single phase cubic spinel structure and absence of any secondary phases. The crystallite size estimated from Scherrer formula for the Gaussian peak (311) has been found to be around 30 nm. The results obtained show that with Al3+ doping, the lattice parameter decreases due to smaller ionic radii of Al3+ ions replaces larger Fe3+ ions. The distribution of Al3+ ions over tetrahedral (A) and octahedral (B) sites is estimated from X-ray intensity calculations. The arrangement of magnetic ions among the tetrahedral and octahedral sites due to the substitution of Al3+ ions modifies the saturation magnetization (Ms) and coercivity (Hc). The room temperature magnetization values increases up to x = 0.1 and then decreases for the increase in aluminum concentration above x > 0.1. Surface topography of the powder samples exhibits nearly spherical shape microstructure and the average grain size has been found to be 180 nm (x = 0.05). The observed variations in the structural and magnetic properties are discussed in the light of existing understanding.

Study of microstructure and augmentation of DC electrical resistivity due to Al3+ substitution in Ni–Zn nano ferrite system synthesized via auto combustion

Nanocrystalline Ni–Zn–Al spinel ferrite was synthesized via citrate-gel auto combustion method. The as-prepared powders have been separated into two batches in which one batch of powders were sintered at 1000∘C for 4 h and the other batch were pressed into pellets and were sintered at the same temperature. Sintering of the samples was done in air atmosphere followed by natural cooling to room temperature. The heat treated powders have then been characterized using TG–DTA, XRD, SEM and TEM for thermal, structural and microstructural aspects while the DC electrical resistivity measurements were carried out on the sintered pellets. The X-ray diffraction patterns displayed the formation of the spinel phase for all powders and the lattice parameter was obtained using Bragg’s law. The crystallite size for all compositions were found to be in nano dimensions and obtained from the Williamson–Hall method. TG–DTA analysis of the undoped Ni0.5Zn0.5Fe2O4 indicated the formation of the spinel phase is around 400∘C while almost uniform microstructure with a more or less spherical grains has been noticed in the SEM micrograph. An enhancement in the DC electrical resistivity ( ≥ 108Ω-cm) has been observed in Ni0.5Zn0.5Fe2O4 synthesized using this technique in comparison with that processed through conventional ceramic technique and a modification in the resistivity has been observed on substituting Al3+ in place of Fe3+. High electrical resistivity makes these ferrites suitable for high-frequency applications due to possible reduction of the eddy current losses. The observed variation in resistivity has been discussed on amendments in structure, microstructure and unavailability of Fe3+ ions with increasing Al3+ ions in the light of existing understanding. The decrease in resistivity with increasing temperature confirms the semiconducting behavior of all samples. Activation energies for conduction were obtained from the slope of the log ρ versus 1/T plots and observed to be in the range of 0.6–0.45 eV. The variation in the activation energy for conduction followed a similar trend as the DC resistivity. The drift mobility decreases with increasing Al3+ ions concentration and increases with increasing temperature.

Effect of Sintering Temperature on the Micro Strain and Magnetic Properties of Ni-Zn Nanoferrites

In this study, nanocrystalline ferrite powders with the composition Ni 0.5 Zn 0.5 Fe₂O₄ were prepared by the autocombustion method. The obtained powders were sintered at 800℃, 900℃ and 1,000℃ for 4 h in air atmosphere. The as-prepared and the sintered powders were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and magnetization studies. An increase in the crystallite size and a slight decrease in the lattice constant with sintering temperature were observed, whereas microstrain was observed to be negative for all the samples. Two significant absorption bands in the wave number range of the 400 cm ?1 to 600 cm ?1 have been observed in the FT-IR spectra for all samples which is the distinctive feature of the spinel ferrites. The force constants were found to vary with sintering temperature, suggesting a cation redistribution and modification in the unit cell of the spinel. The M-H loops indicate smaller coercivity, which is the typical nature of the soft ferrites. The observed variation in the saturation magnetization and coercivity with sintering temperature has been attributed to the role of surface, inhomogeneous cation distribution, and increase in the crystallite size.

Effect of Cr3+ Substitution on Structural, Magnetic and Dielectric Properties of Nanocrystalline Ni0.5Zn0.5CrxFe2-xO4 Ferrite System

Polycrystalline Ni–Zn–Cr ferrite nanoparticles with composition Ni0.5Zn0.5CrxFe2-xO4 (for x in the range of 0.00–0.25 insteps of 0.05) were prepared through the nitrate-citrate gel autocombustion method. The as-obtained powders were ground and annealed at 1000°C for 4 h and characterized for spinel phase using X-ray diffraction, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). X-ray diffraction revealed the existence of single phase spinel while the TEM micrographs showed the presence of nearly spherical particles with soft agglomeration. Pelleted and toroidal samples of the same ferrite compound were sintered at 1000°C for 4 h. The dielectric constant and the dielectric loss tangent decreased with frequency in the range of 1 kHz–13 MHz while the initial permeability remained almost constant at lower measuring frequencies and thereafter increased with frequency for all samples. The observed variations have been discussed in the light of existing understanding.

Effect of sintering temperature on micro structural and impedance spectroscopic properties of Ni0.5Zn0.5Fe2O4 nano ferrite

Ni-Zn nanoferrite Ni0.5Zn0.5Fe2O4 is prepared by citrate gel auto combustion method and sintered at various temperatures 800, 900, 1000, 1100 and 1200°C. The room temperature x-ray diffraction conforms that the single phase spinel structure is formed. Crystallite size and density were increased with increasing of sintering temperature. From Raman spectroscopy all sintered samples are single phase with cubic spinel structure belong to Fd3m space group. From surface morphology studies it is clearly observed that the particle size increased with increasing of sintering temperature. Impedance spectroscopy revel that increasing of conductivity is due to grain resistance is decreased with increasing of sintering temperature. Cole-Cole plots are studied from impedance data. The electrical modulus analysis shows that non-Debye nature of Ni0.5Zn0.5Fe2O4 ferrite.

Structural, dielectric and impedance spectroscopic studies of Ni0.5Zn0.5−xLixFe2O4 nanocrystalline ferrites

Nanocrystalline lithium substituted Ni–Zn ferrites with composition Ni0.5Zn0.5−xLixFe2O4 (x = 0.00–0.25 in steps of 0.05) were synthesized by the citrate gel auto-combustion method and were sintered at 1000∘C for 4 h in air atmosphere. The structural, dielectric, impedance spectroscopic and magnetic properties were studied by using X-ray diffraction, impedance analyzer and vibrating sample magnetometer respectively. The X-ray diffraction patterns confirm that all samples exhibit a single phase cubic spinel structure. Suitable cation distribution for all compositions has been proposed by using the X-ray diffraction line intensity calculations and the theoretical lattice parameter for each composition was observed in close agreement with the experimental ones and thereby supporting the proposed distribution. An increase in the saturation magnetization was observed up to x = 0.10 level of Li+ substitution and thereafter magnetization reduced for higher concentrations to the highest level of Li+ substitution. The dielectric constant and the DC resistivity of Ni–Zn–Li ferrites were noticed to decrease with increase in the Li+ ion concentration. The impedance spectroscopic studies by using the Cole–Cole plots were studied in order to obtain the relaxation time, grain resistance and grain capacitance. AC conductivity initially remained almost independent of frequency for lower frequencies and thereafter for higher frequencies the AC conductivity increased with increase of Lithium concentration.

DC electrical resistivity studies and structure of Ni0.5Zn0.5CrxFe2-xO4 nanoferrites

Chromium substituted Ni–Zn nano ferrite samples with composition Ni0.5Zn0.5Crx⋅Fe2-xO4 (x = 0.00 to 0.25 in steps of 0.05) have been prepared by autocombustion method employing citric acid as an oxidant. Structural, morphological and microstructural characterizations were done on the sintered powders at 1000°C while the DC electrical resistivity studies have been performed on the pellets sintered at the same temperature. The thermogravimetric and differential thermal analysis (TG-DTA) studies indicated the presence of broad exotherm and valley centered about temperature of 400°C. The formation of single-spinel phase has been observed for all the samples. The ionic packing coefficient, vacancy parameter and fulfillment coefficient have all been obtained for each composition. The scanning electron micrographs showed the formation of fine grains with size very less than 1 μm in all samples with a little agglomeration. The observed variation in DC electrical resistivity and activation energy with Cr3+ substitution are attributed to the modifications in microstructure, structural defects and the influence of Cr3+ ions with support of hopping mechanism.