Metal-insulator Transition Temperature Research Articles

Influence of Discharge Current on Phase Transition Properties of High Quality Polycrystalline VO₂ Thin Film Fabricated by HiPIMS.

To fabricate high-quality polycrystalline VO2 thin film with a metal–insulator transition (MIT) temperature less than 50 °C, high-power impulse magnetron sputtering with different discharge currents was employed in this study. The as-deposited VO2 films were characterized by a four-point probe resistivity measurement system, visible-near infrared (IR) transmittance spectra, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. The resistivity results revealed that all the as-deposited films had a high resistance change in the phase transition process, and the MIT temperature decreased with the increased discharge current, where little deterioration in the phase transition properties, such as the resistance and transmittance changes, could be found. Additionally, XRD patterns at various temperatures exhibited that some reverse deformations that existed in the MIT process of the VO2 film, with a large amount of preferred crystalline orientations. The decrease of the MIT temperature with little deterioration on phase transition properties could be attributed to the reduction of the preferred grain orientations.

Structural, magnetic and transport properties of the La0.7Sr0.3MnO3-ZnO nanocomposites

La0.7Sr0.3MnO3 (LSMO)-ZnO composites with varying concentration of the ZnO have been synthesized by the sol-gel method. The effect of ZnO addition on the structural, magnetic and transport properties of the LSMO have been studied in detail. XRD data confirms the coexistence of both phases in the composites. The calculated average particle size of the composite samples was found to be in the range of 16–25nm. The magnetization measurement shows a decrease in magnetization and shift in the transition temperature towards a lower temperature with an increase in ZnO content. The electrical resistivity studies show that the resistivity increases and the metal-insulator transition temperature decreases with increase in the amount of ZnO in LSMO. The effect of ZnO on the transport behavior is explained using different conduction mechanism in different temperature regions. The magnetoresistance measurement shows the enhancement of the low field magnetoresistance and magnetic field sensitivity in the LSMO-ZnO composite samples compared to pure LSMO. The LSMO-ZnO composite sample shows 32.8% MR for an applied magnetic field of 1T at 10K.

Postsynthetic Route for Modifying the Metal—Insulator Transition of VO2 by Interstitial Dopant Incorporation

The thermally driven orders-of-magnitude modulation of resistance and optical transmittance observed in VO2 makes it an archetypal first-order phase transition material and underpins functional applications in logic and memory circuitry, electromagnetic cloaking, ballistic modulation, and thermochromic glazing to provide just a few representative examples. VO2 can be reversibly switched from an insulating to a metallic state at an equilibrium transition temperature of 67 °C. Tuning the phase diagram of VO2 to bring the transition temperature closer to room temperature has been a longstanding objective and one that has tremendous practical relevance. Substitutional incorporation of dopants has been the most common strategy for modulating the metal—insulator transition temperature but requires that the dopants be incorporated during synthesis. Here we demonstrate a novel postsynthetic diffusive annealing approach for incorporating interstitial B dopants within VO2. The postsynthetic method allows for the tr...

Understanding thermomagnetic hysteresis in La1−x−yPryCaxMnO3 thin films

The present work reports the scaling behaviour of thermomagnetic hysteresis in temperature and magnetic field dependent resistivity [(ρ–T) and (ρ–H)] measured during cooling/warming and H increasing/decreasing cycles in single crystalline La0.21Pr0.42Ca0.37MnO3 thin films. The zero-magnetic field (H = 0) insulator–metal transition temperature (IMT) measured in warming cycle ~ 166 K is higher than that in the cooling cycle = 128 K and the difference between them shrinks as H is increased. The two IMTs scale with H as . Here is the H-independent contribution, and the constants, pre-factor and exponent determine the magnetic field dependent part. The ρ–T loop area (AT) diminishes with the increasing H as the magnetic liquid is extremely unstable with respect to external H (H < 30 kOe) and consequently AT shows an exponential decay given by . Here, is the zero-field normalized area and Γ is a constant related to the degree of phase separation. The analysis of the isothermal ρ–H loop area, which increases with H shows scaling behaviour of the type . Here, the constant HIM corresponds to the magnetic field that induces AFMI to FMM phase transition and decreases with temperature, while the exponent ‘η’ measures the degree of phase separation. The value of η is found to be temperature dependent and hence related to the relative fraction of the two coexisting phases.

Electronic phase diagram of oxygen-deficient SmNiO3−δ epitaxial thin films

Epitaxial SmNiO3−δ thin films were fabricated under various oxygen partial pressures () on (0 0 1)-oriented LaAlO3 substrates by using the pulsed laser deposition method. Continuous control of the metal–insulator transition temperature (TMI) from 350 K to 85 K was achieved by varying from 26 Pa to 0.5 Pa. The reduction of TMI can be attributed to the straightening-out of the out-of-plane Ni–O–Ni bond angle due to the elongation of unit cell volume with decreasing . When > 3 Pa, resistivity anomalies around 200 K are observed in the insulating phase of strained SmNiO3−δ films, suggesting the onset of antiferromagnetic ordering of the Ni sublattices. An electronic phase diagram of the oxygen-deficient SmNiO3−δ films has been established in this work based on the results of the transport measurements.

Room temperature metal-insulator transition observed in Pb substituted lanthanum manganite

Abstract Structural, magnetic and transport property of Pb substituted LaMnO 3 (La 1−x Pb X MnO 3 ; x=0.1, 0.2, 0.3, 0.4, 0.5) prepared by solid state reaction route have been studied systematically. X-ray diffraction,SEM, VSM,magneto-transport measurements were carried out on prepared samples. All the samples below x=0.4 composition were crystallized into single phase with R-3C space group of trigonal crystal class. The explored physical properties like magnetoresistance (MR), resistivity, metal–insulator transition temperature (T P ), magnetization (M s )and Curie temp. (T C ) varies with Pb concentration. Higher T P (298 K) and minimum resistivity was recorded for x=0.3 sample whereas no distinct peak corresponding to metal-insulator transition was observed for La 0.5 Pb 0.5 MnO 3 . Appreciable magnetoresistance (MR%) values were obtained for all the samples at 300 K.X-ray photoelectron spectroscopy (XPS) confirms the +2 oxidation state of Pb in all substituted samples. Fine tuning of T c could be achieved by varying Pb concentration in substituted LaMnO 3 samples, hence, it can find application in the field of cancer treatment i.e. Magnetic Hyperthermia.

The structure and electrical properties of PbPdO2 thin films with preferred orientation prepared by PLD

We have fabricated PbPdO2 nano-particles and PbPdO2 films with (211) preferred orientation (sample A) and with (002) preferred orientation (sample B), respectively. The structures and physical properties were measured by XRD, Raman, SEM, XPS and AFM. The experimental results indicate that the PbPdO2 films have granular structure with the grain size of about 200 nm and some Pb vacancies exist in both samples. It is found that PbPdO2 with (002) preferred orientation has near zero-gap, while PbPdO2 with (211) preferred orientation has larger band gap. This surface anisotropy band gap is also proved by our first principles band structure calculations. Moreover, one notices that the sample B possesses lower resistivity than sample A, and both samples possess a wide insulator-metal transition temperature (TMI) with around 370 K.

Effects of copper doping on the structure, electrical and low-field magnetoresistance properties of (1–x)La0.67Sr0.33MnO3/xCu (x=0–0.15) composite coatings

The doping effect of metallic Cu on the performances of manganite coatings has been investigated. The (1–x)La0.67Sr0.33MnO3/xCu (x=0–0.15) composites were prepared by the sol-gel based screen printing method. XRD and SEM measurements revealed CuO phase to form by oxidation and to distribute mainly at grain boundaries (GBs) of the manganite matrix. Excess Cu entered the manganite lattice without changing its rhombohedral structure, resulting in a slight lattice expansion. The surface morphology of composite samples changed dramatically with different Cu contents, and average particle size refined with a low concentration of Cu. As the Cu content increased, the resistivity of the samples increased while the metal–insulator transition temperature decreased. The enhanced magnetoresistance (MR) effect close to the percolation threshold has been ascribed to the introduction of local disorder at GBs.

Thermal conductivity of (VO2)1-xCux composites across the phase transition temperature

The thermal conductivity across the metal-insulator transition (MIT) of hot-pressed polycrystalline vanadium dioxide (VO2) samples is studied. The change in the total thermal conductivity (k) of hot-pressed VO2 is insignificant across the MIT temperature. By adding copper (Cu) to make (VO2)1-xCux composites with x from 0 to 0.5, we find an increase in the electrical conductivity from 4 × 104 S m−1 to 1 × 106 S m−1 at 120 °C, resulting in an electronic thermal conductivity increase from 0.38 W m−1 K−1 for x = 0 to 3.8 W m−1 K−1 for x = 0.3, which is a significant increase. However, the total thermal conductivity did not increase due to the decrease in the value of the Lorenz number by an order of magnitude than its standard value using the Wiedemann-Franz relationship. On the basis of our experimental result, an empirical model is proposed to explain the thermal conductivity behavior of all (VO2)1-xCux samples with different Cu concentrations.

High contrast switchability of VO2 based metamaterial absorbers with ITO ground plane.

A metamaterial consisting of an array of gold micro-disks, separated from a ground plane of indium tin oxide (ITO) by a thin film of vanadium dioxide (VO2), behaves as a perfect absorber at infrared (IR) frequencies at room temperature. This metamaterial, which is transparent to visible light, can be switched to a highly reflecting state for IR light by heating the metamaterial to temperatures larger than the metal-insulator phase transition temperature 68°C of VO2. For a disk diameter of 1.5 μm and VO2 film thickness of 320 nm, two absorption bands are obtained: one, that arises from the metamaterial resonance; and a second peak that arises principally from a Fabry-Pérot resonance. A large change (>78%) occurs in the reflectivity between the low and high temperature phases. IR emittance of the metamaterial was measured with IR cameras and shown to be switchable to result in low emittance at high temperature. Optical readout of the state of VO2 within the metamaterial is demonstrated.

Electric transport and magnetic properties of La0.7Sr0.3MnO3 thin films grown on PLZST ceramics

La0.7Sr0.3MnO3 (LSMO) polycrystalline thin films were deposited respectively on (Pb0.97La0.02)(Zr0.58Sn0.3025Ti0.1175)O3 (PLZST) ceramics and Si wafer by RF magnetron sputtering. Their microstructure, electric transport and magnetic properties were investigated. Compared with the LSMO thin films on Si, the films on PLZST show excellent electrical and magnetic transport properties. The resistivity of LSMO on PLZST reduced obviously compared to that of LSMO on Si. The LSMO films on PLZST show higher Curie temperature and metal-insulator transition temperature (TC=340K, TMI=260K). Meanwhile, the magnetoresistance exhibits excellent thermal stability in a broad temperature range even under 6T magnetic field.

Large magnetocaloric and magnetoresistance effects in metamagnetic Sm0.55(Sr0.5Ca0.5)0.45MnO3 manganite

The metamagnetic transition, magnetocaloric and magnetoresistance effects are investigated in polycrystalline Sm0.55(Sr0.5Ca0.5)0.45MnO3 (SSCMO) manganite. A sharp magnetization jump at Curie temperature (TC) 73.5K with large thermal hysteresis is observed. Magnetic measurements and Arrott plots analysis indicate that the transition is first order in nature. Under a low magnetic field change of 1T, the magnetic entropy change exhibits a peak value of −ΔSMmax(T)=4.01J/kg K with relative cooling power (RCP) value of 44.1J/kg in the vicinity of TC. The mechanism of charge conduction in insulator phase is polaron transport. Large negative magnetoresistance ratio with value of ~99% is obtained within a broad temperature range below metal insulator transition temperature under 1T magnetic field. These results indicate the potential applications of SSCMO in magnetic refrigeration and spintronics devices.

Mechanisms of spin-flipping and metal-insulator transition in nano-Fe3O4

Fe3O4 is a half-metallic ferrimagnet with K exhibiting metal-insulator transition (MIT) at ∼120 K. In bulk form, the saturation magnetization is 0.6 Tesla (∼471 emu cm−3). A recent experimental study has shown that the saturation magnetization of nano-Fe3O4 thin films can achieve up to ∼760 emu cm−3, attributed to spin-flipping of Fe ions at tetrahedral sites assisted by oxygen vacancies (VO). Such a system has shown to have higher MIT temperature (∼150 K). The spin-flipping is a new phenomenon in Fe3O4, while the MIT is a long-standing one. Here, we propose a model and calculations to investigate the mechanisms of both phenomena. Our results show that, for the system without VO, the ferrimagnetic configuration is energetically favorable. Remakably, upon inclusion of VO, the ground-state configuration switches into ferromagnetic. As for the MIT, by proposing temperature dependences of some hopping integrals in the model, we demonstrate that the system without and with VO undergo the MIT in slightly different ways, leading to higher MIT temperature for the system with VO, in agreement with the experimental data. Our results also show that the MIT in both systems occur concomitantly with the redistribution of electrons among the three Fe ions in each Fe3O4 formula unit. As such temperature dependences of hopping integrals may arise due to dynamic Jahn–Teller effects, our phenomenological theory may provide a way to reconcile existing theories relating the MIT to the structural transition and the charge ordering.

Temperature dependence of optical constants of La0.7Sr0.3MnO3 thin films

La0.7Sr0.3MnO3 films were prepared on MgO substrates using direct current (DC) magnetron sputtering, and the optical indices ranging from ultraviolet to infrared wavelengths (0.03–5.0eV) were evaluated using spectroscopic ellipsometry at sample temperatures between 278K and 373K. The free-carrier concentration and mobility were estimated using fitted parameters of the Drude term obtained from analysis of the ellipsometric angle spectra and DC electrical resistivity. The mobility decreased with increasing sample temperature up to ∼320K, which was nearly equal to the metal–insulator transition temperature of the film. The mobility became constant at higher temperatures, whereas the concentration was almost constant relative to the sample temperature.

In situ nanomechanical behaviour of coexisting insulating and metallic domains in VO2 microbeams

Measuring the electrical and mechanical responses of coexisting phases at nanoscale provides a platform to engineer micro-/nanoscale pattern of metallic and insulating domains with control over properties to make novel devices. Here, we employ several in situ characterization techniques, namely Raman, optical imaging and electrical measurements, to identify the phase coexistence of metallic and insulating domains. Further, we performed site-specific in situ nanoindentation to address the spatial variation in nanomechanical properties in vanadium dioxide (VO2) single-crystal microbeams in proximity to metal–insulator transition temperature. We also investigated load or contact depth dependence on elastic modulus at various temperatures to avoid the interference of indentation size effect on nanomechanical properties across the phase transition. The obtained results confirm the abrupt increase in elastic modulus (~17 GPa) and nanohardness (1 GPa) across the transition from monoclinic (insulator) to rutile (metal) phase.

Enhanced magnetoresistance at wide temperature range in [Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3]20 superlattice on (001) MgO

[Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3]20 superlattices were epitaxially fabricated on (001) MgO substrates with 24 nm La0.5Ca0.5MnO3 (LCMO) buffer layer by pulsed laser deposition. As the thickness of Pr0.7Sr0.3MnO3 (PSMO) layer and LCMO layer is not identical, non-uniformity in the superlattice period thickness will occur and result in lower metal–insulator transition temperature and larger magnetoresistance (MR) at a wide low-temperature range. Furthermore, the percolation model was used to quantitatively understand the transport mechanism of superlattice. Our research results demonstrate that inhomogeneous layer thickness accompanied with charge ordering phase separation can strongly influence the transport properties of superlattice. Enhanced magnetoresistance at a wide temperature range provides possibilities for practical application in manganite-based film devices.

Structural, magnetic and electrical properties of Fe-doped perovskite manganites La0.8Ca0.15Na0.05Mn1−xFexO3 (x = 0, 0.05, 0.10 and 0.15)

Here, we report the structural, magnetic and electrical properties of new series of Fe-doped perovskite manganites La0.8Ca0.15Na0.05Mn1-xFexO3. Rietveld analysis of powder X-ray diffraction data revealed that all the compounds crystallize in the orthorhombic symmetry with Pnma space group. The lattice parameters and the unit cell volume increases slightly with Fe-doping. Temperature dependent magnetization studies showed that all our investigated compounds exhibit a paramagnetic–ferromagnetic transition with Curie temperature (TC) decreases with increasing x from 253 K to 130 K. All the phases show metal-insulator transition and the values of metal-insulator transition temperature (TMI) are nearly approachable to TC, reflecting strong correlations between the magnetic and electrical properties in our samples. The transport properties are dominated by the Polaron hopping mechanism in temperature range above TMI with increase in polaron hopping energy with Fe content.

SYNTHESIS, CHARACTERISATION AND RESISTIVITY STUDY OF BI DOPED AT MN SITE OF PrO.6SrO.4MnO3 MANGANITES

We report structural and electrical studies of Pr0.6Sr0.4Mn1-xBixO3 (x=0,0.1 and 0.2) manganites. In manganitesMn ions play a crucial role in shaping the physical properties. Possible reasons for the observed structural, changes of resistivity behavior with Bi doping are discussed. Replacing trivalent Bi ion with Mn ion provides us a system to understand weakened double-exchange (DE) interaction (Mn3+-O-Mn4+). X-ray powder diffraction showed that all synthesized samples are single phase and crystallize in the orthorhombic symmetry with Pnmaspace group. The unit cell volume decreases with increasing Bi content. The value of metal-insulator (M-I) transition temperature (TMI) decreases and resistivity increases as doping level x increases. The drop in resistivityatTMI results from the reduced contribution of magnetic scattering. Ferromagnetic metallic resistivity is observed well below the metal-insulator transition temperature. Above the metal insulator transition temperature the electrical conductivity is dominated by small polaron hopping (SPH) model.

The role of 1-D finite size Heisenberg chains in increasing the metal to insulator transition temperature in hole rich VO2.

VO2 samples are grown with different oxygen concentrations leading to different monoclinic, M1, and triclinic, T, insulating phases which undergo a first order metal to insulator transition (MIT) followed by a structural phase transition (SPT) to the rutile tetragonal phase. The metal insulator transition temperature (Tc) was found to be increased with increasing native defects. Vanadium vacancy (VV) is envisaged to create local strains in the lattice which prevents twisting of the V-V dimers promoting metastable monoclinic, M2 and T phases at intermediate temperatures. It is argued that MIT is driven by strong electronic correlation. The low temperature insulating phase can be considered as a collection of one-dimensional (1-D) half-filled bands, which undergo a Mott transition to 1-D infinitely long Heisenberg spin ½ chains leading to structural distortion due to spin-phonon coupling. The presence of VV creates localized holes (d0) in the nearest neighbor, thereby fragmenting the spin ½ chains at the nanoscale, which in turn increases the Tc value more than that of an infinitely long one. The Tc value scales inversely with the average size of the fragmented Heisenberg spin ½ chains following a critical exponent of ⅔, which is exactly the same as predicted theoretically for the Heisenberg spin ½ chain at the nanoscale undergoing SPT (spin-Peierls transition). Thus, the observation of MIT and SPT at the same time in VO2 can be explained from our phenomenological model of reduced 1-D Heisenberg spin ½ chains. The reported increase (decrease) in the Tc value of VO2 by doping with metals having valency less (more) than four can also be understood easily with our unified model, for the first time, considering finite size scaling of Heisenberg chains.

Growth, Magnetic and Electrical Transport Properties of La$lt;inf$gt;0.7$lt;/inf$gt;Sr$lt;inf$gt;0.3$lt;/inf$gt;MnO$lt;inf$gt;3$lt;/inf$gt; Thin Films on PLZST Ceramics

La0.7Sr0.3MnO3 (LSMO) thin films with different thicknesses were deposited on (Pb0.97La0.02) (Zr0.58Sn0.3025Ti0.1175)O-3 (PLZST) ceramics by RF magnetron sputtering, and their microstructure, magnetic and electrical transport properties were investigated. Microscopy observations show that LSMO thin films are perovskite structure without obvious impurity phase. All the LSMO thin films display smooth surface with uniform, and roughness is as low as 2.93 nm for LSMO thin films at the thickness of 20 nm. Furthermore, large magnetoresistance (MR) effect was observed in LSMO thin films in a broad temperature range of 10-300 K. Particularly the MR of LSMO thin films with 20 nm in thickness exhibits excellent temperature stability. Moreover, the Curie temperature, metal-insulator transition temperature, saturation magnetization and electrical conductivity decrease as the film thickness increases, which is attributed to the diffusion of Pb, Sn, Zr, etc. in the samples, resulting in the distortion of MnO6 octahedron.