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Monophasic and polycrystalline double perovskite Eu2CoMnO6 has been synthesized, and its structural characterization, frequency and temperature-dependent dielectric relaxation have been studied. Observed thermally activated dielectric relaxation was explained using the empirical Havriliak–Negami (HN) dielectric relaxation function with an estimated activation energy E ∼ 0.22 eV and attempt frequency f 0 ∼ 2.46 × 109 Hz. The frequency-dependent AC conductivity data, over a wide range of temperature (100–325 K), followed the empirical universal power law behavior (∼ f n , n is the constant exponent) showing two different frequency exponents, respectively, in the high- and low-temperature regions. The high-temperature (> 275 K) conductivity data followed the continuous time random walk (CTRW) approximation model proposed by Dyre. However, this model failed to reproduce the observed conductivity spectra in the low-temperature side (< 200 K). Interestingly, both the high- and low-temperatures' conductivity data can be scaled to the master curve with suitably chosen scaling parameters. [ABSTRACT FROM AUTHOR]

We report multifunctional properties of the charge disproportionate correlated electronic system La0.5Ba0.5FeO3 synthesized by conventional solid state reaction route. The bulk sample undergoes semiconductor-metal like transition at TSM = 102 K showing appreciable magnetoresistance (∼20%) around TSM even at a low field (∼0.5 T). A crossover of electrical conduction mechanism from Mott's variable range hopping to small polaron hopping occurs around 210 K, well above TSM. Analysis of the transport, magnetic susceptibility, and magnetization data in the light of de Gennes proposal indicates the formation of canted antiferromagnetic spin order preceded by charge ordering in La0.5Ba0.5FeO3. Nonsaturation behavior and nonlinearity of the M-H curve at low temperature and at high field (∼5 T), confirms the canted spin order. The observed resistive transition is considered to be associated with the onset of weak ferromagnetic ordering in the canted antiferromagnetic spin state of the materials. Moreover, novel ferroelectric like behavior showing electric field-polarization hysteresis loop below the charge ordering temperature along with anomalous magnetocapacitance response representing magnetoelectric coupling is also observed. In the low temperature regime spin lattice coupling might be responsible for exhibiting magnetocapacitance in La0.5Ba0.5FeO3. Such multifunctional character implies fundamental as well as technological importance of this sample. [ABSTRACT FROM AUTHOR]

Interesting low temperature memory phenomena have been observed from equilibrium and out of equilibrium magnetic measurements on the La0.7Ca0.3MnO3 nanomanganite system. The observed phenomenon were screened for atomic spin glass (SG), super spin glass (SSG), cluster glass (CG), and superparamagnetic behavior. The results evidences of SG like behavior at low temperature (<40 K) in this manganite system consisting of ferromagnetic nanoparticles. In the temperature region between 40 K and the ferromagnetic Curie point TC∼217 K, a ferromagnetic CG state develops with a relatively weaker interparticle interaction than that of the low temperature SG phase. The dynamic magnetization shows aging, chaos and memory effects. Moreover, we have also noticed asymmetric response in magnetic relaxation in response to positive and negative temperature cycling protocols. The origin and nature of the low-temperature SG state in this system is discussed within the framework of hierarchical organization of metastable states. The results show existence of various time and length scales in the system, which can be explained by considering the nanoparticles with grain boundary spin disorder and the presence of noncompact ferromagnetic clusters. [ABSTRACT FROM AUTHOR]

Internal nanodomains in the form of stripe or nanostripe domains (NSDs) have been found in high permittivity CaCu3Ti4O12 (CCTO) prepared by ceramic route. High resolution transmission electron microscopy study detects plane defects along the domain boundaries. Electron diffraction patterns indicate the existence of probable twinning in CCTO, resulting plane defects. X-ray photoelectron spectroscopy studies also detect Cu1+ and Ti3+ demonstrating the presence of compositional disorder in CCTO which leads to lattice dislocations along the domain boundary regions. Highly disordered plane defects are responsible for creation of NSDs and act as insulating barriers. Thus, high permittivity is achieved through internal barrier layer capacitance within CCTO grains. Present observation of NSDs in CCTO and their origin finally confirm the extrinsic mechanism of the huge dielectric response of CCTO ceramic. [ABSTRACT FROM AUTHOR]

Homogeneous thick film (∼0.10 mm) of high dielectric K0.05Ti0.02Ni0.93O; abbreviated as KTNO/polyvinylidene fluoride (PVDF) composite has been prepared by hot-molding technique. The frequency and temperature dependent dielectric behavior of this composite has been studied by varying the KTNO volume fraction (fKTNO). Near the percolation threshold (fKTNO=0.40), a large enhancement of effective dielectric permittivity ([variant_greek_epsilon]eff∼400 which is 40 times higher than that of pure PVDF) with low loss (∼0.20 at 1 kHz) is observed. The experimental [variant_greek_epsilon]eff data have been fitted with different theoretical models and found to follow percolation theory successfully. Such a high [variant_greek_epsilon]eff and low loss flexible dielectric material appears to be suitable for technological applications. [ABSTRACT FROM AUTHOR]

The magnetocaloric effect (MCE) has been observed in three well-defined temperature intervals (region I: 2–115 K, region II: around charge order transition TCO, and region III: around room temperature) in polycrystalline Nd0.5Ca0.5MnO3 (NCMO) manganite system showing a first order (CO) transition at a temperature, TCO∼250 K. The magnitude of ΔSM(H) increases monotonically with applied magnetic field but does not reach saturation even at fields as high as 60 kOe. In the three given temperature regions, negligible magnetic and thermal hysteresis are found, which satisfies the requirements of using NCMO as an effective magnetocaloric material for magnetic refrigeration. [ABSTRACT FROM AUTHOR]

Complex impedance spectroscopic study has been made to elucidate the conductivity mechanism and dielectric relaxations in a low loss giant dielectric ([variant_greek_epsilon]′∼104) KxTiyNi1-x-yO (KTNO) system with x=0.05–0.30 and y=0.02 over a wide temperature range (200–400 K). Below ambient temperature (300 K), dc conductivity follows variable range hopping mechanism. The estimated activation energy for dielectric relaxation is found to be higher than the corresponding polaron hopping energy, which is attributed to the combined effect of K-doped grains and highly disordered grain boundary (GB) contributions in KTNO. Observed sharp fall of [variant_greek_epsilon]′ below ∼270 K is ascribed to the freezing of charge carriers. Comparatively lower value of relaxation time distribution parameter β of KTNO than that of the CaCu3Ti4O12 (CCTO) system reveals more disorder in KTNO. It is also found that KTNO is structurally more stable compared to the CCTO system, both having giant [variant_greek_epsilon]′ value. [ABSTRACT FROM AUTHOR]

We report a low field (H=0.5 T) magnetoresistance of colossal magnetoresistive (CMR) composites prepared with ferromagnetic (FM)—antiferromagnetic (AFM) coupled system (100-x) La0.7Pb0.3MnO3(LPMO)+xPr0.63Ca0.37MnO3 with x=0–85 wt %. A large increase (∼16% maximum) of magnetoresistance (MR) (∼10% for pure LPMO) is observed at a relatively higher optimum concentration (x=70% AFM). Interesting double peaks are observed for lower AFM concentrations (x<=35%) in the resistivity-temperature [ρ(T)] curve: one due to the usual intrinsic double exchange interaction (between Mn3+ and Mn4+) mechanism and the other due to the stronger FM-AFM coupling for lower concentration. Unlike the usual ceramic CMR material, thermoelectric power (TEP) in this system is also nonadditive in character, indicating the presence of additional defects and strain in this composites. A two-channel (conducting and semiconductor) model has been used which fit well with the ρ(T) data near the optimum concentration x=70%. Around this highest AFM concentration, showing maximum MR, the corresponding TEP is also higher than those of the individual parent compound, which indicates the presence of maximum grain boundary effect and defects. Positron annihilation lifetime data of the composites also support the presence of such defects and strain in the system with maximum around the critical concentration xC=70%. [ABSTRACT FROM AUTHOR]

From the low-temperature (down to 10 K) dc-magnetization data of the La0.7Sr0.3MnO3 (LSM), La0.7Ca0.3MnO3 (LCM), and Pr0.7Ca0.3MnO3 (PCM) systems, we estimated the critical exponents β, γ, and hence δ from the analysis of the modified Arrot plots. The exponent β estimated for the LCM system is less than that predicted by Heisenberg model and resides within the zone predicted by Ising model while for the LSM sample, β is higher than that predicted from the Heisenberg model which is considered to be due to the presence of dipole-dipole interaction arising from the large spin moment in the LSM system. The magnetization data of the PCM system cannot be fitted to the modified Arrot plots, which suggest highly inhomogeneous ground state even under 5 T magnetic field. Both LSM and LCM have almost equal values of γ. Seebeck coefficient data indicate a crossover from higher-temperature n-type to lower-temperature p-type conductivity behavior in both LSM and LCM systems, while the semiconducting PCM system shows p-type conductivity throughout the temperature range (300–80 K). It is noticed that for LSM system TC (Curie temperature) and TMI (metal-insulator transition temperature) are almost equal (∼360 K), whereas for the LCM system there exist a large difference between TC and TMI (TC∼245 K and TMI∼265 K), which may give some idea regarding the critical behavior of the respective samples. [ABSTRACT FROM AUTHOR]

Using the concept of the Preisach model for a ferroelectric capacitor, we have analyzed the behavior of frequency (f)-dependent polarization reversal in surface-stabilized ferroelectric liquid crystals (SSFLCs) under an external field. At a fixed temperature, the peak height of the capacitance-voltage hysteresis loop is found to be decreasing with an increasing number of polarization cycles up to a certain typical value of frequency (e.g., fc) being different for different ferroelectric liquid crystals (FLCs). We also observed an inversion (a well instead of a peak) in the hysteresis loops appearing above a typical higher frequency denoted fi. The values of capacitance of the FLC capacitors became almost independent of the voltage for the frequencies between fc and fi. The frequency dependence of dielectric biaxiality in the SSFLCs is directly attributed to the appearance of such behavior. [ABSTRACT FROM AUTHOR]

We report that vacuum-deposited amorphous MoO3–TeO2 films of different thickness (469–1185 nm) and compositions show metal-insulator transition (MIT) in the temperature (Tp) range 341–231 K (depending on thickness and composition). Interestingly, the corresponding thick bulk glass plates (∼0.2–0.5-mm thickness) of same compositions behave like the usual semiconducting transition metal oxide glasses over the entire range of temperature (80–400 K) and follow small polaron hopping (SPH) conduction mechanism. Temperature-dependent resistivity curves of the films showed peaks (maxima) around the respective MIT temperature Tp, which shifted to the lower-temperature region with the increase of MoO3 content. Fitting of the conductivity data of the film in the high-temperature (T>Tp) semiconducting phase shows a crossover around Tc (>Tp) from the Mott variable range hopping to SPH behavior, which is in sharp contrast to the behavior of the bulk glass. In these films, the estimated localization length (ξ) diverges as MIT is approached from the high-temperature phase. Low-temperature (Tp) magnetic-field-independent metallic part of the resistivity (ρ) data of higher (>=60%) TeO2 containing films can be fitted with ρ=ρ0+ρ2T2 indicating the importance of electron-electron scattering (second term) mechanism of conduction. The corresponding metallic behavior of the comparatively high-resistive and lower (<=50%) TeO2 containing film is, however, more complicated. All these different features of these glassy films suggest that their glass network structure is different from that of the corresponding bulk glass (indicating a change of glass network structure in the film phase), which is considered to be responsible for the metallic transition in the films. [ABSTRACT FROM AUTHOR]

Transport properties (resistivity and thermoelectric power) of Pb doped LaNiO3 viz. La1-xPbxNiO3-δ (0.0<=x<=0.1) show metallic behavior over a wide range of temperature (10–550 K). Pb doping (up to 10%) at the La site does not destroy the metallic behavior of LaNiO3. The paramagnetic susceptibility χ decreases with Pb doping. Above 50 K, χ is almost temperature independent and exhibits Pauli like features with a small additional Curie law contribution. The resistivity ρ increases with Pb doping though the thermoelectric power does not change proportionately, indicating that Pb doping does not introduce much disorder in the system. A linear T dependence of ρ observed above 150 K suggests the importance of electron–phonon (el–ph) interactions but at temperature below 150 K, ρ follows a T1.5 dependence. The estimated el–ph interaction constant λ increases (0.80–2.53) with Pb doping (x=0.0–0.1). No saturation of resistivity has been observed even up to 550 K with x<=0.1. The phonon frequency νph∼3444 cm-1 of the undoped sample calculated from the absorption peak of the Fourier transform infrared spectra agrees well with that at ∼3025 cm-1 estimated from the reported heat capacity data. [ABSTRACT FROM AUTHOR]

Magnetization and transport properties of La[sub 0.7]Pb[sub 0.3]MnO[sub 3]+x Ag (x=0-20 wt %) nanocomposites have been reported. In this Ag-containing colossal magnetoresistive (CMR) La[sub 0.7]Pb[sub 0.3]MnO[sub 3] composite (referred to as CMR-Ag), conductivity (σ) and metal–insulator transition temperature (T[sub p]) increase with increasing Ag. Electron microscopy and elemental mapping indicated a uniform distribution of Ag nanoparticles/clusters. The enhancement of T[sub p] is accompanied by a reduction of the c-axis lattice constant. Coexistence of interfacial tunneling with intrinsic transport behavior has been observed at the grain boundaries in the samples with a higher (>=10 wt %) Ag content. Due to the presence of nonmagnetic Ag in the ferromagnetic La[sub 0.7]Pb[sub 0.3]MnO[sub 3] material, dc magnetization decreases but the corresponding T[sub p] increases. In the low-temperature (TT[sub p]), conductivity data follow the adiabatic polaron hopping conduction mechanism. Unlike the Ag free sample, the variable-range-hopping model is found to be inapplicable for the present Ag containing nanocomposites. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Frequency (10 Hz-1 MHz) and temperature (22- 80 °C) dependent dielectric constant (both real and imaginary) of a ferroelectric liquid crystal ZLI-4851 film (21 μm thickness) showing a second order SmC*-SmA phase transition around 63.2 °C have been reported. The variation of dielectric constant as a function of temperature follows the generalized Landau theoretical model. Thermal variation of the complex dielectric constant has also been measured with different bias field strengths (0-20 V) for both the SmC* and the SmA phases. Interestingly, soft mode is observed in the SmC* phase only by applying a bias field perpendicular to the helix axis. Due to the application of the bias field, the SmC* phase loses stability of the helical structure, which is considered to be responsible for the appearance of soft mode in the present ferroelectric liquid crystal system. [ABSTRACT FROM AUTHOR]

Pulsed magnetic field (0-4.4 T) was used to study the magnetic field dependent resistivity (12-350 K) and thermoelectric power (0-1.5 T) of Na and K-doped La[SUB0.7]Ca[SUB0.7-y]A[SUBy]MnO[SUB3] (0.0≤y≤0.3, A=Na, K) system showing semiconducting to metallic transitions around temperature T[SUBp]. Na/K-doping increases both conductivity and T[SUBp]. In La[SUB1-x]Ca[SUBx]MnO[SUB3], an increase of T[SUBp] and conductivity with an increase of Ca (for x≤0.33) are small and the small polaron coupling constant (γ) and hence the electron-lattice (phonon) interaction is strong. But in the Na/K doped system, γ is small and for y≥0.05, Motts' condition of strong el-ph interaction breaks down in the high temperature (T>T[SUBp]) phase. Increase of conductivity in the Na/K doped system is caused by the)decrease of γ, binding energy (W[SUBp]), hopping energy (W[SUBH]), and effective mass (m[SUBp]) of the polarons leading to the melting (we call it) of polarons in the T>T[SUBp] phase. This melting results in an increase)of exchange coupling constant between spins. Field dependent thermoelectric power (TEP) of the samples (measured between 80-300 K) also supports the small polaron hopping conduction. The resistivity data are well fitted with the variable range hopping model for a limited range of temperature (T[SUBp]θ[SUBD]/2. With the application of a magnetic field, the density of states at the Fermi level increases. The TEP data indicate the importance of electron-magnon contribution in the low temperature (TT[SUBp]. [ABSTRACT FROM AUTHOR]

Partial substitution of Cu by K in multicomponent glassy precursor for high T[SUBC] superconductor viz. Bi[SUB2-x]Pb[SUBx]Sr[SUB2]Ca[SUB2]Cu[SUB3]O[SUBδ](x=0.1-0.3) is found to destroy superconductivity of the corresponding annealed phase. These glasses viz. Bi[SUB2-x]Pb[SUBx]Sr[SUB2]Ca[SUB2]Cu[SUB3-y]K[SUBy]O[SUBz](x=0.1-0.3, y=0.2), however, show ferroelectric transitions (between 500 and 530 K depending on x). The corresponding K free glasses are not ferroelectric but they are good precursors for high T[SUBC] superconductor. Nanocrystalline particles/clusters (∼10-50 nm size) embedded in the K doped glasses, observed from transmission electron microscopic studies, are considered to be responsible for the ferroelectric behavior of the bulk glass nanocrystal composite (GNC). Unlike K free glasses, the K-containing glasses do not become superconductor by annealing them at higher temperature. This indicates suppression of superconductivity by the precipitation of ferroelectric nanoparticles in the K doped GNC. Though their dielectric behavior is different, both the K free and K containing glassy systems are semiconductors and follow the similar nonadiabatic small polaron hopping conduction mechanism (obeying Holstein condition). A comparative study of the properties of the two glassy systems showing distinguishing behavior has been made. [ABSTRACT FROM AUTHOR]

Magnetic-field (B=0-1.5T) dependent thermoelectric power (TEP) of the Na-doped La[sub 0.7]Ca[sub 0.3-y]Na[sub y]MnO[sub 3] (0.0≤y≤0.3) system has been studied in the temperature range 80-300 K. X-ray diffraction studies indicate a rhombohedrally distorted perovskite structure of the samples. The observed sharp peak in the resistivity (ρ) versus temperature (T) curve falls and shifts to higher temperature with increasing Na concentration (y). In the low-temperature ferromagnetic (FM) regime, thermopower (Seebeck coefficient, S) obeys the expression S=S[sub 0] + S[sub 1.5]T[sup 1.5] + S[sub 4]T[sup 4] over the entire range of y. Electron-magnon scattering is found to dominate the low-temperature resistivity and TEP data. High-temperature TEP data can be well fitted with Mott's small polaron hopping model. The activation energy (E[sub S]) and polaron hopping energy (W[sub H]) decrease with increasing Na content. Both E[sub S] and W[sub H] decrease while polaron radius (r[sub p]) increase with the application of a magnetic field. Field-dependent TEP data also indicate the suppression of spin fluctuations in the presence of a magnetic field. In the low-temperature FM region, both magnon drag and phonon drag effects coexist. [ABSTRACT FROM AUTHOR]

The effect of particle size on the transport properties (resistivity and thermopower) of La[sub 0.5]Pb[sub 0.5]MnO[sub 3] has been investigated both in the presence and in the absence of magnetic field B=0.0–1.5 T (maximum). Grain size, dc conductivity; and the metal–insulator transition temperature T[sub p] of the sample increase with increasing annealing time. Grain size has, however, comparatively little effect on the Seebeck coefficient S. Magnetoresistance is higher for the samples with smaller grain sizes. dc magnetic susceptibility also increases with increasing grain size. High temperature (T>θ[sub D]/2) resistivity data well fit the small polaron hopping model. Polaron hopping energy W[sub H] decreases but polaron radius r[sub p] increases with the increase of grain size. In the metallic regime (for TT>θ[sub D]/2), it is hard to justify this model from the temperature dependent thermopower data. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Two different phases of LaMnO[sub 3+δ] [one showing a metal-insulator transition (MIT), referred to as LaMn-C, and the other not showing a MIT, referred to as LaMn-S] have been clearly observed to follow two different conduction mechanisms. Interestingly, small polaron hopping models of Mott, Schnakenberg, and Emin are found to fit the conductivity data of all the samples above the corresponding MIT temperature. The conductivity data of the insulating (semiconducting) LaMn-S followed a nonadiabatic hopping conduction mechanism while LaMn-C and the Pb doped samples viz. La[sub 1-x]Pb[sub x]MnO[sub 3] (x=0.05-0.5) showed a similar type of MIT and followed an adiabatic small polaron hopping conduction mechanism in the high temperature paramagnetic phase (above the respective MIT temperature). Activation energy (W), density of states at the Fermi level N(E[sub F]), Debye temperature (θ[sub D]), electron-phonon interaction constant (γ[sub P]), etc. of LaMn-S showed appreciable differences from those of LaMn-C and La[sub 1-x]Pb[sub x]MnO[sub 3], which show a MIT. Polaron hopping conduction is also supported by thermoelectric power (TEP) measurements. An observed small but appreciable magnetic field dependence of the TEP data (measured at B=1.5 T) is considered to be associated with magnetic polarons. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports on transport properties of chromium containing multicomponent oxide glasses. Glass transition and crystallization temperatures obtained from differential thermal analysis curves of glass samples; Room temperature thermoelectric power measurement; Adiabatic polaron-hopping mechanism for glasses.