Broad Metal-insulator Transition Research Articles

Enhanced room temperature coefficient of resistivity (RT-TCR) and broad metal-insulator transition temperature (TMI) of La0.67Ca0.33-Ag MnO3 polycrystalline ceramics

Abstract Polycrystalline La0.67Ca0.33-xAgxMnO3 (LCAMO, x = 0, 0.06, 0.15, 0.18, and 0.24) ceramics were fabricated by conventional sol-gel route at relatively low sintering temperature of 1100 °C for 12 h. Effects of silver content (x) on crystal structure, grain size, resistivity and magnetic properties of as-prepared LCAMO specimens were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), elemental mapping and energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and standard four-probe method (ρ-T). The data from XPS, XRD and EDS revealed that silver existed as Ag+ ions in the lattice matrix position of LCAMO ceramics. Broad metal-insulator transition temperature (TMI) values ranging from 267.0 K (x = 0) to 302.6 K (x = 0.24) were obtained with LCAMO specimens prepared with variable Ag+ added contents. Peak temperature coefficient of resistivity (TCR) enhanced from 4.1% K−1 at 263.2 K (for x = 0) to 10.9% K−1 at 278.5 K (for x = 0.18), and reached 7.5% K−1 at room-temperature (295.9 K) for x = 0.24. Meanwhile, magnetoresistance (MR) of materials reached 17.7% at room-temperature (299.2 K) for x = 0.24. Overall, these findings demonstrated that Ag doping was beneficial for improving electrical and magnetic properties of LCAMO materials. In summary, LCAMO ceramics achieved RT-TCR and MR at optimal Ag stoichiometric ratio, promising for applications in infrared bolometers or magnetic sensors.

Investigation of electronic phase segregation in La0.75Ca0.15Sr0.10MnO3 manganite

The effect of electronic phase segregation in a broad metal-Insulator transition (MIT) observed in La0.75Ca0.25-xSrxMnO3 (x=0.1) composition is investigated using heat capacity, magnetization, electrical resistivity and magnetoresistance measurements. The negative magnetoresistance of 65% in an applied magnetic field of 12T and 15% in 1T with a broad working range of 18K around 300K which is beneficial for room temperature colossal magnetoresistance (CMR) applications. The broad transition in temperature dependent zero field resistivity measurement is analyzed in the light of percolation model indicates the abundance of insulating/metallic clusters in metallic/insulating region. A significant difference between the metallic fraction around the MIT and the ferromagnetic phases observed around the Curie temperature demonstrates the interplay between volume of itinerant and polaronic electronic phases.

Wide range magnetoresistance and high temperature coefficient of resistance in La0.7Sr0.3−xAgxMnO3 system

We report observation of composition and process parameter dependant wide range magnetoresistance and high temperature coefficient of resistance in La0.7Sr0.3−xAgxMnO3 (0 ≤ x ≤ 0.3) system. The polycrystalline samples synthesized through solid state reaction method exhibited rhombohedral structure with systematic change in lattice parameters. The samples with x ≤ 0.2 showed metallic behavior below room temperature. A broad metal-insulator transition is observed for x = 0.3, which becomes sharp for the samples sintered at high temperature. The broad transition resulted in magnetoresistance (MR) value around 35% over a wide temperature range whereas the sharp transition results in MR value as high as 85%. The temperature coefficient of resistance value of 11% is seen as a consequence of sharp transition. The magnetic transition was sharp for the samples sintered at high temperature.

Magnetoresistive studies on nanocrystalline La 0.8Sr 0.2MnO 3+δ manganite

Low-temperature magnetoresistance (MR) measurement has been carried out on nanocrystalline La 0.8Sr 0.2MnO 3+ δ manganites prepared by combustion synthesis. This preparation method yields voluminous powders with large surface area (∼40 m 2/g) having crystalline nanosize (∼50 nm) products. Formation and homogeneity of the solid solutions have been confirmed by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). Rietveld refinement of X-ray data indicates that as-formed compound exhibits cubic phase with space group Pm3 m. However, calcined sample transforms into rhombohedral phase with space group R-3c. The stabilization of the cubic phase in as-formed manganite is due to the substitution of Sr 2+ on La 3+ sites, resulting in higher Mn 4+ content. The low-temperature resistivity measurements down to 70 K exhibit a broad metal–insulator transition ( T M−I) at around 257 K. MR measurements on sintered pellets show ∼5% MR at 1 T, whereas for 4 and 7 T, the %MR values are found as 22% and 28%, respectively, at T M−I.

The first organic paramagnetic metal containing the aminoxyl radical

We have prepared an organic magnetic anion, PROXYL-4-CONHCH2SO3− (1). The electrocrystallisation of BEDT-TTF with PPh41 gave the first purely organic paramagnetic metal, β″-(BEDT-TTF)2(1). The salt shows a broad metal–insulator transition at approximately 210 K.

Temperature-dependent photoemission and x-ray absorption studies of the metal-insulator transition inBi1−xLaxNiO3

Perovskite-type $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_{3}$ is an insulating antiferromagnet in which a charge disproportionation occurs at the Bi site. La substitution for Bi suppresses the charge disproportionation and makes the system metallic, and for $0.05\ensuremath{\le}x\ensuremath{\le}0.1$ a broad metal-insulator transition (MIT) occurs as a function of temperature. We have measured the temperature dependence of the photoemission and x-ray absorption (XAS) spectra of ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}\mathrm{Ni}{\mathrm{O}}_{3}$ to investigate how the electronic structure changes across the MIT. From the $\mathrm{Ni}\phantom{\rule{0.2em}{0ex}}2p$ XAS spectra of $x=0.05$, we found almost no change in the valence of Ni across the MIT. In the valence-band photoemission spectra, the Fermi cutoff disappeared for $x=0.05$ at a low temperature, whereas for $x=0.1$ and 0.2, it remained at all temperatures but the intensity at the Fermi level decreased gradually with decreasing temperature. Our experimental results suggest that the MIT is caused by the localization of holes in the $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ band and that the ``insulating'' phase below the MIT is indeed a mixture of insulating and metallic regions.

Structural, magnetic and magneto-transport studies on the (La2/3Sm1/3)0.67Ba0.33−xSrxMnO3 (x = 0, 0.1, 0.2 and 0.33) manganites

The effect of substituting Sr for Ba on the magneto-transport and magnetic properties of the (La2/3Sm1/3)0.67Ba0.33MnO3 system has been investigated. The samples of (La2/3Sm1/3)0.67Ba0.33−xSrxMnO3 (x = 0.0, 0.1, 0.2 and 0.33) are synthesized by the citrate gel route and crystallize in an orthorhombic structure. The unit cell volume decreases while the metal–insulator transition temperature (TMI) increases with increasing Sr content. The atomic disorder, which is represented by the size variance, increases with Ba content. The small anomaly in ρ(T) due to the atomic disorder which leads to the different magnetic ordering followed by a broad metal–insulator transition is observed in Ba-rich systems. The peak at T* (small anomaly) is presumed to be due to spin-dependent interfacial tunnelling that causes the difference in magnetic order between the surface and the core. The suppression in the resistivity values with increasing Sr content could be due to the more symmetric MnO6 octahedra network. The paramagnetic phase coexists with the magnetically ordered phase between T* (∼TC) and TMI. The suppression of the metastable magnetic phase leads to the high magnetoresistance in Ba-rich samples. The transport data has been analysed using a small polaron correlated model. The magnetic behaviour may be understood by the Ising model. The overall magnetic behaviour could be accounted for by the competing antiferromagnetic and ferromagnetic interactions.

Magnetoresistance studies on barium doped nanocrystalline manganite

An energetically attractive, simple, fast and a novel low temperature (300 °C) solution combustion route for the synthesis of crystalline and homogeneous nanoparticles of lanthanum barium manganese oxide La 0.9Ba 0.1MnO 3+ δ (LBMO) is reported. Formation and homogeneity of the solid solutions have been confirmed by powder X-ray diffraction (PXRD) and energy dispersive X-ray analysis (EDS) respectively. The Rietveld analysis shows both as-formed as well as calcined samples are in cubic phase with space group pm3 m. The microstructure and agglomerated particle size of the compounds are examined by scanning electron microscope. Infrared spectroscopy revealed that both Mn O Mn bending mode and Mn O stretching mode are influenced by calcination temperature. The magnetoresistance measurement on sintered LBMO pellet exhibits a broad metal–insulator transition ( T M-I) at around 228 K. At 1 T applied magnetic field, LBMO shows magnetoresistance (MR) of 10%, whereas for 4 and 7 T, the negative magnetoresistance values are in the range 51 and 59% respectively at T M-I. The experimental resistivity data of the present investigation are fitted to a simple empirical equation in order to understand conduction mechanism in this compound.

Magnetic and transport properties in the Ti doped cobaltite Ca3Co4−xTixO9 (⩽x⩽0.8) single crystals

Single crystals of Ca3Co4−xTixO9 (x=0, 0.2, 0.4, 0.6, and 0.8) are grown by flux growth technique. The effect of Ti doping at Co site on magnetic and transport properties is studied. All samples show Curie-Weiss behavior at temperatures above 50K except for the x=0.8 sample, and the effective magnetic moment μeff increases with increasing Ti content. The free-doped sample Ca3Co4O9 shows a ferrimagnetic transition at 19K. However, a spin-glass transition at Tf(∼20K) appears for the Ti doped samples and the transition temperature Tf shifts to higher temperature as the Ti content increases. The magnetic moment values M at Tf and 5K under the applied field of 4.5T increase with increasing Ti doping content. The variation of the magnetic properties may be related to the induced extra Co2+ ions in the CoO2 layer by substituting Ti4+ for Co3+. The resistivity ρ of the x=0, 0.2, 0.4, and 0.6 samples shows metalliclike behavior at high temperatures and a broad metal-insulator (M-I) transition appears as the temperature decreases to a certain value Tmin. The value of resistivity and the ratio of ρ5K∕ρ300K increase with increasing Ti content. The results are discussed according to the decrease of charge carrier concentration in the samples, which is confirmed by the Hall coefficient measurement. For the x=0.8 sample, there exists another M-I transition in the vicinity of 269K, which is suggested to originate from the spin-state transition of Co3+ or Co4+ ions.

Enhancement of magnetoresistances at room temperature in YSZ doping La 0.67Sr 0.33MnO 3 system

The temperature dependence of the resistance of composite samples (1− x)La 0.67Sr 0.33MnO 3+ xYSZ with different YSZ doping level x was investigated at magnetic fields 0–3 T, where YSZ represents yttria-stabilized zirconia. Results show that the YSZ dopant does not only adjust the metal–insulator transition temperature, but also increases the magnetoresistance effect. With increase of YSZ doping level for the range of x<2%, the metal–insulator transition temperature values T P of the composites decrease, but T P increases with increase of x further for the range of x>2%. Meanwhile, in the YSZ-doped composites, a broad metal–insulator transition temperature region was found at zero and low magnetic field, which results in an obvious enhanced magnetoresistance in the temperature range 10–350 K. Specially, a larger magnetoresistance value was observed at room temperature at 3 T, which is encouraging with regard to the potential application of magnetoresistance materials.

Distinct insulating state below the Curie point inPr0.7Ba0.3MnO3

Magnetization, resistivity, thermal expansion, thermopower, and neutron powder diffraction data are presented for the perovskite manganite ${\mathrm{Pr}}_{0.7}{\mathrm{Ba}}_{0.3}{\mathrm{MnO}}_{3}.$ A distinct ferromagnetic insulating state, similar to that observed in the ${\mathrm{La}}_{0.88}{\mathrm{Sr}}_{0.12}{\mathrm{MnO}}_{3}$ system, is observed in the temperature range between the paramagnetic-ferromagnetic transition at ${T}_{\mathrm{FM}}\ensuremath{\approx}180 \mathrm{K}$ and the broad metal-insulator transition near ${T}_{p}=120 \mathrm{K}.$ The large A-site cation-size mismatch appears to suppress the ferromagnetic double-exchange interactions relative to the competing ferromagnetic superexchange interactions, thereby giving rise to the ferromagnetic insulating state.

Beta' '-(BEDT-TTF)4[(H3O)Cr(C2O4)3]CH2Cl2: effect of included solvent on the structure and properties of a conducting molecular charge-transfer salt.

The new charge-transfer salt β″-(BEDT-TTF)[(HO)Cr(CO )]S, where BEDT-TTF = bis-(ethylenedithio)tetrathiafulvalene and S = CHCl has a broad metal-insulator transition at approximately 150 K. This differs from similar salts in the BEDT-TTF tris(oxalato) metalate series, notably the superconducting materials with S = PhCN and the salt with S = CHN, which has a sharp metal-insulator transition.

Extrinsic magnetoresistance in La 2/3Ca 1/3MnO 3 thick films

Thick films of La 0.66Ca 0.33MnO 3 have been obtained on alumina substrate by the “paint-on” method. They exhibit a clear extrinsic magnetoresistance (broad metal–insulator transition with peak temperature lower than Curie temperature, magnetoresistance at low temperatures and magnetic fields, etc.). This behavior seems to be related to the competition between tunneling magnetoresistance (TMR) at the grain boundaries and double exchange in the grains. An anomaly is observed in the resistance vs. applied magnetic field at low temperatures, which support the presence of TMR. EDAX analysis performed to the films shows elemental composition deviations near to the grain boundaries.

Growth of epitaxial and polycrystalline thin films of the electron doped system La1−xCexMnO3 through pulsed laser deposition

The polycrystalline La1−xCexMnO3 manganites do not exist in single phase in bulk under the preparation conditions so far employed, but their polycrystalline and epitaxial films deposited by the pulsed laser deposition (PLD) technique form readily in single phase. The cerium oxide (CeO2) remains partially unreacted when the bulk sample is prepared through the solid state reaction route. The resistivity of the bulk La0.7Ce0.3MnO3 sample shows a broad metal insulator transition (MIT) clearly resolved into two peaks, suggesting the presence of a second (impurity) phase, which is identified as unreacted CeO2 by the intensity analysis of the x-ray diffraction (XRD) data. However, when prepared as thin films by PLD, La0.7Ce0.3MnO3 forms in single phase, as corroborated by the uniqueness and sharpness of the MIT peak and also by the XRD patterns of the polycrystalline films. We also performed a detailed study of the epitaxial films by a high-resolution XRD system with a four-circle goniometer and did not find any impurity phase. The magnetization data shows a very sharp transition followed by a sharp MIT in resistivity at the same temperature in the epitaxial thin film. These results suggest that PLD can be used as a useful technique to synthesize unconventional compounds, which do not form easily in bulk.

Structural Properties of the Superconducting Salt (BEDT-TTF)3Cl2·(H2O)2 at Low Temperatures

The X-ray crystal structure of (BEDT-TTF)3Cl2·(H2O)2 [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene] has been determined at 190, 130, and 10 K. The broad metal–insulator transition (100–160 K) observed in the resistivity data at ambient pressure, and the pressure induced superconductivity (Tc=5 K at 1.2 GPa) can be associated with temperature-dependent motion of the molecules within the unit cell and charge redistribution among the three independent BEDT-TTF molecules. The motion can be considered as a dimerization of the BEDT-TTF along the a-axis. The thermal expansion tensor in the range 12 to 295 K is anisotropic and reveals no sharp anomaly. The system remains two dimensional at all temperatures, in agreement with the electrical conductivity measurements.

Studies of the microwave resistivity of κ-(BEDT-TTF)2Cu[N(CN)2]Cl

Abstract We have performed temperature dependent measurements of the surface impedance of κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl in both directions, parallel and perpendicular to the highly conducting planes. In both orientations the microwave resistivity basically follows the dc behavior for T > 15 K: the crystal undergoes a broad metal-insulator transition at around 50 K. While there are no indications of superconductivity at ambient pressure found in the dc resistivity and by ac susceptibility, the microwave resistivity clearly drops at 13 K and again at 5 K, indicating two superconducting phases. When pressure is applied by embedding it in grease, only the well known superconducting phase transition at T c = 13 K remains.

Effect of Growth Conditions and Buffer Layers on the Metal-Insulator Transition in V2O3 Thin Films

AbstractThin films of V2O3 with thickness from 20 to 450 nm were deposited on (0001) oriented sapphire substrates by reactive e-beam evaporation. LEED, x-ray diffraction and AFM studies show highly oriented grains with a lateral size of 50 to 800 nm, dependent on substrate temperature and deposition rate. The films were characterized by optical and infrared transmission, electrical resistance and Hall effect measurements. Films grown directly on the Al2O3-substrate show a very broad metal-insulator (MI) transition as a function of temperature. The width of the transition decreases with increasing film thickness. The insertion of Cr2O3 buffer layers decreases the transition width by a further factor of three. The electronic properties of the films can be drastically influenced by the growth conditions.

Electrical Properties of the Square-Antiprismatic Chain Compounds M4Br4Os (M = Y, Er) and M4Te4Z (M = Nb, Ta; Z = Si, Cr, Fe, Co)

The resistivities of the rare-earth metal halides, R[sub 4]Br[sub 4]Os (R = Y, Er) and several of the ternary tellurides M[sub 4]Te[sub 4]Z (M = Nb, Z = Si, Fe, Co; M = Ta, Z = Si, Cr, Co) were measured using standard four-probe ac and dc techniques at temperatures ranging from 4.2 to 350 K. The rare-earth halides R[sub 4]Br[sub 4]Os show semiconducting behavior: conductivity increases exponentially with temperature. Activation energies (E[sub a]) for conduction in Y[sub 4]Br[sub 4]Os and Er[sub 4]Br[sub 4]Os are 0.16 and 0.11 eV, respectively. The resistivity of Er[sub 4]Br[sub 4]Os is 2 orders of magnitude smaller than that of Y[sub 4]Br[sub 4]Os at 273 K. The behavior of these compounds is in accord with the calculated band structure presented. The isoelectronic compounds M[sub 4]Te[sub 4]Si (M = Ta, Nb) behave as electronic conductors at high temperature. Ta[sub 4]Te[sub 4]Si shows a resistivity maximum at 160 K and below 60 K exhibits a broad metal-insulator transition. The absolute resistivity measured for Ta[sub 4]Te[sub 4]Si is 2 orders of magnitude higher than the other tellurides. The analogue Nb[sub 4]Te[sub 4]Si shows a similar anomalous resistivity maximum near 280 K and at low temperature ([approximately]60 K) exhibitsmore » a gradual rise in resistivity that suggests a semiconducting ground state. Transition-metal-centered M[sub 4]Te[sub 4]Z compounds all behave as simple metals in the temperature range 4.2-300 K; metal-insulator transitions are not seen at temperatures greater than 4.2 K. Nb[sub 4]Te[sub 4]Fe shows a small resistivity anomaly near 25 K. 30 refs., 10 figs., 2 tabs.« less

On the nature of the metal-insulator transition in β-(BEDT-TTF) 4Pt(CN) 4

Tight-binding electronic band structures of the organic conductor β-(BEDT-TTF) 4Pt(CN) 4 were calculated for the crystal structures determined at 293 and 135 K to examine a probable cause for its broad metal-insulator transition that begins at ∼250K.