K. B. Modi

Magnetic and electrical properties of Al3+-substituted MgFe2O4

The structural, magnetic and electrical properties of the Al3+-substituted disordered spinel system Mg(Fe2−xAlx)O4 have been investigated by X-ray diffraction, magnetization, a.c. susceptibility and electrical resistivity measurements. The cation distribution derived from the X-ray diffractometry data was found to agree very well with the cation distribution obtained through Mössbauer spectroscopy. The variation of saturation magnetization per formula unit as a function of aluminium context, x, has been satisfactorily explained on the basis of Neels collinear spin model and the slight discrepancy between the observed and calculated nB values can be explained in terms of a random canting model. The Néel temperatures calculated theoretically by applying molecular field theory agreed well with the experimentally determined values from thermal variation of susceptibility and electrical resistivity. An unusual metal-like thermo-electric behaviour was found for the compositions with x⩾ 0.3 which was attributed to the decrease in the Fe-Fe separation distance arising from aluminium substitution.

Study on the electrical and dielectric behaviour of Zn-substituted cobalt ferrialuminates

The structural, electrical and dielectric properties of the spinel ferrite system ZnxCo1−xFeAlO4 (x = 0.2–0.5) have been studied by means of various experimental techniques. Polycrystalline samples of this series have been prepared by the double sintering ceramic method. The structural parameters such as the lattice constant, x-ray density, pore fraction, cation distribution, interionic distances and polaron radius have been determined for better understanding of the electrical and dielectric behaviour. The compositional and thermal variations of the electrical properties have been studied by means of dc resistivity, thermoelectric power, I–V and C–V measurements. The electrical parameters such as the activation energy, Fermi energy, charge carrier concentration and mobility have been determined and a probable conduction mechanism in the system has been suggested. It is found that the ferrites are electronic conductors, and various results confirm the formation of small polarons. Current controlled negative resistance (CCNR)-type switching is observed in samples with x = 0.3–0.5. The dielectric behaviour of the system has been studied by measuring the dielectric constant, complex dielectric constant and conductivity in the frequency range 100 Hz–1 MHz at selected temperatures. The compositions exhibit normal dielectric behaviour, attributed to Maxwell–Wagner type interfacial polarization. Broadband dielectric spectroscopy in the form of an electric modulus has been applied to investigate the electrical spectroscopy in a wide temperature range. The results clearly indicate the presence of the non-Debye type of dielectric relaxation in these materials.

Magnetic and electrical properties of aluminium and chromium co-substituted yttrium iron garnets

Abstract Polycrystalline yttrium aluminium-chromium iron garnets (Y 3 Al x Cr x Fe 5 − 2 x O 12 ) with varying Al-Cr substitution (0 ≤ x ≤ 1.0) have been prepared in the pellet form, and studied by X-ray diffraction, magnetization, a.c. susceptibility and electrical resistivity measurements. The lattice constants are determined and the applicability of Vegards law has been tested. The saturation magnetization (4π M s ) decreases linearly with increasing x from x = 0.1 to 1.0 indicating reduction in ferrimagnetic behaviour. Variation of saturation magnetic moment per formula unit at 300 K with x can be explained satisfactorily assuming the collinear spin ordering model. The Curie temperature ( T C ) decreases with increasing x which is consistent with the observed decrease in 4π M s with x . The activation energy ( E ) initially increases up to x = 0.6 and thereafter it levels off for further increase in x > 0.6.

Study of cation distribution in lithium doped nickel ferrite

The structural and magnetic properties of ferrites are sensitive to the nature, the valence state and distribution of metal ions over tetrahedral (A) and octahedral (B) sites in the spinel lattice. Therefore, the knowledge of cation distribution and spin alignment is essential to understand the magnetic properties of spinel ferrites. The outstanding fact about the ferrites is that they combine extremely high electrical resistivity with good magnetic properties. This means that the ferrites are more suitable for high frequency and low loss applications as compared to ferromagnetic metals. For more than a decade, lithium ferrite materials have dominated in the field of microwave applications. Several series of lithium ferrite covering wide range of properties have become commercially available. Suitable substituents in lithium ferrites have made it possible to tailor the materials properties for a variety of diverse requirements of microwave devices. Lithium ferrites also have the inherent properties of high Neel temperatures, rectangular hysterisis loop and high dielectric constant combined with low cost. There are many research reports in the literature on the studies of Li-Zn, Li-Mg, Li-Ti, Li-Cd ferrites [1– 4]. However, no information is available in the literature regarding the Li-Ni ferrite and relative site preference of Li+1, Fe+3 and Ni+2 in the spinel lattice. The spinel system Li0.5Fe2.5−x Mx O4 (where M = V, Cr and Rh) has been studied by Blasse [5]. It has been observed that substitution of V, Cr and Rh causes migration of Li ions for B-sites to A-sites. It has also been observed that Li ions occupy B-sites in pure Li-ferrite. The above results indicate that the Li ions have B-site preference and therefore it is interesting to study the sites occupancy of Li ions in the presence of the cations having strong B-site preference. Kapitonove [6] studied the distribution of cations in Li0.5Fex Ga2.5−x O4 with varying composition and preparation conditions. Nickel ferrite (NiFe2O4, x = 0.0) is an inverse spinel with all the Ni ions on B-sites. In order to study the relative site preferences for Li1+ and Ni2+ ion for octahedral site in spinel lattice, the spinel series Li0.5x Ni1−x Fe2+0.5x O4 with x = 0.0–0.8 has been prepared. The cation distributions have been determined through X-ray diffraction and confirmed by magnetization measurements. It is found that a greater percentage of Li1+ ions occupy the A-sites in the Ni-rich samples. This is reflected in the variation of lattice constant, magnetization and Neel temperature with Li-substitution supported by IR spectroscopy. The thermal variation of low field a.c. susceptibility exhibits normal ferrimagnetic behavior. Eight compositions of Li-Ni system were prepared by standard ceramic technique where stoichiometric proportions of Li2Co3, Fe2O3 and NiO powder were ground in acetone. The homogeneous mass was pelletized using 2% solution of polyvinyl acetate as binder medium. The pellets were presintered at 1000 ◦C for 12 h. In the final sintering process the materials were held at 1200 ◦C for 12 h and furnace cooled at the rate of 2 ◦C per min to room temperature. The X-ray diffraction patterns were recorded for all the samples on a Philips diffractometer model PW 1710. The magnetization measurements were made at 300 K using the high field hysterisis loop tracer. The low field (39.8 A/m) a.c. susceptibility measurements of powdered samples were taken in the temperature range 300–900 K using the double coil set-up, operating at a frequency of 263 Hz. The infrared spectra were recorded at 300 K on Perkin-Elemer IR spectrometer in KBr medium, in the frequency range 1000 cm−1–400 cm−1. The X-ray diffraction (XRD) patterns showed that the samples were single phase spinels. No reflections other than those belonging to a spinel structure were observed in the patterns. The variation of lattice constant a (nm) with content x is shown in Fig. 1. From Fig. 1 it is seen that the lattice constant value is not much influenced by Li1+ substitution (within the error) in NiFe2O4 (x = 0.0) up to x = 0.3, thereafter, it gradually decreases with the average rate of 0.0008 nm per 10% of Li1+-substitution. This happens for the Ni-rich samples (x 0.3, a greater fraction of Li1+ migrates to octahedral (B) sites with decreasing Ni concentrations. The X-ray density (dx) decreases with Li1+ substitutions because of the fact that the rate of decrease of molecular weight is faster than that of the volume of the unit cell per 10% of lithium concentration. The XRD intensities were calculated by using the formula suggested by Burger [7].

Effect of Al3+ substitution on the transport properties of copper ferrite

This study presents the compositional and the temperature-dependent thermoelectric power and dc resistivity measurements on polycrystalline spinel ferrite system: CuAlxFe2−xO4 (x = 0.0, 0.2, 0.4 and 0.6). The probable conduction mechanisms in the system are Fe2+ ↔ Fe3+ + e− (n-type) at the octahedral sites and Cu2+ ↔ Cu1+ + e+ (p-type) at the tetrahedral sites. The cationic concentrations of Cu1+, Cu2+, ferrous and ferric ions have been deduced from Seebeck coefficients which, in turn, are used to determine the actual cation distribution, oxygen deficiency and to explain the variation and origin of dc resistivity with Al concentration. The Fermi energy, charge carrier concentration and mobility of charge carrier have been determined to explain the charge transport phenomena.

Study of cation distribution and macro-magnetic properties of magnesium and aluminum co-substituted lithium ferrite

Abstract The structural and bulk magnetic properties of mixed spinel ferrite system Mg x Al 2 x Li 0.5(1− x ) Fe 2.5(1− x ) O 4 ( x =0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) have been investigated by means of X-ray diffraction, magnetization and a.c. susceptibility measurements. The cation distribution determined through X-ray data are supported by the magnetization and susceptibility measurements. The study has revealed the strong octahedral site preference of Li + and Al 3+ as compared to Mg 2+ . It is also found that the magnetic ordering in the system is of collinear type and the variation of magnetization can be explained on the basis of Neels collinear spin model. The Neel temperatures calculated theoretically are in good agreement to those deduced experimentally.

Structural and elastic properties of Ca-substituted LaMnO3at 300 K

The compositional dependence of structural and elastic properties of Ca2+ substituted manganite perovskite system with a general formula La1−xCaxMnO3 (x = 0.00, 0.25, 0.33, 0.50 and 1.00) have been studied by means of x-ray diffraction, scanning electron microscopy and ultrasonic pulse transmission technique at 300 K. The elastic moduli of the specimens have also been computed and these are corrected to the void free state. The lattice parameters calculated theoretically are in agreement with those determined experimentally. The structural–elastic properties correlations have been studied. The observed enhancement of elastic constants up to x = 0.33 and reduction with a further increase in calcium concentration has been explained in the light of change in the strength of interatomic bonding caused by change in interatomic distances, microstructure and electronic configuration. The applicability of the heterogeneous metal mixture rule for theoretical estimation of elastic constants has been tested.

Raman and Mossbauer Spectroscopy and X-ray Diffractometry Studies on Quenched Copper–Ferri–Aluminates

Four spinel ferrite compositions of the CuAl(x)Fe(2-x)O4, x = 0.0, 0.2, 0.4, 0.6, system prepared by usual double-sintering ceramic route and quenched (rapid thermal cooling) from final sintering temperature (1373 K) to liquid nitrogen temperature (80 K) were investigated by employing X-ray powder diffractometry, (57)Fe Mossbauer spectroscopy, and micro-Raman spectroscopy at 300 K. The Raman spectra collected in the wavenumber range of 100-1000 cm(-1) were analyzed in a systematic manner and showed five predicted modes for the spinel structure and splitting of A1g Raman mode into two/three energy values, attributed to peaks belonging to each ion (Cu(2+), Fe(3+), and Al(3+)) in the tetrahedral positions. The suppression of lower-frequency peaks was explained on the basis of weakening in magnetic coupling and reduction in ferrimagnetic behavior as well as increase in stress induced by square bond formation on Al(3+) substitution. The enhancement in intensity, random variation of line width, and blue shift for highest frequency peak corresponding to A1g mode were observed. The ferric ion (Fe(3+)) concentration for different compositions determined from Raman spectral analysis agrees well with that deduced by means of X-ray diffraction line-intensity calculations and Mossbauer spectral analysis. An attempt was made to determine elastic and thermodynamic properties from Raman spectral analysis and elastic constants from cation distribution.

Structural properties of magnesium and aluminium co-substituted lithium ferrite

The structural properties of Mg2+ and Al3+ co-substituted Li0.5Fe2.5O4 are studied by synthesizing the spinel solid solution series MgxAl2xLi0.5(1−x)Fe2.5(1−x)O4. Polycrystalline samples of this series with x=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 have been prepared by double-sintering ceramic method. The structural details like: lattice constant and distribution of cations in the tetrahedral and octahedral interstitial voids have been deduced through X-ray diffraction (XRD) data analysis. The x dependence of bond length, oxygen positional parameter, site ionic radii, bulk density, porosity and shrinkages have also been determined.

Far-infrared spectral studies of magnesium and aluminum co-substituted lithium ferrites

Polycrystalline MgxAl2xLi0.5(1−x)Fe2.5(1−x) O4 (x = 0.0, 0.2, 0.5, 0.6 and 0.7) ferrites were prepared by standard ceramic method, and characterized by X-ray diffraction and infrared absorption spectroscopy. The spectra show two significant absorption bands in the wave number range of 400–1000 cm−1 arising from interatomic vibrations in the tetrahedral and octahedral coordination compounds. The decrease in intensity and increase in broadness of bands with concentration (x) are explained on the basis of cation distribution. The force constants and bulk modulus are found to decrease with Mg-Al content (x) which suggested weakening of interatomic bonding. An alternate method for the determination of bulk modulus, longitudinal and transverse velocities is suggested. The magnetic and electrical properties of these compounds are explained in the light of structural and optical properties.