H. H. Joshi

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.

The magnetic properties of the MgZn ferrite system by Mössbauer spectroscopy

Abstract Samples of the magnetism-zinc ferrite series ZnxMg1−xFe2O4 (x = 0.0 to 1.0) have been studied by the Mossbauer effect technique at 77 K. Mossbauer spectra for x = 0.0 to 0.6 suggest the existence of two hyperfine fields, one due to the Fe3+ tetrahedral ions (A-sites) and the other due to the Fe3+ octachedral ions (B-sites), while for x=0.7 it shows relaxation behaviour and for x⩾0.8 it exhibits a paramagnetic quadrupole doublet. The variation of nuclear magnetic fields at the A and B sites is explained on the basis of the AB and BB supertransferred hyperfine interactions. Analysis of the average Mossbauer line width as a function of zinc concentration suggests that the relaxation spectrum observed at x=0.7 (77 K) is possibly due to domain wall oscillations.

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].

Mössbauer study of the spinel system TixCo1+xFe2−2xO4

Abstract Based on 57 Fe Mossbauer effect studies conducted at room-temperature the magnetic properties of the spinel solid-solution series Ti x Co 1+ x − Fe 2−2 x O 4 have been investigated. Mossbauer spectra for x =0.1 to 0.4 suggest the existence of two hyperfine fields, one due to the Fe 3+ octahedral ions (B-sites) and the other due to the Fe 3+ tetrahedral ions (A-sites), while for x =0.5 it shows relaxation effects and x ⩾0.6 it exhibits a paramagnetic quadrupole doublet. The variation of the hyperfine fields at the A and B sites is explained on the basis of the A-B and B-B supertransferred hyperfine interactions. The relaxation behaviour observed for x =0.5 may be due to lack of long range ordering which may be attributed to the localized canting structure of the spins.

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.

The effect of Zn+2 substitution on some structural properties of CuFeCrO4 system

The interesting physical and chemical properties of the spinel ferrites are governed by the type of magnetic ions residing on the tetrahedral (A) and octahedral (B) sites and the relative strength of the inter (JAB) and intra sublattice (JBB, JAA) interactions. The distribution of cations among A and B sites and physical parameters such as density, porosity, shrinkage, particle size, etc. can be controlled by preparation parameters like temperature, synthesis technique, pressure and suitable choice of substituent ions. Ferrites containing Cu have shown interesting electrical and magnetic properties [1]. The potential applications of Cu, Ni and Co based soft ferrites are well known, especially where these elements are partially substituted by zinc and cadmium which occupy tetrahedral position in the spinel lattice and strongly affect the magnetic properties of ferro spinels [2, 3]. Such properties are affected and governed by structural parameters. Recently, some work has been reported in the literature on the magnetic properties of slowly cooled [4] and quenched [5] samples of Znx Cu1−x FeCrO4 spinel ferrite system. No systematic study of the concentration dependence of structural properties such as lattice constant, ionic radii, density, porosity, shrinkage, particle size etc. has been reported in the literature. In the present communication we report the effect of Zn+2 substitution for Cu+2 in Znx Cu1−x FeCrO4 (x = 0.0, 0.2, 0.4 and 0.6) on some structural properties with the aim to develop sintered materials whose properties can be tailored to the requirements of the device engineers. The four samples of the spinel series were prepared by usual double sintering ceramic method. The starting materials were AR grade oxides ZnO, CuO, Fe2O3 and Cr2O3 produced from Thomas & Backer. These oxides were mixed in proper proportion and pre-sintered at 950 ◦C for 12 h. In final sintering process, the materials were held at 1100 ◦C for 12 h and slowly cooled to room temperature. The X-ray diffractograms were recorded using Cu Kα radiation in a Philips X-ray diffractometer, model PM 9220. Typical X-ray diffractograms for the samples with x = 0.2, 0.4 and 0.6 are shown in Fig. 1. The diffractograms showed the presence of cubic spinel phase with no extra lines corresponding to any other phase. The X-ray lines were found to be sharp, which makes the detection of any impurity phase easy. The values of lattice constant a were determined with an accuracy of ±0.0002 nm. The variation of lattice constant with zinc concentration, x , is shown in Fig. 2 and the values are tabulated in Table I. The lattice constant increases linearly with increasing x thus obeying the Vegard’s law [6]. Usually in a solid solution of spinels within the miscibility range, a linear change in the lattice constant with the concentration of the components is observed [6]. The slow linear increase in lattice constant is due to the replacement of smaller Cu+2 ions (0.069 nm) by the slightly larger Zn+2 ions(0.070 nm) in the system Znx Cu1−x FeCrO4. The X-ray density (dx) and bulk

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.

Cation distribution of the system CuAlxFe2-xO4 by X-rays and Mossbauer studies

The structural and Mossbauer spectroscopy studies have been performed on the spinel solid solution series CuAlxFe2−xO4 (0.0 ≤ x ≤ 1.0). All the compounds with 0.0 ≤ x ≤ 1.0 crystallised with cubic spinel structure. Lattice constant values calculated from XRD analysis were found to decrease on increasing x, linearly obeying Vegards law. The X-ray intensity calculations indicated that Cu2+ prefers to occupy octahedral (B) site, where as Al3+ ions replace Fe3+ ions from both tetrahedral (A) and octahedral (B) sites. Mossbauer spectra at room temperature display magnetic sextets corresponding to A and B-sites superimposed on each other. The data shows that Al-possesses greater preference for B-site compared to A-site, and iron exists in high spin ferric Fe3+ state. The hyperfine fields for both A and B-sites decrease with increasing x. The cation distribution calculated from X-ray intensity data agrees with the Mossbauer results.

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.

Magnetic properties of magnesium-cobalt ferrites synthesized by co-precipitation method

The alternating current (a.c.) low field susceptibility vs temperature, magnetization and57Fe Mössbauer effect measurements are reported for the spinel solid solution series MgxCo1−xFe2O4 synthesized by a wet-chemical method before and after high temperature annealing. The observed features for the wet samples, such as the coexistence of paramagnetic doublet and magnetic sextets in Mössbauer spectra and lower saturation magnetization values confirm small particle ferrite behaviour. Especially, Mössbauer spectra of wet samples reveal the presence of superparamagnetic particles which exist simultaneously with ferrimagnetic regions in the materials well supported by a.c. susceptibility data. The high temperature annealing changes the wet-prepared ferrites into the ordered magnetic structure of ceramic ferrites.