Metal Insulator Transition Temperature TMI Research Articles

Structural and Magneto-Electrical Properties of (1-x) La0.70Sr0.30MnO3- xBaTiO3 (x = 0.10 and 0.20)

(1-x) La0.70Sr0.30MnO3 (LSMO) – x BaTiO3 (BTO) (x = 0.10 and 0.20) ceramics were synthesised by chemical route as well as solid-state route for preparation of various compositions in LSMO-BTO (x = 0.10 and 0.20) systems. In this process, LSMO have been prepared by chemical route method and BTO synthesised by solid-state method. The room temperature structure characterisation of all the samples of LSMO-BTO for x = 0.00, 0.10 and 0.20 was carried out. The LSMO have rhombohedral symmetry with space group R-3C; BTO has tetragonal symmetry with space group P4mm. Among all the samples studied, pure and composite samples have shown that the lowest Metal–Insulator (MI) transition temperature occurs for x = 0.20 at 187 K for their temperature-dependent resistivity having the largest value (~ 170 Ω cm). The peak resistivity, ρmax increases as the MI transition temperature TMI, goes decreases. Percentage magnetoresistance (MR) variation of LSMO-BTO (x = 0.10 and 0.20) with temperature has also been studied. Analytical analysis shows the % MR data is normally distributed the value of test is greater than 0.05.

Structural and transport properties of composite of La0.8Sr0.2MnO3 and BaTiO3 materials

We studied the effect of BaTiO3 phase on the structural, vibrational, and electrical properties of La0.8Sr0.2MnO3. The parent samples of La0.8Sr0.2MnO3, BaTiO3 and their composite (0.85) La0.8Sr0.2MnO3 + (0.15) BaTiO3 were synthesized by solid-state reaction route. Structural study using X-ray diffraction (XRD) for La0.8Sr0.2MnO3 phase revealed a rhombohedrally distorted (space group R3c) structure, while BaTiO3 compound was found to have crystallized in tetragonal structure (space group P4mm). The XRD results were verified using Rietveld refinement. The composite sample (0.85) La0.8Sr0.2MnO3 + (0.15) BaTiO3 display the presence of both of La0.8Sr0.2MnO3 and BaTiO3 phases. The addition of insulating BaTiO3 phase in the ferromagnetic metallic phase of La0.8Sr0.2MnO3 which exhibits well-characterized metal-to-insulator transition in the vicinity of 350 K enhances the resistivity and shifts the metal–insulator transition temperature TMI towards lower temperature. Further, with the application of magnetic field, resistivity of the composite sample is found to decrease and TMI in composite sample is observed to shift towards higher temperature. The magnetoresistance (MR) of composite sample (0.85) La0.8Sr0.2MnO3 + (0.15) BaTiO3 decreases monotonously in 200–300 K regime under magnetic field of 8 T. The analysis of electrical resistivity data in the metallic region (T TMI) is well described using Small polaron hopping (SPH) and Mott's variable range hopping (VRH) models.

Temperature dependence of thermodynamic properties of spinel CuIr2S4

We have investigated the thermodynamic and electrical properties of spinel CuIr2S4. The temperature, T, dependence of the electrical resistivity ρ(T), thermal conductivity, κ(T), and thermoelectric power, S(T), show the abrupt jumps with the relative large hysteresis at a metal-insulator transition temperature TMI∼230K, with decreasing T. The acoustic-Debye model can fit κ(T) in the insulating phase (T<TMI). On the other hand, κ(T) in the metallic phase (TMI<T) can be reproduced by the Debye model adopting the optical phonon contributions. S(T) is described qualitatively by the one calculated using the simplified diffusion thermoelectric power formula with the density of state. The enhancement of S(T) in T<TMI is caused by the existence of the small gap at the Fermi level. The dimensionless figure of merit, ZT, from ρ(T), κ(T), and S(T) shows abrupt jump at TMI, and the linear increase with increasing T.

Strain effect on the transport properties of epitaxial PrNiO3 thin films grown by polymer-assisted deposition

Epitaxial PrNiO3 (PNO) thin films were grown on single-crystal LaAlO3, (LaAlO3)0.3(SrAlTaO6)0.7, and SrTiO3 substrates, respectively, by the polymer-assisted deposition method. It is found that compared with that of PNO bulk, the c-axis parameter of PNO film increases under compressive strain and decreases under tensile strain. Moreover, under tensile strain, the c-axis first decreases and then increases. All the PNO films exhibit metal–insulator (MI) transition with a thermal hysteresis in the temperature-dependent resistivity. At room temperature, the resistivity of the PNO films shows an increasing trend with the lattice mismatch strain changing from compressive to tensile. The MI transition temperature TMI of the PNO films under both compressive and tensile strains is lower than that of PNO bulk. However, the trend of TMI dependent on the lattice mismatch between the film and the substrate under tensile strain is decreasing. The conduction mechanism fits very well with the Mott’s variable range hopping (VRH) model in temperatures below 35 K. It is suggested that strain relaxation increases with the strain increasing from compressive to tensile, which results in the increase of oxygen vacancies compensated by more Ni2+ changed from Ni3+ in the film. The competing and joint effects of the bending of the Ni–O–Ni bond angle, the elongation of the Ni–O bond length, and the change of Ni valence dominate the transport properties of the PNO films.

Microstructural properties, electrical behavior and low field magnetoresistance of (1−x)La0.67Sr0.33MnO3 (LSMO)+(x)Ni0.5Zn0.5Fe2O4 (NZFO) composites

Abstract The composites with composition of (1−x)La0.67Sr0.33MnO3 (LSMO)+(x)Ni0.5Zn0.5Fe2O4 (NZFO) with x=0.0 (S1), 0.04 (S2), 0.07 (S3), 0.10 (S4), 0.30 (S5) and 1.0 (S6) were synthesized by solid-state reaction route. Structural study using Rietveld refinement of X-ray diffraction (XRD) pattern indicates a rhombohedrally-distorted structure (space group R3c) for LSMO phase while to that NZFO compound crystallizes in cubic structure (space group Fd3m). XRD patterns and microstructural analysis show that LSMO and NZFO phase exists independently in Ni0.5Zn0.5Fe2O4 doped composites. The transport properties of the compositions x=0.0, x=0.04, and x=0.07 showed that NZFO phase improves the resistivity and shifts the metal–insulator transition temperature TMI towards lower temperature. The magnetoresistance (MR) of composite samples with x=0.04 and x=0.07 decreases monotonously from 200 to 300 K in a magnetic field of 8 T. At lower temperatures (~5 K), a sharp drop of negative MR at low fields (H 1 T) where MR is almost linear with applied magnetic field. Temperature dependence of resistivity for composites samples with x=0.04 and x=0.07 has been best fitted by small Polaron hopping (SPH) and variable range hopping models (VRH).

Search for high temperature coefficient of resistance La2/3Ca1/3MnO3 polycrystalline ceramics

La2/3Ca1/3MnO3 polycrystalline ceramics were synthesized by sol–gel method. Sharp temperature coefficient of resistance (TCR) variation (with peak value up to 22 %) has been observed near the metal-insulator transition temperature T MI (273 K) for the sample sintered at 1,450 °C. This TCR value is much higher than the previously reported values for the undoped and Ag-doped La0.67Ca0.33MnO3 samples and is comparable to the optimized thin films. It was concluded that the improved physical properties of the La0.67Ca0.33MnO3 material are due to its improved microstructure and homogeneity.

Nonlinear transport in nanoscale phase separated colossal magnetoresistive oxide thin films

We report a study of the I-V characteristics of 2.5–5.4 nm epitaxial La1−xSrxMnO3 (x = 0.33 and 0.5) and La0.7Ca0.3MnO3 thin films. While La0.67Sr0.33MnO3 films exhibit linear conduction over the entire temperature and magnetic field ranges investigated, we observe a strong correlation between the linearity of the I-V relation and the metal-insulator transition in highly phase separated La0.5Sr0.5MnO3 and La0.7Ca0.3MnO3 films. Linear I-V behavior has been observed in the high temperature paramagnetic insulating phase, and an additional current term proportional to Vα (α = 1.5–2.8) starts to develop below the metal-insulator transition temperature TMI, with the onset temperature of the nonlinearity increasing in magnetic field as TMI increases. The exponent α increases with decreasing temperature and increasing magnetic field and is significantly enhanced in ultrathin films with thicknesses close to that of the electrically dead layer. We attribute the origin of the nonlinearity to transport through the nanoscale coexisting metallic and insulating regions. Our results suggest that phase separation is not fully quenched even at low temperatures and high magnetic fields.

Study of the transport properties of La0.8Ba0.2MnO3/SrTiO3:Nd heterojunctions under reverse bias

Heterojunctions composed of P-type La0.8Ba0.2MnO3 and N-type Nd doped SrTiO3 are successfully fabricated via chemical solution deposition. Good rectifying property is obtained. Detailed studies of the transport properties under reverse bias show that above the metal-insulator transition temperature TMI, the Frenkel–Poole emission dominates the leakage current and the emission barrier height was extracted. When placed in magnetic field, the junctions show different transport behaviors below and above TMI. It shows negative magnetoresistive effect below TMI and positive one above TMI. This is explained by the phase separation mechanism and the band structure of manganites.

Structural and magnetic inhomogeneity, phase transitions, magnetoresonance and magnetoresistive properties of La0.6 − x Pr x Sr0.3Mn1.1O3 (x = 0–0.6)

Ceramic samples of manganite perovskites La0.6 − xPrxSr0.3Mn1.1O3 (x = 0−0.6) have been studied using the X-ray diffraction, resistive, magnetic (χac, 55Mn NMR), microscopic, and magnetoresistive methods. It has been found that an increase in the praseodymium concentration x leads to a transition from the rhombohedral R\(\bar 3\)c (x = 0–0.3) to orthorhombic Pbnm (x = 0.4–0.6) perovskite structure. It has been shown that the real perovskite structure contains anion and cation vacancies, whose concentrations increase with an increase in the praseodymium concentration x. A decrease in the metal-insulator phase transition temperature Tmi and the ferromagnetic-paramagnetic phase transition temperature Tc with increasing x correlates with an increase in the concentration of vacancies weakening the high-frequency electronic exchange Mn3+ ↔ Mn4+. For compositions with x = 0 and 0.1, when the lattice contains not only vacancies but also nanostructured clusters with Mn2+ in the A-positions, there is an anomalous hysteresis. An analysis of the asymmetrically broadened 55Mn NMR spectra of the compounds has revealed a high-frequency electronic exchange of the ions Mn3+ Mn4+ in the B-positions and a local heterogeneity of their surrounding by other ions (La2+, Pr3+, Sr2+) and vacancies. The phase diagram has demonstrated that there is a strong correlation between the composition, imperfection of the perovskite structure, phase transition temperatures Tmi and Tc, and magnetoresistive properties.

Ultrasonic, Transport, and Magnetic Properties of Nd0.5Sr0.5Mn1−x Cr x O3 Perovskites

The resistivity, magnetization and longitudinal ultrasonic velocity (V l) have been measured in single-phase polycrystalline Nd0.5Sr0.5Mn1−x Cr x O3 (x=0, 0.02, 0.05, 0.1) to clarify the Cr doping effect on the metal-insulator and charge ordering transitions. For x=0 sample, the V l shows two large stiffening just below the metal-insulator transition temperature T MI and the charge ordering temperature T CO, respectively. It is suggested that the former ultrasonic anomaly may originate from the spin-lattice effect and the dynamic Jahn-Teller effect, and the latter is due to the electron-lattice effect arising from the static Jahn-Teller effect of Mn3+. With the Cr doping, the T MI shifts to lower temperature, the maximum of magnetization becomes smaller and the resistivity remains metallic type. Furthermore, only one stiffening of ultrasonic velocity is observed below T MI in these doping samples. The analysis suggests that the Cr doping enhances the double exchange between Mn3+ and Mn4+, and thoroughly destroys the charge ordering state.

Evidence of direct correlation between out-of-plane lattice parameter and metal-insulator transition temperature in oxygen-depleted manganite thin films

We report on the role of oxygen content alone on structural and transport properties of La0.65Sr0.35MnO3−δ (LSMO) thin films. Identical films were deposited side-by-side during a single deposition run and subsequently post-annealed separately in vacuum to systematically vary the oxygen content. All films remained coherently strained to the SrTiO3 substrate, with no broadening of rocking curve widths after post-anneal. As oxygen content decreases, the LSMO unit cell expands while the metal-insulator transition temperature TMI decreases. A linear correlation between the out-of-plane lattice parameter and the metal-insulator transition temperature was observed.

Influence of Gd-doping in La0.7Ca0.3MnO3 on its structural and magneto-electrical properties

Bulk samples of gadolinium doped manganites with compositional formula La0.7−xGdxCa0.3MnO3 (x=0.0 and 0.1) were prepared by conventional solid state reaction method. After characterizing the samples by X-ray diffraction, scanning electron microscope and energy dispersive X-ray spectrometer, a systematic investigation of electrical and magneto-transport properties has been investigated. The replacement of La ion by Gd results in a decrease of the metal–insulator transition temperature TMI and the magnetoresistance as well as the resistivity are found to increase. The electrical resistivity in the entire temperature range fit well with the phenomenological percolation model, which is based upon an approach that the system consists of the phase separated ferromagnetic metallic and paramagnetic insulating regions.

Preparation and electric field driven magnetoelectric effect for multiferroic La2/3Sr1/3MnO3/BaTiO3 composite films

Using pulsed laser deposition, multiferroic La2/3Sr1/3MnO3(LSMO)/BaTiO3(BTO) composite films are deposited on LaAlO3 (LAO)(001) substrate. X-ray diffraction results show that LSMO and BTO films exhibit only (001) orientation. Film smoothness is verified by their low root-mean-square surface roughness values as 1.4 nm from atomic force microscope study. The magnetic and the electric properties of these composite films are investigated. Furthermore, the variations of resistivity and metal-insulator transition temperature TMI of LSMO, induced by the external electric field, are studied. The resisitivity is reduced while the TMI is enhanced for hole accumulation state which is induced by negative electric field across BTO layer. In contrast, the resistivity is enhanced while the TMI is reduced for hole depletion state, which shows coupling between magnetic and electric order parameters, i.e., there is a magnetoelectric effect induced by electric field.

Fabrication of 0-3 type manganite/insulator composites and manipulation of their magnetotransport properties

In order to promote the technological applications of perovskite manganites, a great fundamental interest has been devoted to tailoring and/or enhancing their magnetotransport properties. Design and fabrication of manganite-based nanocomposites offer great potential to tailor the magnetotransport properties. In this work, we illustrate the promising concept of 0-3 type manganite/insulator composites (where manganite nanoparticles are uniformly and discretely embedded in a three-dimensional-connectivity insulator matrix) through the example of the La0.67Ca0.33MnO3 (LCMO)/MgO system. We present a promising strategy, which is based on the creation of core (LCMO)–shell (MgO) composite powders, for the synthesis of 0-3 type LCMO/MgO composites. A modified polyacrylamide gel method has been developed to prepare the core-shell structured LCMO/MgO composite powders. Besides its ability to create well-defined core-shell composite structures, the present gel method also allows the production of nanopowders with uniform particle size and in spherical shape. In our (1−x)LCMO/xMgO composite system, the lattice of LCMO is found to expand with rising MgO concentration x, yielding a bulk tensile strain. The increase in the tensile strain with x yields to a structural phase transition in the LCMO phase from an orthorhombic Pnma structure (x≤0.2) to a rhombohedral R3¯c structure (x≥0.3), and this structural transition leads to a relaxation of the strain. The strain effects induced by the MgO second phase exert a great influence on the ferromagnetic transition temperature TC. The composite system exhibits a percolative behavior in the conductivity, and the metal-insulator transition temperature TM-I decreases with x and is finally suppressed as the MgO content increases up to x=0.3. An enhancement in magnetoresistance (MR) is observed at low temperatures for the composites, and the low-field sensitivity of MR gets enhanced with the increase in MgO content. This work provides a promising method for manipulating the magnetotransport properties of manganites by composites with a proper insulator phase in a 0-3 connectivity fashion.

Electrical transport and low-field magnetoresistance properties of (La1–xYbx)2/3Ca1/3MnO3 (x=0 to 0·12) ceramics

Colossal magnetoresistance ceramics with the nominal composition of (La1–xYbx)2/3Ca1/3MnO3(x=0 to 0·12) were synthesised, and their electrical transport and magnetoresistance behaviours were investigated. Experimental results showed that the metal–insulator transition temperature TMI of the ceramic is largely dependent on Yb substitution. Electrical resistivity as a function of temperature in zero field, ρ(0,T) gradually increased with Yb content up to x=0·1 but drastically decreased at x=0·12. The metallic region of the ρ(0,T) curve (T<TMI) of all Yb substituted samples was fitted to the model for electron–electron and electron–magnon scatterings while the insulating region was fitted to a model of polaron hopping with lowest activation energy Ea observed at x=0·10. The largest MR effect of 47% was observed at 0·4 T external field for x=0·10. It is suggested that the increase in ρ(0,T) is related to enhanced Jahn–Teller effect while the MR response is due to magnetic changes as a result of the introduction of Yb3+ magnetic ions.

Thermal expansion of filled skutterudite compound GdRu4P12

We have performed X-ray diffraction and thermal expansion measurements at low temperatures of the filled skutterudite compound GdRu4P12 in order to investigate structural changes related to the antiferromagnetic phase transition at TN=22K. No significant change in the X-ray powder diffraction pattern of GdRu4P12 was detected down to 10K. The variation of thermal expansion is continuous at TN. Therefore, the transition is not accompanied with any crystallographic transformation. The temperature dependence of the linear thermal expansion coefficient shows large jump (1.2 × 10-6 K-1) at TN. This value is comparable to that of isostructual compound SmRu4P12 at the metal-insulator transition temperature TMI=16K.

Photoinduced resistivity change of electron-doped La0.8Te0.2MnO3 film

In this paper, electron-doped La0.8Te0.2MnO3 film has been prepared on SrTiO3 (100) substrate by pulsed laser deposition method, and the photoinduced resistivity change and colossal magnetoresistance effect have been investigated. The results show that the film has a high metal-insulator transition temperature TMI of ∼283 K and the maximum magnetoresistance ratio is ∼24% at 0.7 T. Under laser irradiation, TMI varies from 283 to 243 K and resistivity increases below TMI, while it decreases above TMI. The maximum photoinduced resistivity change in resistivity (LR%) is 134.8%. The photoinduced relaxation character of the film indicates that the relaxation of photoinduced character relates to the excitation of small polarons.

Transport properties and backward diode-like behaviour in manganite-based p–i–n junctions

We report the effect of metal–insulator transition on electrical transport properties in manganite-based p–i–n heterojunctions, consisting of p-type La0.67Sr0.33MnO3−δ/Pr0.7Ca0.3MnO3−δ and n-type Si with a buffer layer of SiO2. As the temperature is higher than the metal–insulator transition temperature TMI of manganite films, the current–voltage relation of these p–i–n junctions shows good forward rectifying behaviour. Once the temperature is below TMI, the La0.67Sr0.33MnO3−δ/SiO2/Si p–i–n junction behaves like a backward diode. This backward diode-like behaviour in p–i–n junctions based on manganite can be understood by the change in electronic band structure of manganite films with temperature. We also found that the electric current in these p–i–n junctions is dominated by the tunnelling process at higher bias voltages and the space-charge-limited current process at lower bias voltages in these heterojunctions. This work reveals the various properties and the key role of metal–insulator transition in manganite based diodes.

Scanning Tunneling Spectroscopy on Pr$_{0.68}$Pb$_{0.32}$MnO$_{3}$ Single Crystals

Scanning tunneling microscopy/spectroscopy studies were carried out on single crystals of colossal magnetoresistive manganite Pr0.68Pb0.32MnO3 at different temperatures in order to probe their spatial homogeneity across the metal-insulator transition temperature TM-I(~255K). A metallic behavior of the local conductance was observed for temperatures T<TM-I. Zero bias conductance (dI/dV)V=0, which is directly proportional to the local surface density of states at the Fermi level, shows a single distribution at temperatures T< 200 K suggesting a homogeneous electronic phase at low temperatures. In a narrow temperature window of 200 K <T<TM-I, however, an inhomogeneous distribution of (dI/dV)V=0 has been observed. This result gives evidence for phase separation in the transition region in this compound

Electrical Conductivity and Low-Field Magnetoresistance in La&lt;sub&gt;2/3&lt;/sub&gt;(Ca&lt;sub&gt;0.60&lt;/sub&gt;Ba&lt;sub&gt;0.40&lt;/sub&gt;)&lt;sub&gt;1/3&lt;/sub&gt;Mn&lt;sub&gt;1-x&lt;/sub&gt;V&lt;sub&gt;x&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Prepared by Sol-Gol

La2/3(Ca0.6Ba0.4)1/3Mn1-xVxO3 (x=0, 0.03, 0.05, 0.07, 0.10, 0.15, 0.20) nanoparticles were synthesized using sol-gel technology. The experimental results reveal that, (1) the substitution of V for Mn in La2/3(Ca0.6Ba0.4)1/3Mn1-xVxO3 lowers the Curie temperature TC and the metal–insulator transition temperature TMI; (2) there exists the evident difference between the TC and the TMI for different V substitution ratio; (3) the low-temperature tunneling magnetoresistance and maximum magnetoresistance near Tc increase with the enhancement of V-doping content. Based on the tunneling magnetoresistance model and the percolation model near Curie temperature, the experimental results are explained well.