Metal-insulator Transition Temperature Research Articles

Effect of TiO<sub>2</sub> Nanoparticle on the Structural and Electrical Properties of Nd<sub>0.67</sub>Sr<sub>0.33</sub>MnO<sub>3</sub> /TiO<sub>2</sub> Composites

In this work, (1-x) (Nd0.67Sr0.33MnO3): x (TiO2) composites with x = 0, 0.1, 0.2, 0.3 and 0.4 have been prepared to investigate the structural and electrical properties. Nd0.67Sr0.33MnO3 (NSMO) was synthesised via the solid-state reaction method before incorporated with TiO2. The addition of TiO2 nanoparticle as the secondary phase in manganite composite would favour the spin-polarized tunnelling near to the grain boundary and thus enhance the extrinsic magnetoresistance. Nevertheless, nanoparticle addition might contribute to substitution and diffusion with manganite compound as reported in literature. The effect of the TiO2 nanoparticle addition into NSMO composites has been examined by an X-ray diffractometer (XRD) and a four-point probe (4PP) system. From the thermogravimetric analysis (TGA), NSMO phase formation occurred in between 756.45 - 977.59 °C. XRD patterns showed that there is no peak shift when the TiO2 concentration increases. It can be deduced that TiO2 was segregated at the NSMO grain boundary region and its grain surface. However, a small amount of Ti atoms are expected to replace the Mn atoms in NSMO crystal system and has caused the increase in crystallite size. The electrical study showed that the presence of TiO2 nanoparticle and substitution of Ti in Mn sites have weaken the double exchange (DE) mechanism and suppressed the metal-insulator transition temperature (TMI). In addition, the insulating behaviour of TiO2 has also caused the resistivity of composites to increase drastically.

Electroresistance Effect due to Mo Substitution at Mn-Site in Monovalent Doped La<sub>0.85</sub>Ag<sub>0.15</sub>Mn<sub>1-</sub><i><sub>x</sub></i>Mo<i><sub>x</sub></i>O<sub>3</sub> (<i>x </i>= 0.00 and 0.05) Manganites

The electroresistance, ER effect of La0.85Ag0.15Mn1-xMoxO3 (x = 0.00 and 0.05) samples prepared using solid method are investigated. The increased of applied current from 5 mA to 10 mA does not change the metal-insulator transition temperature, TMI for both samples however decreased the resistivity in the temperature region of 50 K – 300 K. Both samples exhibit large ER effect at low temperature region. At TMI, the ER value is 75.5% (x =0) and decrease to 34.15% (x = 0.05). However, at 300 K, the value of ER increases to 57 % for Mo substituted sample, and the value decreases to 6.4% for the x =0 sample. The enhanced ER effect at 300 K may be due to the growth of conductive filaments under increased applied current. The increase of applied current may perturb the arrangement of magnetic inhomogeneity induced by Mo substitution, result in reduction of resistivity and lead to the observation of ER effect. These findings suggest potential application of La0.85Ag0.15Mn1-xMoxO3 (x = 0.05) in spintronic devices.

Enhancing visible-light transmittance while reducing phase transition temperature of VO2 by Hf–W co-doping

In order to enhance the visible-light transmittance while reducing the insulator–metal transition (IMT) temperature, Hf–W co-doping is designed for modification of VO2. We grow high-quality HfxWyV1−x−yO2 (HfWVO2) alloy films on c-plane sapphire substrates by pulsed laser deposition, and test structural, electrical, and optical properties of the films by various techniques. The Hf–W co-doped VO2 films exhibit outstanding thermochromic performances with a high luminous transmittance up to 41.1%, a fairly good near-infrared modulation capacity of 13.1%, and a low phase transition temperature of 38.9 °C. The enhanced luminous transmittance along with reduced IMT temperature in HfWVO2 is attributed to the co-doping synergetic effect of Hf and W, which effectively improves the optical bandgap and donates extra electrons into the system, respectively, while largely retaining the near-infrared modulation capacity of VO2. Our work provides an effective strategy in tailoring VO2 toward practical smart-coating applications by Hf–W co-doping.

Studies on Colossal Magnetoresistance Behaviour of Pr0.6Sr0.4MnO3/Pr0.5Ca0.5MnO3 Heterostructure Films

Heterostructure bilayer thin films of Pr0.6Sr0.4MnO3/Pr0.5Ca0.5MnO3 were prepared by pulsed laser deposition, and the structural, magnetic and magnetotransport properties were studied, and the results were compared to that of Pr0.6Sr0.4MnO3 and Pr0.5Ca0.5MnO3 single-layer thin films. Structural analysis of the heterostructure reveals c-axis orientation for both the manganite layers. The heterostructure exhibits a higher metal-insulator transition temperature and a larger colossal magnetoresistance near room temperature than that of the single-layer counterparts. The electrical conduction above the metal-insulator transition could be explained within the realm of the adiabatic small polaron hopping mechanism, with lesser activation energies compared to the single-layer films. Magnetization measurements portray the presence of an unsaturated symmetric hysteresis loop for the heterostructure with the Curie temperature of ~270 K corresponding to the ferromagnetic ordering emanating from Pr0.6Sr0.4MnO3. These results could be qualitatively understood considering the combined effect of strain and ferromagnetic proximity of Pr0.6Sr0.4MnO3 with charge-ordered Pr0.5Ca0.5MnO3 manganite.

The Physical Properties of Submicron and Nano-Grained La0.7Sr0.3MnO3 and Nd0.7Sr0.3MnO3 Synthesised by Sol–Gel and Solid-State Reaction Methods

La0.7Sr0.3MnO3 (LSMO) and Nd0.7Sr0.3MnO3 (NSMO) possess excellent colossal magnetoresistance (CMR). However, research work on the neodymium-based system is limited to date. A comparative study between LSMO and NSMO prepared by sol–gel and solid-state reaction methods was undertaken to assess their structural, microstructural, magnetic, electrical, and magneto-transport properties. X-ray diffraction and structure refinement showed the formation of a single-phase composition. Sol–gel-synthesised NSMO was revealed to be a sample with single crystallite grains and exhibited intriguing magnetic and electrical transport behaviours. Magnetic characterisation highlighted that Curie temperature (TC) decreases with the grain size. Strong suppression of the metal–insulator transition temperature (TMI) was observed and attributed to the magnetically disordered grain surface and distortion of the MnO6 octahedra. The electrical resistivity in the metallic region was fitted with theoretical models, and the conduction mechanism could be explained by the grain/domain boundary, electron–electron, and electron–magnon scattering process. The increase in the scattering process was ascribed to the morphology changes. Enhancement of low-field magnetoresistance (LFMR) was observed in nano-grained samples. The obtained results show that the grain size and its distribution, as well as the crystallite formation, strongly affect the physical properties of hole-doped manganites.

The Insulator–Insulator and the Insulator–Mixed‐Metal–Insulator Transitions in Self‐Heated AlxV1−xO2 Single Crystals and Their Visualization

In addition to the well‐known insulator–metal transition above room temperature in pure VO2, AlxV1−xO2 exhibits an insulator–insulator transition at temperatures above room temperature and below the insulator–metal transition temperature, consisting of an increase in resistance by a factor ≤2. The wealth of static and dynamic domain structures discovered in the mixed‐metal‐insulator state of free‐standing, high‐quality, pure VO2 single crystals via optical microscopy synchronous with dc I–V tracing has inspired the use of this technique in these more complex materials. Reported herein are I(V) measurements conducted on crystals of AlxV1−xO2 (0.007≤ × ≤0.02) at various ambient temperatures (To). Prominent features (switching) on the I(V) loops are easily identified with the onset of the insulator–insulator and of the mixed‐metal‐insulator states, both induced by self‐heating (Joule heating). The switching results from instabilities of two different types of negative differential resistance regimes. The applied power at the onset of each switching scales with the excess temperature above ambient. For both transitions, optical microscopy synchronous with dc I–V tracing visualizes the expansion of each phase within the background upon the current increasing, and its contraction upon the current decreasing.

In situ spectroscopic ellipsometry as a pathway toward achieving VO2 stoichiometry for amorphous vanadium oxide with magnetron sputtering

As a special class of materials, transition metal oxides exhibit in their crystalline phase a variety of interesting properties, such as metal–insulator transition, ferroelectricity, magnetism, superconductivity, and so forth. However, for industrially widely applied methods such as room temperature magnetron sputtering, during initial fabrication steps of these materials, they are mostly amorphous, and control of stoichiometry during fabrication is challenging. It is, therefore, of pivotal importance to control the stoichiometry of transition metal oxides during growth in the amorphous state. One particularly important example for the necessity of stoichiometry control is vanadium dioxide (VO2), where small deviations in stoichiometry during fabrication result in unfavorable changes in the electronic and structural properties, for example, the metal–insulator transition temperature and optical permittivity. In this work, the stoichiometry of amorphous vanadium oxides is adjusted to VO2 using in situ spectroscopic ellipsometry (in situ SE) and verified by x-ray photoelectron spectroscopy. After an annealing process, a monoclinic VO2 crystalline structure is observed through x-ray diffraction at 30 °C. At an elevated temperature of 150 °C, which is higher than the typical metal–insulator transition temperature in VO2 of around 67 °C, a rutile crystalline structure is observed, which verifies the correctness of the stoichiometry of VO2. A Mott metal–insulator transition is revealed by the change in the imaginary part of optical permittivity through SE as well.

Spin injection into vanadium dioxide films from a typical ferromagnetic metal, across the metal–insulator transition of the vanadium dioxide films

A vanadium dioxide VO2 film shows metal-insulator transition (MIT) induced by changing environmental temperature. We report the temperature dependence of electromotive force properties generated in VO2/Ni80Fe20 bilayer junctions under the ferromagnetic resonance (FMR) of the Ni80Fe20 layer. An electromotive force generated in a VO2/Ni80Fe20 bilayer junction under the FMR showed a small change across the MIT temperature of the VO2 film, while the VO2 film resistance drastically changed. This behavior was not only explained with the temperature dependence of the electromotive force property generated in the Ni80Fe20 film itself under the FMR, but also with the generated electromotive forces due to the inverse spin-Hall effect (ISHE) in the VO2 film under the FMR of the Ni80Fe20 film. That is, we successfully demonstrated the spin injection from a Ni80Fe20 film into a VO2 film across the MIT temperature of the VO2 film.

Tunable metal-insulator transition of V2O3 thin films strained by controlled inclusion of crystallographic defect

Current understanding of the metal-insulator transition (MIT) of V2O3 based on the electronic band structures states that the electrical properties of V2O3 are extremely sensitive to the crystallographic structures. In the present report, we present the systematic tuning of the MIT characteristics through the control of crystallographic structure of the polycrystalline V2O3 thin film. Both crystallographic textures and the residual strains due to the presence of controlled amount of stacking faults in the films could be adjusted with the proper control of deposition conditions and the corresponding changes in the electrical properties have be tracked to establish the tunability of MIT temperature as well as the transition rates. Observed change was understood with the consideration of electronic band structures, through which the effect of atomic distances to the electron band gap was demonstrated. Additionally, it was found that the lattice strain of a strongly textured V2O3 influences both room temperature resistivity and MIT temperature.

Structural and magneto-transport features of cobalt-substituted mixed-valence manganites

We report the results of a detailed investigation of the role of cation substitution Co → Mn on the structural, microstructural, magnetic, and transport properties of the manganite system La2/3Ca1/3Mn1−xCoxO3 (x = 0.0, 0.1, 0.2 and 0.3). It is found that the replacement of Mn with Co does not change the crystal structure of the parent compound La2/3Ca1/3MnO3 which crystallizes in orthorhombic structure with Pbnm space group. From a systematic analysis of the magneto-transport measurements, it has been shown that the cobalt substitution leads to a decrease in the system magnetization and lowering of Curie and metal–insulator transition temperatures. The magnetoresistance percentage (MR%), which is found to change significantly by Co substitution and exceeds 100% for x = 0.3 sample at low temperatures, is also reported at 1 T applied magnetic field.

Reversible optical control of the metal-insulator transition across the epitaxial heterointerface of a VO2/Nb:TiO2 junction

Optical control of exotic properties in strongly correlated electron materials is very attractive owing to their potential applications in optical and electronic devices. Herein, we demonstrate a vertical heterojunction made of a correlated electron oxide thin film VO2 and a conductive 0.05 wt% Nb-doped TiO2 single crystal, whose metal-insulator transition (MIT) across the nanoscale heterointerface can be efficiently modulated by visible light irradiation. The magnitude of the MIT decreases from ~350 in the dark state to ~7 in the illuminated state, obeying a power law with respect to the light power density. The junction resistance is switched in a reversible and synchronous manner by turning light on and off. The optical tunability of it is also exponentially proportional to the light power density, and a 320-fold on/off ratio is achieved with an irradiance of 65.6 mW cm−2 below the MIT temperature. While the VO2 thin film is metallic above the MIT temperature, the optical tunability is remarkably weakened, with a one-fold change remaining under light illumination. These results are co-attributed to a net reduction (~15 meV) in the apparent barrier height and the photo-carrier-injection-induced metallization of the VO2 heterointerface through a photovoltaic effect, which is induced by deep defect level transition upon the visible light irradiance at low temperature. Additionally, the optical tunability is minimal, resulting from the quite weak modulation of the already metallic band structure in the Schottky-type junction above the MIT temperature. This work enables a remotely optical scheme to manipulate the MIT, implying potential uncooled photodetection and photoswitch applications.

Correlation between Symmetry and Phase Transition Temperature of VO2 Films Deposited on Al2O3 Substrates with Various Orientations

AbstractThe structural aspects of the insulator–metal transition (IMT) characteristics of VO2 are sensitive to the octahedral symmetry variation of VO6. By varying substrate orientation (c‐, a‐, and m‐plane Al2O3), the correlation between IMT temperature and local symmetry is investigated. For a VO2 film deposited on m‐plane Al2O3, which has high symmetry due to fewer domain boundaries induced by m‐plane Al2O3, the IMT temperature is low (326.47 K). In contrast, for a film deposited on c‐plane Al2O3 (having lower symmetry), the IMT temperature is the highest (336.74 K) among the films used in this work. Furthermore, temperature‐dependent Raman spectra reveals that the structural phase transition temperature decreases in the order of the VO2 film deposited on c‐, a‐, and m‐plane Al2O3, suggesting that the symmetrical structure reduces the activation energy for IMT by decreasing thermodynamic energy. These results demonstrate that structural symmetry plays a crucial role in lowering the transition temperature.

Spatially Controlled Octahedral Rotations and Metal-Insulator Transitions in Nickelate Superlattices.

The properties of correlated oxides can be manipulated by forming short-period superlattices since the layer thicknesses are comparable with the typical length scales of the involved correlations and interface effects. Herein, we studied the metal–insulator transitions (MITs) in tetragonal NdNiO3/SrTiO3 superlattices by controlling the NdNiO3 layer thickness, n in the unit cell, spanning the length scale of the interfacial octahedral coupling. Scanning transmission electron microscopy reveals a crossover from a modulated octahedral superstructure at n = 8 to a uniform nontilt pattern at n = 4, accompanied by a drastically weakened insulating ground state. Upon further reducing n the predominant dimensionality effect continuously raises the MIT temperature, while leaving the antiferromagnetic transition temperature unaltered down to n = 2. Remarkably, the MIT can be enhanced by imposing a sufficiently large strain even with strongly suppressed octahedral rotations. Our results demonstrate the relevance for the control of oxide functionalities at reduced dimensions.

Electrical, magnetotransport and magnetization studies of (1-x) La0.7Sr0.3Mn0.94Co0.06O3/x ZnO (x = 0, 0.04) composite

We have investigated the electrical transport, magnetotransport and magnetic properties of (1-x) La0.7Sr0.3Mn0.94Co0.06O3(LSMCO)/x ZnO (x = 0, 0.04) composite samples to explore the role of the secondary phase. The results of Rietveld refined X-ray diffraction (XRD) data show that the prepared LSMCO sample possesses a rhombohedral structure in the hexagonal lattice and the same reveal that both the compounds in the composite maintain their identities, that is, there is no evidence of reaction between the LSMCO and ZnO. It has been observed that the addition of the ZnO phase into the pure LMSCO sample lowers the metal–insulator transition temperature (TMI). The electrical resistivity was explored by various theoretical models below & above the metal–insulator transition temperature. In the case of a composite sample (x = 0.04), the high field magnetoresistance has been observed at around room temperature. In the magnetic study, the saturation of magnetization decreases in the case of ZnO added LSMCO sample.

Structural and Magneto-Transport Properties of (1-x)La0.70Sr0.30MnO3 – (x)ErMnO3 (x = 0.00, 0.10, 0.20, and 0.30) Composite

(1-x)La0.70Sr0.30MnO3 (LSMO) – (x)ErMnO3 (EMO) (x = 0.00, 0.10, 0.20, and 0.30) ceramics were synthesized by chemical route. The structure characterization of all the samples was carried out at room temperature. The LSMO has rhombohedral symmetry with space group R-3C and EMO has hexagonal symmetry with space group P63/mcm. The temperature-dependent resistivity behavior of pure and composite samples has shown metal-insulator (MI) transition. The peak resistivity ρmax increases with decrease in MI transition temperature. Percentage magneto-resistance (MR) variation of LSMO-EMO (x = 0.10, 0.20, and 0.30) with temperature has also been studied. The result of ln (ρ) vs. Tn shows that conduction occurs due to variable range hopping (VRH) of electrons for x = 0.20 and 0.30. % MR data has been statically analyzed which shows the normal distribution.

Strain engineering of optical properties in transparent VO2/muscovite heterostructures.

Transparent VO2/muscovite heterostructures have attracted considerable attention because of their unique chemical and physical properties and potential practical applications. In this paper, we investigated the influence of uniaxial mechanical strain on the optical properties of VO2/muscovite heterostructures through Raman scattering and optical transmittance measurements. Under applied strain, linear shifts in peak positions of Raman-active phonon modes at approximately 340, 309, and 391 cm-1 were observed. The extracted Grüneisen parameter values were approximately between 0.44 and 0.57. Furthermore, a pronounced strain-induced change in the metal-insulator transition (MIT) temperature was observed, which decreased under compressive strain and increased under tensile strain. The rates of MIT temperature variation reached 4.5 °C per % and 7.1 °C per % at a wavelength of 1200 nm during heating and cooling processes, respectively. These results demonstrate that the modulation of the optical properties of VO2/muscovite heterostructures is controllable and reversible through strain engineering, opening up new opportunities for applications in flexible and tunable photonic devices.

Influence of sintering temperature on the structural, electrical transport and magnetic behaviour of La0.7Sr0.3MnO3 prepared through co-precipitation technique

In this communication, report of nanostructured polycrystalline La0.7Sr0.3MnO3 (LSMO) prepared through a co-precipitation technique at 800 °C, 900 °C and 1000 °C is discussed in detail. The structural, electrical transport, magnetoresistance and magnetic properties of LSMO samples were found strongly dependent on the sintering temperature (TS). Structural properties were studied through XRD and SEM. The results of Rietveld refined XRD data show that all the samples are in tetragonal arrangement and having single-phase structure with space group I4/mcm (140). The average size of synthesized nanoparticles was found increasing (21.05 nm to 46.06 nm) with increase in sintering temperature. A significant increase in metal–insulator transition temperature (TMI) was observed at higher sintering temperatures. Higher sintering temperature results in the enhancement of resistivity. To understand the metallic part of resistivity (T < TMI) including upturn behavior, resistivity data with and without magnetic field were fitted using Kondo like spin-dependent scattering, electron–electron (e-e), electron–phonon (e-p) and electron-magnon (e-m) scattering. The decrement in magnetoresistance (MR) in the wide temperature range has been explained by spin-polarized tunnelling in this communication. Magnetic properties were examined through M−H measurements and found the highest value of magnetization for the sample sintered at 1000 °C.

Synthesis of Tungsten-Doped Vanadium Dioxide Using a Modified Polyol Method Involving 1-Dodecanol.

The doping of tungsten into VO2 (M) via a polyol process that is based on oligomerization of ammonium metavanadate and ethylene glycol (EG) to synthesize a vanadyl ethylene glycolate (VEG) followed by postcalcination was carried out by simply adding 1-dodecanol and the tungsten source tungstenoxytetrachloride (WOCl4). Tungsten-doped VEGs (W-VEGs) and their calcinated compounds (WxVO2) were prepared with varying mixing ratios of EG to 1-dodecanol and WOCl4 concentrations. Characterizations of W-VEGs by powder X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, and infrared and transmittance spectroscopy showed that tungsten elements were successfully doped into WxVO2, thereby decreasing the metal-insulator transition temperature from 68 down to 51 °C. Our results suggested that WOCl4 variously combined with 1-dodecanol might interrupt the linear growth of W-VEGs, but that such an interruption might be alleviated at the optimal 1:1 mixing ratio of EG to 1-dodecanol, resulting in the successful W doping. The difference in the solar modulations of a W0.0207VO2 dispersion measured at 20 and 70 °C was increased to 21.8% while that of a pure VO2 dispersion was 2.5%. It was suggested that WOCl4 coupled with both EG and 1-dodecanol at an optimal mixing ratio could improve the formation of W-VEG and WxVO2 and that the bulky dodecyl chains might act as defects to decrease crystallinity.

Investigation of metal-insulator transition temperature and magnetic properties of NdNiO3 nanoparticles

The control of metal–insulator transition (MIT) and magnetic properties of rare-earth nickelates remains a fundamental problem in material science. Here, we report a possible way to tune the MIT as well as the magnetic properties of NdNiO3 by reducing the crystalline size to the nanoscale. NdNiO3 nanoparticles of different sizes have been synthesized by a simple aqueous solution-based chemical method in the atmospheric pressure under oxygen. The x-ray diffraction study shows the formation of orthorhombic nanocrystals of NdNiO3 with Pbnm space group, presence of oxygen vacancies and higher octahedral distortion. The morphology and the particle size distribution have been evaluated by FESEM and TEM. The temperature dependent resistivity measurement shows the metal to insulator transition, which depends on the particle size; lower the particle size, higher the MIT temperature (TMI). Further study on the temperature dependent magnetic susceptibility shows the signature of magnetic phase transition at a temperature (Néel temperature TN), lower than the TMI, which indicates the existence of second order magnetic phase transition in our NdNiO3 nanoparticles. We have attributed the higher TMI and the origin of second order magnetic phase transition predominantly to the higher NiO6 octahedral distortion and the narrowing of bandwidth of the nanocrystalline NdNiO3, which leads to the opening of the charge transfer gap at the temperature higher than the bulk.

Enhancement of intrinsic magnetoresistance in Zn doped La0.9Sr0.1MnO3 epitaxial films

Mn-site doping is an effective avenue to tune the physical properties of manganites, but the underlying mechanism has rarely been explored in the epitaxial film systems doped with closed shell metal ions. Here, high-quality epitaxial La0.9Sr0.1Mn1-yZnyO3 (LSMZO) thin films have been successfully prepared by magnetron sputtering on LaAlO3 substrate, and the effect of Zn doping on the structural, transport and magnetic properties of the LSMZO films was investigated systematically. As Zn content increases, Curie temperature (TC), saturation magnetization (MS) and metal-insulator transition temperature (TMI) are reduced, and the room-temperature resistivity is raised. Analysis on Mn3+/Mn4+ ratio shows the substitution of Mn3+ ion by Zn2+ ion reduces Mn3+/Mn4+ ratio deviating from the optimal value and thus weakening double exchange interaction. Besides that, the electron-lattice coupling stemming from the lattice distortion and Jahn-Teller (JT) effect also has a significant impact. Most significantly, Zn doping induces an enhancement in the intrinsic magnetoresistance (MR) due to the enhanced magnetic disorder, and a fairly large MR of 26.4% is obtained under a low field of 0.3 T at 210 K in the LSMZO (y = 0.20) film. Our study reveals that Zn doping at Mn site can regulate intrinsic physical properties of manganite films by modifying double exchange and electron-lattice interaction.