Giant Magnetoresistance and Related Properties of Rare Earth Manganates and Other Oxide Systems

Giant magnetoresistance (GMR), which was until recently confined to magnetic layered and granular materials, as well as doped magnetic semiconductors, occurs in manganate perovskites of the general formula Ln(1-x)A(x)MnO(3) (Ln = rare earth; A = divalent ion). These manganates are ferromagnetic at or above a certain value of x (or Mn4+ content) and become metallic at temperatures below the curie temperature, T-c. GMR is generally a maximum close to T-c or the insulator-metal (I-M) transition temperature, T-im. The T-c and %MR are markedly affected by the size of the A site cation, [r(A)], thereby affording a useful electronic phase diagram when T-c or T-im is plotted against [r(A)]. We discuss GMR and related properties of manganates in polycrystalline, thin-film, and single-crystal forms and point out certain commonalities and correlations. We also examine some unusual features in the electron-transport properties of manganates, in particular charge-ordering effects. Charge ordering is crucially dependent on [r(A)] or the e(g) band width, and the charge-ordered insulating state transforms to a metallic ferromagnetic state on the application of a magnetic field.

Variable-Temperature EPR and Transport Studies on Poly(4,4'-methylenedianiline): Evidence for Bipolarons

The correlation between magnetic and transport properties is examined by studying poly(4,4'-methylenedianiline)(PMDA) salts and their bases using EPR and conductivity measurements. Five different PMDA salts (doped polymers)were prepared by chemical polymerization of 4,4'-methylenedianiline using different protonic acids. The PMDA bases were obtained by dedoping the salts using ammonium hydroxide. Ambient temperature electrical conductivity measurements show evidence for the doped PMDA system to be highly disordered. The EPR spectra of the samples were recorded in the range 20-200 "C, and the results were analyzed on the basis of the polaron-bipolaron model, which is typical of nondegenerate systems. Both PMDA salts and their bases consist of self-trapped, highly mobile polarons or radical cations. EPR studies on PMDA salts show evidence for the presence of thermally activated and temperature independent (or Pauli type) paramagnetism while the bases show thermally activated, Pauli and Curie-Weiss types of paramagnetism. The paramagnetism arises due to polarons.It is proposed that charge transport takes place through both polarons and bipolarons.

Charge transport in transparent conductors: A comparison

A comparative investigation of charge transport properties is presented, for polymeric [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)], single-wall carbon nanotube (SWNT) and inorganic (indium tin oxide, ITO), transparent conducting electrodes. The polymeric and nanotube systems show hopping transport at low temperatures, in contrast with the disordered-metal transport in ITO. The low temperature magnetotransport (up to 11 T) and high electric-field transport (up to 500 V/cm) indicate the significant role of nanoscopic scale disorder for charge transport in polymer and nanotube based systems. The results show that characteristic length scales like localization length correlates with the nanomorphology in these systems. Further, the high frequency conductivity measurements (up to 30 MHz) in PEDOT:PSS and SWNT follow the extended pair approximation model [σ(ω)=σ(0)[1+(ω/ω0)s].

Electrical transport in layered crystalline semiconductors GeS doped with Ag, P impurities at high pressure

A study of the transport properties of layered crystalline semiconductors GeS (undoped and doped with Ag, P impurity) under quasihydrostatic pressure using Bridgman anvil system is made for the first time. Pressure-induced effects in undoped crystals reveal initial rise in resistivity followed by two broad peaks at higher pressures. Silver doping induces only minor changes in the behaviour except removing the second peak. Phosphorous impurity is found to have drastic effect on the transport properties. Temperature dependence of the resistivity exhibits two activation energies having opposite pressure coefficients. Results are discussed in the light of intrinsic features of the layered semiconductors.

The problem of liquid droplet combustion-reexamination

The simple quasi-steady analysis of the combustion of a liquid fuel droplet in an oxidising atmosphere provides unsatisfactory explanations for several experimental observations. It's prediction of values for the burning constant (K), the flame-to-droplet diameter ratio ( ) and the flame temperature (Tf) have been found to be amgibuous if not completely inaccurate. A critical survey of the literature has led us to a detailed examination of the effects of unsteadiness and variable properties. The work published to date indicates that the gas-phase unsteadiness is relatively short and therefore quite insignificant.A new theoretical analysis based on heat transfer within the droplet is presented here. It shows that the condensed-phase unsteadiness lasts for about 20â??25% of the total burning time. It is concluded that the discrepancies between experimental observations and the predictions of the constant-property quasi-steady analysis cannot be attributed either to gas-phase or condensed-phase unsteadiness.An analytical model of quasi-steady droplet combustion with variable thermodynamic and transport properties and non-unity Lewis numbers will be examined. Further findings reveal a significant improvement in the prediction of combustion parameters, particularly of K, when consideration is given to variations of cp and λ with the temperature and concentrations of several species. Tf is accurately predicted when the required conditions of incomplete combustion or low ( ) at the flame are met. Further refinement through realistic Lewis numbers predicts ( ) meaningfully.

Anderson transitions in Ln1-xSrxCoO3 (Ln=Pr or Nd)

Electrical conductivity measurements show that Ln1-xSrxCoO3, (Ln = Pr or Nd) undergoes a non-metal-metal transition when x-0 3. The d.c. conductivity of compositions with 0properties suggest that Co4+ ions in Ln1-xSrxCoO3 (0

Local structure and magneto-transport in Sr2FeMoO6 oxides

Double perovskite oxides Sr2FeMoO6 have attracted a great interest for their peculiar magneto-transport properties, and, ill particular, for the large values of low-field magneto-resistance (MR) which remains elevated even at room temperature, thanks to their high Curie temperature (T-c > 400 K). These properties are strongly influenced by chemical cation disorder, that is by the relative arrangement of Fe and Mo on their sublattices: the regular alternation of Fe and Mo enhances the M R and saturation magnetization. On the contrary the disorder generally depresses the magnetization and worsen the MR response. In this work the X-ray absorption fine structure (XAFS) technique has been employed in order to probe the cation order from a local point of view. XAFS spectra were collected at the Fe and Mo K edges on Sr2FeMoO6 samples with different degree of long-range chemical order. The XAFS results prove that a high degree of short-range cation order is preserved, despite the different long-range order: the Fe-Mo correlations are always preferred over the Fe-Fe and Mo-Mo ones in the perfectly ordered as well as in highly disordered samples.

Heisenberg-like critical properties in ferromagnetic Nd1-xPbxMnO3 single crystals

Static magnetization for single crystals of insulating Nd0.85Pb0.15MnO3 and marginally conducting Nd0.70Pb0.30MnO3 has been studied around the ferromagnetic to paramagnetic transition temperature T-C. Results of measurements carried out in the critical range vertical bar(T - T-C)/T-C vertical bar <= 0.1 are reported. Critical exponents beta and gamma for the thermal behaviour of magnetization and susceptibility have been obtained both by modified Arrott plots and the Kouvel-Fisher method. The exponent delta independently obtained from the critical isotherm was found to satisfy the Widom scaling relation delta = gamma/beta + 1. For both compositions the values of exponents are consistent with those expected for isotropic magnets belonging to the Heisenberg universality class with short-range exchange in three dimensions. Correspondingly, the specific heat displays only a cusp-like anomaly at the critical temperature of these crystals which is consistent with an exponent alpha < 0. The results show that the ferromagnetic ordering transition in Nd1-xPbxMnO3 in the composition range 0.15 <= x <= 0.40 is continuous. This mixed-valent manganite displays the conventional properties of a Heisenberg-like ferromagnet, irrespective of the differing transport properties and in spite of low ordering temperatures T-C = 109 and 147.2 K for x = 0.15 and 0.30, respectively.

Room-temperature gas sensors based on gallium nitride nanoparticles

Room-temperature sensing characteristics for H-2, ethanol, NH3, H2S and water have been investigated with thick-film sensors based on GaN nanoparticles, prepared by a simple chemical route. In general, GaN nanoparticles exhibit satisfactory sensor properties for these gases and vapors even at room temperature. The sensitivity for ethanol is found to be highest, the sensitivity and recovery times being smallest. Gas sensor properties of GaN seem to be related to intrinsic defects, which act as sorption sites for the gas molecules. (c) 2010 Elsevier Ltd. All rights reserved.

Temperature dependent electrical transport behavior of InN/GaN heterostructure based Schottky diodes

InN/GaN heterostructure based Schottky diodes were fabricated by plasma-assisted molecular beam epitaxy. The temperature dependent electrical transport properties were carried out for InN/GaN heterostructure. The barrier height and the ideality factor of the Schottky diodes were found to be temperature dependent. The temperature dependence of the barrier height indicates that the Schottky barrier height is inhomogeneous in nature at the heterostructure interface. The higher value of the ideality factor and its temperature dependence suggest that the current transport is primarily dominated by thermionic field emission (TFE) other than thermionic emission (TE). The room temperature barrier height obtained by using TE and TFE models were 1.08 and 1.43 eV, respectively. (C) 2011 American Institute of Physics. doi: 10.1063/1.3549685]

Synthesis and properties of nanodispersed granular materials

Nanometric granular materials represent a new class of materials with significant promise. We shall discuss in this paper two phase granular materials where one of the phases having nanometric dimension is embedded in a matrix of larger dimension. These materials show many interesting properties which include structural, magnetic and transport properties, The phase transformation of the embedded particles shows distinctive behavior and yields new insight. We shall first highlight the strategy of synthesis of these materials through rapid solidification. This will be followed by three examples where the nanoscale dimension of the embedded particles play a unique role. These are melting and solidification of the nanodispersed embedded particles and the superconducting transition. (C) 1997 Elsevier Science S.A.

Applications of solid electrolytes in galvanic sensors

The concepts and theoretical origins of conduction domains for solid electrolytes and electrode polarization are outlined briefly. The point electrode made of the ' solid electrolyte material is useful for deflecting the semipermeability flux away from the electrode. The emf of galvanic sensors consisting of two solid electrolytes in intimate contact with each other and in which transport occurs by a common ion is reviewed. The voltage of such cells depends on the chemical potential of the active species at the interface between the two electrolytes, which can be evaluated from the transport properties of electrolytes using a numerical procedure. The factors governing the speed of response of solid electrolyte gas sensors are analyzed. A rigorous expression for the emf of non-isothermal galvanic sensors and the criterion for the design of temperature compensated reference electrodes for nonisothermal galvanic sensors are outlined. Non-isothermal sensors are useful for the continuous monitoring of concentrations or chemical potentials in reactive systems at high temperatures. The principles of operation of galvanic sensors for oxygen, sulphur, oxides of sulphur (SOx,x=2,3), carbon, oxides uf carbon (COx,x= 1,2), oxides of nitrogen (NOx,x= 1,2) and silicon are discussed. The use of auxiliary electrodes in galvanic sensors to expand the detection capability of known solid electrolytes to a large number of species is explained with reference to sensors for sulphur and oxides of sulphur (SOx,x=2,3).Finally the cause of the common errors in galvanic measurements and test for the correct functioning of galvanic sensors is given.

Anderson transitions in Ln1-x Sr x CoO3 (Ln═Pr or Nd)

Electrical conductivity measurements show that Ln1-x Sr x CoO3, (Ln = Pr or Nd) undergoes a non-metal-metal transition when x≈0 3. The d.c. conductivity of compositions with 0properties suggest that Co4+ ions in Ln1-x Sr x CoO3 (0

Anomalous low-temperature magnetic and magnetotransport properties in Ru-deficient SrRuO3

SrRuO3 is a well-known itinerant ferromagnet with many intriguing characteristics. The Ru deficiency in this system is believed to play a pivotal role in influencing many of its magnetic and transport properties. The present study involves the magnetic and transport properties of the Ru-deficient SrRu0.93O3 sample to gain more insight into the unusual low-temperature behavior. The ac susceptibility study reveals a sharp ferromagnetic transition at 150 K followed by a hump at T-h similar to 50 K, which has anomalous frequency dependence. Besides, the T-h shifts to lower temperatures with an increase in the superposed dc-biasing field and adheres to H-2 dependence, in accordance with the Gabay and Toulouse line for the Heisenberg spin glass systems. We also observe a pronounced memory effect toward the low-temperature side, signifying the characteristic of glassy behavior. The temperature-dependent magnetoresistance indicates the signature of an additional ordering toward the low-temperature side. All of the interesting findings combined unveil the existence of low-temperature cryptic magnetic phase in SrRu0.93O3. (C) 2012 American Institute of Physics. doi:10.1063/1.3673427]

Experiments on and thermodynamic analysis of a turbocharged engine with producer gas as fuel

This article presents the studies conducted on turbocharged producer gas engines designed originally for natural gas (NG) as the fuel. Producer gas, whose properties like stoichiometric ratio, calorific value, laminar flame speed, adiabatic flame temperature, and related parameters that differ from those of NG, is used as the fuel. Two engines having similar turbochargers are evaluated for performance. Detailed measurements on the mass flowrates of fuel and air, pressures and temperatures at various locations on the turbocharger were carried out. On both the engines, the pressure ratio across the compressor was measured to be 1.40 +/- 0.05 and the density ratio to be 1.35 +/- 0.05 across the turbocharger with after-cooler. Thermodynamic analysis of the data on both the engines suggests a compressor efficiency of 70 per cent. The specific energy consumption at the peak load is found to be 13.1 MJ/kWh with producer gas as the fuel. Compared with the naturally aspirated mode, the mass flow and the peak load in the turbocharged after-cooled condition increased by 35 per cent and 30 per cent, respectively. The pressure ratios obtained with the use of NG and producer gas are compared with corrected mass flow on the compressor map.