M. Nadeem

Investigation of conduction and relaxation phenomena in LaFe0.9Ni0.1O3 by impedance spectroscopy

Polycrystalline LaFe0.9Ni0.1O3 is prepared by the solid state reactions route. X-ray diffraction is used to analyse the phase purity of the compound. Impedance spectra of LaFe0.9Ni0.1O3 over the frequency range of 1?Hz to 10?MHz are investigated at different temperatures from 100 to 373?K. Two relaxation processes with different relaxation times are observed at each temperature. An equivalent circuit model (R1Q1) (R2Q2) is applied to explore the physical parameters associated with grains and grain boundaries. Frequency and temperature dependence of the relaxation processes and extracted parameters are discussed in terms of hopping between Fe+4 and Fe+3 and trap state scattering at grain boundaries. A change in conduction mechanism from variable range hopping to adiabatic small polaron hopping at 296?K is evident from these results. Traps are active below 296?K in capturing the charge.

Structural and electrical properties of polyaniline/silver nanocomposites

Polyaniline (PANI)/Ag nanocomposites were prepared by separate synthesis of silver nanoparticles by inert gas condensation, incorporating in the 1-methyl-2-pyrrolidinone (NMP) solution of polyaniline emeraldine base (PANIEB) and then cast into films at 120 °C. X-ray diffraction confirmed the presence of ~67 nm silver nanoparticles in the polyaniline matrix. From the thermogravimetric analysis it is observed that the nanocomposite films have a higher degradation temperature than the pure PANI film. Scanning electron microscopy showed a uniform distribution, with spherical and granular morphology for low concentration of Ag nanoparticles, whereas for higher concentration (1.0% Ag) nanorods are formed. The impedance spectroscopic studies of NMP plasticized nanocomposite films suggest microphase separation into reduced and oxidized repeat units. Incorporation of silver nanoparticles in PANI reduces the charge trapping centres and increases the conducting channels, which causes a tenfold decrease in the real part of impedance.

Effects of iron doping on the transport and magnetic behaviour in La0.65Ca0.35Mn1-yFeyO3

We report on the magnetic and transport measurements in an Fe-doped colossal magnetoresistance compound (La0.65Ca0.35MnO3). Increased spin disorder and a decrease of Tc with increasing Fe content are evident. We find that the resistivity data above Tp fits better to a variable range hopping model and the localization length decreases with Fe content. The variations in the critical temperature Tc, confinement length, magnetic moment and magnetoresistance show a rapid change at about 4-5% Fe. The maximum magnetoresistance is seen to increase consistently with the addition of Fe and increases up to 400% for 8% Fe. The effect of Fe is seen to be consistent with the disruption of the Mn-Mn exchange, possibly due to the formation of magnetic clusters.

Colossal resistivity with diminished tangent loss in Zn?Ni ferrite nanoparticles

We have investigated the electrical and magnetic response of the sol‐gel synthesized ZnxNi1−xFe2O4 (x = 0.0, 0.5 and 1) nanoparticles. The ratio of A-site sextet intensity to that of B-site sextet is featured in terms of divergence in coordination of Fe 3+ ions from four-fold (A-site) to six-fold (B-site). Canted spin structure and weakening of Fe 3+ (A)‐Fe 3+ (B) interactions at the surface of the nanoparticles assign the reduced value of room temperature magnetization in these nanoparticles. Shift of the blocking temperature with Zn content is ascribed to the change in the magnetic anisotropy. Colossal resistivity and reduced dielectric constant are discussed on the basis of dangling bond, superparamagnetic character, canted spin structure and polarizability of the cations. Diminished tangent loss is stipulated in terms of decrease in magnetocrystalline anisotropy and collapse of long-range magnetic order. We report colossal resistivity (i.e. 3.15 × 10 9 � cm), reduced dielectric constant (3.97) and diminished tangent loss (0.07) for Ni0.5Zn0.5Fe2O4 nanoparticles. (Some figures in this article are in colour only in the electronic version)

Investigation on the structural, dielectric and impedance analysis of manganese substituted cobalt ferrite i.e., Co1−xMnxFe2O4 (0.0 ≤ x ≤ 0.4)

Manganese substituted cobalt ferrites, i.e., Co1−xMnxFe2O4 (0.0 ≤ x ≤ 0.4) were prepared by a solid state reaction method. XRD analysis confirmed the formation of a single-phase cubic spinel structure for all of the synthesized compositions, whereas an SEM study revealed that Mn substitution changes the microstructure. 57Fe Mossbauer spectroscopy measurements suggested that Fe3+ cations progressively migrate with Mn addition from tetrahedral (A) sites to octahedral (B) sites which have a relatively smaller covalency. Therefore, the distribution of cations between the A- and B-sites changed with increasing x. Moreover, interestingly, the Fe2+/Fe3+ cation ratio remains zero and high spin Fe3+ is the only oxidation state observed at both sites for all of the synthesized compositions. In order to explore the effects of observed variations in the microstructure and cation distribution on the dielectric and resistive properties, the prepared samples were subjected to impedance spectroscopic experiments in a wide frequency range at room temperature. Mn substitution is found to improve the resistive properties by about two orders of magnitude. This increase in the resistive properties is explained in terms of the variations in the microstructure and decrease in the mobility of the charge carriers associated with the cations redistribution. Similarly, the variation in the dielectric permittivity is also conferred in terms of the change in microstructure and cation redistribution.

Dielectric relaxation with polaronic and variable range hopping mechanisms of grains and grain boundaries in Pr0.8Ca0.2MnO3

Polycrystalline Pr0.8Ca0.2MnO3 has been synthesized through solid state reaction route and phase purity is analyzed using synchrotron XRD. Magnetization M(T) showed Mn spin alignment due to ferromagnetic ordering around 125 K (TC), whereas M(H) showed an onset of non-linear behavior from 200 K and with temperature re-orientation of magnetic moment is discussed. The modulation of relaxation processes revealed different formations of impedance plane plots with temperatures. A change in equivalent circuit models from (R1C1)(R2Q2) to (R1C1)(R2Q2)(R3Q3) at 100 K is being reported. The analysis of the impedance data is carried out by calculating impedance of grains and grain boundaries. Fitted parameters derived from these equivalent circuit parameters showed a change in conduction mechanism from small polaronic hopping model (SPH) to Motts variable range hopping (MVRH) model, around 125 K (TC). Carriers hop to larger distance with multiple activation energies and are described by MVRH below TC. Above this temperature, different trap centers start facilitating these carriers through SPH. Dielectric relaxation shows dispersion around 125 K and the origin of this polarization lies close to the relaxation derived from the grains and their interfaces.

Origin of anomalies and phase competitions around magnetic transition temperature in Pr0.7Ca0.3MnO3

A polycrystalline sample of Pr0.7Ca0.3MnO3 is synthesized by the conventional solid-state reaction method and the phase formation is confirmed by x-ray diffraction. In this work, we addressed the phase competition issues in the vicinity of magnetic transition temperature and also established its correlation with oxygen contents of domains, disorder effects and heterogeneity in the material. The appearance and disappearance of anomaly in the vicinity of TC (128K) with magnetic field is discussed in terms of establishment of short- and long-range networks between Mn 3+ and Mn 4+ . Switching behaviour of two competing phases is analysed qualitatively and quantitatively, using an equivalent circuit model and magnetization analysis. The issue of coexisting phases is further substantiated using a simple depression angle approach of impedance plane plots. variable range hopping is found to be a better model than polaronic for explaining the transport properties of both competing phases below the magnetic transition temperature, 128K. (Some figures may appear in colour only in the online journal)

Atomistic computer simulation studies of Sr2RuO4 and Ca2RuO4

Atomistic static computer simulation techniques have been applied to investigate the energetics of defects and dopants in Sr2RuO4 (SRO) and Ca2RuO4 (CRO). Interatomic potentials have been derived which reproduced the crystal structures of these systems. Solution energies are calculated for different dopant ions to ascertain the site occupied by the dopant ion in the host lattice. Monovalent and divalent ions are predicted to substitute preferentially at the alkaline-earth site in both the systems. Trivalent cations of smaller ionic radii substitute at the Ru sites while those having larger ionic radii prefer to substitute at the Sr or Ca sites in SRO or CRO systems, respectively. In addition, there is a possibility of self-compensation, where a trivalent cation can substitute at both Sr(Ca) and Ru sites. Tetravalent dopants are found to substitute at the ruthenium sites in both systems. q 2002 Elsevier Science Ltd. All rights reserved.

Thermoelectric power measurements in Fe doped La0.65Ca0.35MnO3

We report measurements of the thermoelectric power (TEP) on the La0.65Ca0.35Mn1−x FexO3 system for 0.00 ≤ x ≤ 0.07. The ferromagnetic and metallic transition temperatures are lowered and the TEP shows an increasingly positive trend with the addition of Fe. We also observe a clear magnetic contribution that manifests itself as a peak in the TEP close to the critical temperature. The activation energies determined from the TEP are seen to be insensitive to the Fe content. The data are interpreted firstly as showing a decrease in the density of active holes, i.e. holes that can participate in the hopping process, with increasing Fe content. Secondly the data suggest the role of magnetic scattering due to the clusters formed by the antiferromagnetically coupled Fe. Abrupt changes in the variation of the TEP are observed at the concentration region x ~ 0.04 consistent with the hole density variation and with previously reported transport and magnetic measurements.

Low Temperature Synthesis and Properties of BiFeO3

Extensive efforts have been made to synthesize single phase and stoichiometric BiFeO3. Some modified techniques have been tried in synthesizing BiFeO3 as compared with the conventionally used co-precipitation method. Thermogravimetric Analysis/Differential Scanning Calorimetry and x-ray diffraction experiments were used exclusively to explore the effects of heat treatment temperature and time on crystallographic behavior of the prepared BiFeO3 powder. Field emission scanning electron microscopy was used to explore the microstructure of the synthesized BiFeO3. The appearance of different magnetic phases in 57Fe Mössbauer spectra and field dependent magnetization was confirmed on the basis of particle size distribution. In this research, an easy, low cost and high-yield method for the low temperature single phase and stoichiometric synthesis of BiFeO3 has been suggested.