Insulator Transition Temperature Research Articles

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.

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.

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.

Tuning room temperature Tp and MR of La1-y(Cay-xSrx)MnO3 polycrystalline ceramics by Sr doping

Polycrystalline Sr−doped La1-y(Cay-xSrx)MnO3 (LCSMO, y=3/8) ceramics with x=0, y/25, 2y/25, 3y/25, 4y/25, y/3, y/2, and 2y/3 were prepared by sol−gel methods. X-ray diffraction patterns results indicated that all the bulk samples were single phase without any impurities. The structure of the samples transformed from orthorhombic phase (space group of Pnma) to rhombohedral phase (space group of R3̅c) when the Sr content (x) was equal to or greater than y/2. With the increasing of x content, the temperature coefficient of resistance (TCR) and magnetic resistance (MR) decreased gradually. However, the metal to insulator transition temperature (Tp) and the MR reached at 300.4K and 12% at 300K for the LCSMO (x=4y/25) sample, respectively, which meant the LCSMO material could be used as a candidate for photoelectric (uncooled bolometer or infrared detectors, etc.) or magnetic devices (uncooled magnetic sensors, etc.) at room temperature. Visibly, the results indicated that the room temperature modulation of Tp and MR was observed when the site of the Ca2+ ion was substituted by the Sr2+ ion in LCSMO (y=3/8) polycrystalline ceramics. This gave rise to the increasing size of the A−site average cationic radius <rA> and improving of the Mn4+ ion concentration through double exchange between the Mn3+ and Mn4+ ions, leading to enhance the electrical and magnetic properties of LCSMO ceramics.

Effects of Ag addition on the structural and electrical properties of La0.67Sr0.33MnO3 ceramics

ABSTRACTLa0.67Sr0.33MnO3:mol-%Agx (LSMO:Agx, x = 0, 0.1, 0.2, 0.3 and 0.4) polycrystalline ceramics were prepared by chemical co-precipitation methods. All of the LSMO:Agx samples showed rhombohedral perovskite structure with a space group of . The diffraction peaks of metallic Ag did not appear in the XRD graph, due to the Ag substitution at A sites (La3+ or Sr2+ ions) and the evaporation loss of the element Ag in the process of high-temperature sintering, or the residual amount of Ag beyond the sensitivity of the XRD. With the increase of Ag addition, unit cell volume (V), metal−insulator transition temperature (Tp) and conductivity at Tp (ρTp) of the LSMO:Agx samples increased significantly. At the same time, the temperature coefficient of resistance (TCR%), temperature peak of the TCR% (Tk) and the figure of merit (Z) reached the optimal values of 3.3% K−1, 367 K and 0.049 at x = 0.3. Ag substitution at A sites (La3+ or Sr2+ ions) and gathering at grain boundaries (GBs) were proposed to explain the existence of Ag addition. All results suggested that the enhancement of electrical properties was due to silver-doping and it increased the bond angles of Mn−O−Mn (θMn−O−Mn) and the bond distance of Mn−O (dMn−O), improving Mn4+ concentration, grain sizes and GBs connectivity in LSMO:Agx ceramics.

Giant magneto resistance and temperature coefficient of resistance in Sm0.55Sr0.30Ag0.15MnO3 perovskite

Silver ions substituted samarium strontium manganite (Sm0.55Sr0.30Ag0.15MnO3) pervoskite was synthesized by using respective oxides in stoichiometric ratio through solid state reaction. The as-prepared sample was characterized by various analytical techniques to confirm its formation and understand the effect of monovalent silver ions in pervoskite lattice. X-ray diffraction pattern confirms the single phase formation while grain morphology in SEM image indicates good connectivity among the grains. The enhancement in metal to insulator transition temperature shows quenched disorder and magnetoresistance phenomena. The magnetoresistance (MR) and temperature coefficient of resistance (TCR) emerge from grain growth factor and homogeneity induced by Ag+ ions in the lattice. The reduction in hysteresis loss resulted from antiferromagnetic - ferromagnetic (TN) and ferromagnetic - paramagnetic (Tc) transitions reveals the removal of disorder in perovskite lattice by Ag+ ions substitution. This increases the magnetic moment across distinct ions on the applying magnetic field. The rise in MR% (~99%) with silver doping emerging from smooth spin tunneling of the grains across the boundary and suppression of the disordered magnetic fluctuations with increase in magnetic field has been reported. The present compound exhibits the first order nature of magnetism and observed first time the highest value of TCR ~ 95%.

Structural, magnetotransport and thermal properties of Sm substituted La0.7−xSmxBa0.3MnO3 (0≤x≤0.2) manganites

In the present paper we studied the structural, magnetotransport and thermopower properties of a series of polycrystalline La0.7−xSmxBa0.3MnO3 (0≤x≤0.2) compounds which were synthesized by conventional solid state reaction techniques. The Rietveld refinement of the X-ray diffraction pattern predicts the formation of single-phase compositions and doping of La3+ by Sm3+ ion converted the chemical structure form rhombohedral (R-3C) to orthorhombic (Imma) The scanning electron microscopy show clean smooth and densified structure images be a sign of good crystalline nature of the samples. The temperature dependent electrical resistivity is measured by standard four-probe method between 5 and 300K exhibits Metal–Insulator Transition MIT which shows the decreasing trend with increasing Sm doping content and metallic state (T<Tp2) that signifies importance of electron-electron, electron–phonon and electron-spin fluctuation scattering process. The metal–insulator transition temperature (Tp2) is observed which decreases with the increase in doping content when magnetic field is applied. The resistivity data in high temperature region (T>Tp2) are explained within the framework of variable range hopping (VRH) and small polaron hopping (SPH) models. The application of magnetic field suppresses the resistivity in entire temperature range and with increasing the Sm3+ content the samples showed very high magnetoresistance. The thermoelectric power (S) of the reported samples exhibit a crossover from positive to negative value and it increases in doped samples. The analysis of thermoelectric power data confirms the applicability of the small polaron hopping model in high temperature region and in low temperature fitted to an equation containing diffusion, magnon drag, phonon drag and electron-spin fluctuation term. An appropriate justification for the observed behavior is specified.

Effect of low temperature vacuum annealing on microstructural, optical, electronic, electrical, nanomechanical properties and phase transition behavior of sputtered vanadium oxide thin films

Vanadium oxide thin films were deposited on quartz substrate by pulsed RF magnetron sputtering technique at 400–600 W and subsequently annealed at 100 °C in vacuum (1.5 × 10−5 mbar). Phase analysis, surface morphology and topology of the films e.g., both as-deposited and annealed were investigated by x-ray diffraction, field emission scanning electron microscopy and atomic force microscopy techniques. X-ray photoelectron spectroscopy (XPS) was employed to understand the elemental oxidation of the films. Transmittance of the films was evaluated by UV–vis-NIR spectrophotometer in the wavelength range of 200–1600 nm. Sheet resistance of the films was measured by two-probe method both for as-deposited and annealed conditions. XPS study showed the existence of V5+ and V4+ species. Metal to insulator transition temperature of the as-deposited film decreased from 339 °C to 326 °C after annealing as evaluated by differential scanning calorimetric technique. A significant change in transmittance was observed in particular at near infrared region due to alteration of surface roughness and grain size of the film after annealing. Sheet resistance values of the annealed films decreased as compared to the as-deposited films due to the lower in oxidation state of vanadium which led to increase in carrier density. Combined nanoindentation and finite element modeling were applied to evaluate nanohardness (H), Young’s modulus (E), von Mises stress and strain distribution. Both H and E were improved after annealing due to increase in crystallinity of the film.

Low-temperature fabrication of VO2 thin film on ITO glass with a Mott transition

Polycrystalline Vanadium dioxide (VO2) thin film can be fabricated on glass substrates by high power impulse magnetron sputtering at a relative high temperature. In order to apply an effective bias voltage on substrate and control the energy of the ions impinged to the substrate, conductive indium-tin oxide (ITO) glass was used as the substrate. UV-visible-near IR transmittance spectra and X-ray diffraction (XRD) patterns of the as-deposited films exhibited that M-VO2 thin film with a metal–insulator transition temperature of 37[Formula: see text]C was fabricated successfully at 300[Formula: see text]C with a bias voltage of [Formula: see text]200[Formula: see text]V, and the calculated average crystalline size of this film was about 12[Formula: see text]nm. XRD patterns at varied temperatures showed that the structural change of MIT of the VO2 thin film was suppressed during the phase transition process, and a pure Mott transition was obtained.

Impact of silver substitution on the magnetotransport and thermal behavior of polycrystalline Sm0.55Sr0.45−xAgxMnO3 (x = 0 & 0.15) manganites

We have synthesized a series of Sm0.55Sr0.45−xAgxMnO3 (x = 0 & 0.15) by conventional solid state reaction method to scrutinize the effect of monovalent Ag+ as a replacement of Sr2+ ion on structural, electrical, magnetotransport and thermoelectric properties. The structural parameters obtained using Rietveld refinement of X-ray diffraction data which showed perovskite structure having orthorhombic (Pnma) symmetry without any detectable impurity phase. It is observed that MZFC and MFC curves become more manifested as Ag+ content is increased which indicates an increase in magnetic frustration arising due to bending of Mn-O-Mn bond. The M-H plots show the narrow hysteresis loops which indicate the soft ferromagnetic nature. Magnetic studies also show that the Curie temperature TC is found to decrease with Ag+ content. The metal–insulator transition temperature (TMI) is observed which decreases with the increase of Ag+ content when magnetic field is applied. The resistivity data in high temperature region are explained within the framework of variable range hopping (VRH) and small polaron hopping (SPH) models. The thermoelectric power (S) of the reported samples exhibits a crossover from positive to negative value and value of S increases in doped samples. The analysis of thermoelectric power data confirms that the small polaron hopping model in high temperature region and at low temperature electron–magnon scattering is the dominating in the FM region. With increasing the Ag+ content the samples showed very high magneto-resistance (MR%) and temperature coefficient of resistance (TCR%) which make the compound more promising for scientific and technological applications.

Effect of A-site cationic radius on polycrystalline ceramics La x Sm0.67−x Sr0.33MnO3 prepared by sol–gel technique

In this paper, polycrystalline ceramics LaxSm0.67−xSr0.33MnO3 (x = 0.00, 0.10, 0.20, 0.25, 0.30, 0.365, and 0.40) were prepared by sol–gel technique. The structure was tested by X-ray diffraction, which shows that all samples are single phase, but the progressive substitution of Sm3+ by La3+ enlarges the volume of the unit cell. The surface morphology was investigated by scanning electron microscope, which indicates that the samples with better crystallization have higher density. The electrical property was measured by the standard four-probe method, which demonstrates that the resistivity decreases and the metal–insulator transition temperature shifts to higher temperature with increasing La content. Furthermore, the progressive substitution of Sm3+ by La3+ broadens the metal–insulator transition and reduces the temperature coefficient of resistivity. A mechanism based on double exchange is proposed to interpret the results above.

Metal–insulator transition of valence-controlled VO2 thin film prepared by RF magnetron sputtering using oxygen radical

We have prepared b-axis-oriented VO2 thin films by RF magnetron sputtering using oxygen radicals as the reactive gas. The VO2 thin films consist of a mixed-valence V3+/V4+ state formed by oxygen vacancies. The V3+ ratio strongly depends on the film thickness and the oxygen partial pressure of the radical gun during deposition. The lattice constant of the b-axis increases and the metal–insulator transition (MIT) temperature decreases with decreasing V3+ ratio, although the VO2 thin films with a high V3+ ratio of 42% do not exhibit MIT. The bandwidths and spectral weights of V 3d a1g and bands at around the Fermi level, which correspond to the insulating phase at 300 K, are smaller in the VO2 thin films with a low V3+ ratio. These results indicate that the control of the mixed-valence V3+/V4+ state is important for the MIT of b-axis-oriented VO2 thin films.

Role of spin-glass phase for magnetoresistance enhancement in nickel substituted lanthanum calcium manganite

Effect of Ni substitution in lanthanum calcium manganite (LCMO) has been investigated for change in magnetoresistance (MR). Scanning electron microscopy images revealed decrease in grain size from 3.72µm to 0.55µm by Ni substitution. Maximum increase in MR has been found 28% at low temperature (100K) for x=0.10, Ni substitution at Mn site. Metal insulator transition temperature has been decreased from 253.2K for x=0.0–90K for x=0.10. Above x=0.10, Ni substitution no metal-insulator transition temperature appeared due to the presence of porosity in the samples. Ni substitution lowered the magnetic transition temperature from 255K for x=0.0–125K for x=0.25. Lowering of irreversible temperature (Tirr) from 250K for x=0.0–135.4K for x=0.20 has been obtained by zero field cooled (ZFC) and field cooled (FC) measurements confirm reduction of ferromagnetic clusters and spin-glass phase like behavior due to Ni presence. The spin-glass phase presence allows spin-polarized tunneling even at low magnetic field, which ultimately results in enhancement of MR at low temperature. Core level X-ray photoelectron spectroscopy measurements confirm Ni2+ charge state of Ni ions and increase in Mn4+/Mn3+ ratio with increasing Ni content. Increase in resistivity and weakening of ferromagnetism with Ni substitution at Mn site has been observed due to the reduction in grain size and dilution of double exchange interaction.

Electric, magnetic, and thermo-electric properties of Cr doped La0.8Ca0.2Mn1−Cr MnO3 manganites

Abstract A detailed study of the structural, magnetic, magneto-transport and thermoelectric properties of polycrystalline La 0.8 Ca 0.2 Mn 1− x Cr x MnO 3 (0 x XRD studies confirm that the samples are pure and single phased. We have observed from electrical and magnetic studies that all the samples are exhibiting metal–insulator transition temperature, T MI and ferromagnetic–paramagnetic ( FM–PM ) transition temperature, T C . With increasing Cr-content, both T MI and T C are observed to decrease. The electrical resistivity data is analyzed using different theoretical models at various regions viz., metallic, insulating and percolation region. Analysis in the metallic region ( T T MI ) suggests that the electron–electron scattering in predominant in the region whereas the insulating region ( T > T MI ) is well described using SPH model. However, the resistivity data in the whole temperature range is analyzed using a phenomenological model based on phase segregation of ferromagnetic metallic and paramagnetic insulating regions. Thermoelectric power, S measurements were performed to understand the conduction mechanism and to ascertain the types of charge carrier responsible for conduction. It is observed that pristine as well as Cr-doped compounds show positive value of S which demonstrates that the charge carriers are holes.

Fabrication of La x Nd0.67−x Sr0.33MnO3 polycrystalline ceramics by sol–gel method

A systematic experimental study for La x Nd0.67−x Sr0.33MnO3 polycrystalline ceramic samples (x = 0.00, 0.07, 0.08, 0.09, 0.1, 0.2 and 0.3) were prepared by sol–gel method. The crystal structures were analyzed by X-ray diffraction, which reveals that all the samples are single-phase without any detectable secondary phases. The grain size and surface morphology were investigated by scanning electron microscope, which indicates that the ceramic samples with better crystallization have high density and chemical uniformity. The relation between resistivity and temperature shows that the resistivity decreases and the metal–insulator transition temperature shifts to higher temperature with the substitution La ion for Nd ion, which gives rise to increasing transition broadening temperature, and consequently decreases the temperature coefficient of resistivity. It is mainly due to the substitution La for Nd increases the A-site average cationic radius and thus the Mn–O–Mn angle increases, which gives rise to the double exchange between Mn3+ and Mn4+.

Influence of BiFeO3 Additions on the Electrical and Magnetic Properties of La0.7Ca0.3MnO3 Ceramics

(1 − x) La0.7Ca0.3MnO3–xBiFeO3 (0 ≤ x ≤ 0.1) ceramics were prepared by a solid state reaction method, and the structural, magnetic, electrical and thermoelectric power properties have been investigated. The single-phase (1 − x) La0.7Ca0.3MnO3-xBiFeO3 solid solutions with orthorhombic structure are achieved for x ≤ 0.1, and the lattice parameters of La0.7Ca0.3MnO3 ceramics keep equivalence with BiFeO3 additions. In contrast, both the ferromagnetic–paramagnetic transition temperature and the metal–insulator transition temperature monotonically shift towards lower temperature, and the thermoelectric power is enhanced for all the measured temperatures with increasing BiFeO3 content. In combination with resistivity and Seebeck coefficient data, the small polaron hopping is demonstrated to be responsible for the transport phenomena above the metal–insulator transition temperature.

The electrical transition temperature and magnetoresistance prediction of LaSr2Mn2O7 bilayered manganite

In this paper, the resistivity of bilayer manganite LaSr2Mn2O7 around the metal–insulator-like transition was systematically calculated as a function of temperature and applied magnetic field. A typical mathematical function (Gauss function) was used for fitting the experimental data. The theoretical values simulated in this work such as – metal–insulator transition temperature (Tp) and maximum resistivity (ρmax) showed promising agreement with the experimental data.

Carrier tuning the metal-insulator transition of epitaxial La0.67Sr0.33MnO3 thin film on Nb doped SrTiO3 substrate

La0.67Sr0.33MnO3 (LSMO) thin films were deposited on (001)SrTiO3(STO) and n-type doped Nb:SrTiO3(NSTO) single crystal substrates respectively. The metal to insulator transition temperature(TMI) of LSMO film on NSTO is lower than that on STO, and the TMI of LSMO can be tuned by changing the applied current in the LSMO/NSTO p-n junction. Such behaviors were considered to be related to the carrier concentration redistribution in LSMO film caused by the change of depletion layer thickness in p-n junction which depends greatly on the applied electric field. The phenomenon could be used to configure artificial devices and exploring the underlying physics.