Metal-insulator Transition Temperature Research Articles

Large temperature coefficient of resistivity (TCR) of La0.67Ca0.33MnO3 polycrystalline ceramics improved by optimizing sintering temperatures

This work investigates fabrication of La0.67Ca0.33MnO3 (LCMO) polycrystalline ceramics using a facile sol–gel method at sintering temperatures (Ts) ranging from 1470 °C to 1540 °C and explores relationship between their electrical transport properties and Ts. X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and X-ray photoemission spectroscopy were employed to characterize crystal structure, morphologies, element distributions, and Mn3+/Mn4+ ratio of the as-obtained LCMO ceramics. The influence of Ts on phase purity, microstructure, morphology, and electrical transport properties was also investigated. LCMO ceramics exhibit increased grain size and higher temperature coefficient of resistivity (TCR) at higher temperatures. An increase in Ts can reduce the strength of electron–lattice interactions and theoretical Curie temperature in LCMO ceramic samples, which can in turn optimize their electrical transport properties. Moreover, an increase in Ts enhances double-exchange (DE) effect to some extent. The DE effect further affects resistivity and metal–insulator transition temperature of material. Additionally, small-polaron hopping model and phenomenological percolation model were used to characterize electrical transport in metal–insulator transition region. The temperature at which TCR is maximum (265.78 K) and the largest TCR value (58.16 %) were achieved at a Ts of 1530 °C. By modulating the sintering temperature, we achieved LCMO ceramics with larger grains and enhanced electrical properties. This optimization renders materials suitable for highly sensitive infrared bolometer applications.

Exploring the Magnetism and Magnetocaloric Effect of W‐Doped VO2 Based on J1–J2 Ising Model

W‐doped VO2 exhibits a significant reduction in the metal–insulator transition temperature. In this study, the authors aim to investigate the magnetic influences of different proportions of W doping in VO2 with a rutile structure, using the J1–J2 model. The J1 and J2 values are calculated as 1.21 and −0.024 eV, respectively, using the generalized gradient approximation (GGA)+U method. Additionally, the J1–J2 model without W doping (x = 0) is simulated using the Ising model, and the obtained phase transition temperature of approximately 361 K closely matches the temperature calculated by Zheng et al. based on the spin dimer model. Furthermore, employing this model, the magnetization curves and magnetic entropy change (ΔS) are simulated for different W doping levels (x = 0–0.2). Through comprehensive analysis, it is found that the system exhibits the most promising research potential at a doping level of x = 0.12. These findings deepen the understanding of the mechanisms underlying the magnetic effects of W‐doped VO2 and provide a theoretical model guidance for future development of vanadium oxide materials.

A broadband reconfigurable frequency-selective surface in terahertz band based on different metal-insulator transition temperature of VO<sub>2</sub>

A broadband reconfigurable frequency-selective surface in terahertz band based on different metal-insulator transition temperature of VO<sub>2</sub>

Probabilistic computing with NbOx metal-insulator transition-based self-oscillatory pbit

Energy-based computing is a promising approach for addressing the rising demand for solving NP-hard problems across diverse domains, including logistics, artificial intelligence, cryptography, and optimization. Probabilistic computing utilizing pbits, which can be manufactured using the semiconductor process and seamlessly integrated with conventional processing units, stands out as an efficient candidate to meet these demands. Here, we propose a novel pbit unit using an NbOx volatile memristor-based oscillator capable of generating probabilistic bits in a self-clocking manner. The noise-induced metal-insulator transition causes the probabilistic behavior, which can be effectively modeled using a multi-noise-induced stochastic process around the metal-insulator transition temperature. We demonstrate a memristive Boltzmann machine based on our proposed pbit and validate its feasibility by solving NP-hard problems. Furthermore, we propose a streamlined operation methodology that considers the autocorrelation of individual bits, enabling energy-efficient and high-performance probabilistic computing.

Size-induced metal–insulator transition temperature decrease of VO2 nanoparticles obtained by laser irradiation in water

Vanadium dioxide (VO2) is a highly attractive and multifunctional material for technological applications due to its metal–insulator transition, which is accompanied by several orders of magnitude changes in electrical resistivity and optical transmissivity. In this work, we report on the metal–insulator transition of VO2 nanoparticles which are produced by a simple, efficient, and low-cost two-step preparation method based on infrared laser irradiation of V2O5 powder dispersed in water and subsequent stoichiometric stabilization by vacuum annealing at 773 K. VO2 nanoparticles have an average size of 36 nm with facet-like contours and undergo a structural phase transition from a monoclinic to a tetragonal phase, accompanied by a metal–insulator transition occurring around 323 K, which is about 17 K lower than bulk VO2. No evidence of the formation of intermediary metastable phases is observed. Our findings reveal that the size of VO2 nanoparticles is a key parameter to moving the transition closer to room temperature, which is important for many applications.

Temperature dependence of thermal transport properties in spinel Cu1-xAgxIr2S4

We investigated the thermal transport properties of Cu1-xAgxIr2S4 (0 ≤ x ≤ 0.05). The thermal conductivity of CuIr2S4 exhibits a sudden change at the metal-insulator transition (MIT) temperature, where the phonon thermal conductivity κph decreases sharply from the insulating phase (IP) to the metallic phase (MP). The decrease in κph is attributed to a decrease in the phonon mean free path l from the IP to the MP, which is caused by anomalous phonon scattering in the MP. The decrease in l at the MIT temperature becomes less pronounced with increasing x, and the absolute values of l in the MP for x = 0.025 and 0.05 are higher than that for x = 0. This behavior suggests that the anomalous phonon scattering is suppressed by Ag substitution. Compared with previously reported results for the effect of Zn substitutions at Cu sites, the phonon relaxation time exhibits a distinct behavior for Ag and Zn substitution.

Enhancing thermochromic properties of VO2 amorphous films on glass substrates by Sn-W co-doping

Co-doping is an effective method for VO2 films to meet the requirements of smart window applications. In this research, we prepared pure VO2, Sn-doped, and Sn-W co-doped amorphous films on glass substrates. Compared with pure and Sn-doped VO2 films, Sn-W co-doped samples showed better thermochromic properties. For the 4.8 % Sn-2.1 % W co-doped sample, the luminous transmittance (Tlum) reached 76 %, the metal-insulator transition (MIT) temperature (Tc) decreased to 46.7 °C, the solar transmittance modulation capacity (ΔTsol) was maintained at around 5.2 %, and the near-infrared modulation capacity (ΔTIR) was up to 13.5 %. These results show that co-doping Sn and W into VO2 is feasible for smart window applications.

Lattice stress engineering induced by rare-earth ion in regulating phase transition and thermochromic properties of VO2 films

Reduction of metal-insulator transition temperature (θMIT) of VO2 has been a key topic in turning its potential into practical applications. Electron injection as induced by high-valence ions’ doping of W, Mo and Nb has been studied extensively due to its high efficiency in declining θMIT. However, little attention has been given to the impact of lattice stress/strain caused by larger radius ions on the phase transition and thermochromic properties of VO2. Herein, we investigate the dependence of θMIT on lattice stress by varying the radius of rare-earth (RE) ions in RE-Mo co-doped VO2 films. As the radius of the RE ion increases, the lattice stress gradually intensifies, resulting in a lower θMIT and enhanced luminance transmittance (Tlum) at the same doping concentration. The TEM results reveal that the apical V–O bond length of the VO6 octahedron is significantly influenced by the radius of the RE ion, thereby altering the energy gap between the d// and π∗ bands and adjusting θMIT. These findings demonstrate an additional degree of freedom to manipulate the phase transition of VO2 through lattice stress engineering.

High room temperature magnetoresistance in La0.9Sr0.1Mn1-yZnyO3 epitaxial films

Developing large room temperature magnetoresistance materials has been a major hotspot for a few last decades due to its importance in the practical applications in magneto-resistive random access memory, and it remains a challenge. Here, La0.9Sr0.1Mn1-yZnyO3 (LSMZO) films have been grown on SrTiO3 substrates by magnetron sputtering, showing excellent quality of epitaxy. Upon increasing Zn content, the reduction of Curie temperature (TC) and metal-insulator transition temperature (TMI) is observed, accompanied by the increase of coercive field and room-temperature resistivity. Analysis on the fraction of Mn3+ and Mn4+ indicates that Zn2+ substitution at Mn site results in a higher Mn4+ fraction departing from the optimum value and hence a weaker double-exchange interaction by replacing more Mn3+. Besides that, ionic radius mismatch induced internal strain and substrate induced tensile strain also play a part. Most significantly, the enhanced magnetic disorder gives rise to an increase of intrinsic magnetoresistance around room temperature in LSMZO (y = 10%) film, and a large magnetoresistance of 18.4% is obtained under 0.3 T at 280 K in this sample. Our research provides a simple and effective approach to achieve a large room temperature magnetoresistance in Mn site doped manganite epitaxial films.

Tailoring the thermochromic properties of sputter-deposited VO2 films by O2+ ion irradiation

Vanadium dioxide (VO2) thin film is a well-known phase transition material showing extensive modification in their physical properties, such as electronic, magnetic, and optical characteristics when the metal–insulator transition (MIT) occurs. Many approaches (including doping, micro-structure, external stress, and defect engineering) can enhance the MIT characteristics of vanadium dioxide thin films. More recently, low-energy ion irradiation has been well-established to modify the electrical, optical, and physical properties of metal oxide thin films. In this paper, the effect of low-energy O2+ ion irradiation on the thermochromic properties of VO2 thin films was investigated. From the temperature-dependent near infrared transmittance measurements, both MIT temperature and thermal hysteresis width of VO2 thin films showed an opposite trend and decreased as the ion fluence increased. The values of solar modulation and luminous transmittance of irradiated VO2 films were significantly altered by introducing low-energy ion irradiation, compared to the pristine VO2 thin films. According to the XPS analyses, the vanadium ion are in mixed of two valence states (V4+ and V5+). Moreover, the concentration of oxygen vacancies in VO2 films increases as the fluence of low-energy O2+ ion increases. Our study concludes that low-energy O2+ ion irradiation is an effective means to optimize the MIT characteristics of VO2 thin films to a certain extent.

Anisotropic magnetic and magnetotransport properties in morphologically distinct Nd0.6Sr0.4MnO3 thin films

We investigate the magnetic and magnetotransport properties of nanostructured Nd0.6Sr0.4MnO3 (NSMO) thin films grown on (100) oriented SrTiO3 (STO) substrates. The thin films of 100 nm thickness fabricated using the pulsed laser deposition technique possess two distinct surface morphologies—granular and nano-rod type. The morphological change present in the system significantly affects the magnetic and magnetotransport properties of the thin films. Magnetization measurements revealed that the films with rod-type morphology exhibit improved in-plane magnetic anisotropy. The colossal magnetoresistance (∆R/R(H = 0)) of the granular sample is ∼91 %, and the rod morphology sample is ∼97 % at 3 T magnetic field. Additionally, magnetotransport studies revealed that the granular thin films display a characteristic butterfly-shaped low-field magneto-resistive (LFMR) behavior with the value of LFMR of up to ∼10 %. Furthermore, it is observed that the thin film’s morphology has a significant effect on the anisotropic magnetoresistance ratio (AMR). Thin films with rod-type morphology show an enhanced AMR of ∼30 % around its metal-insulator transition temperature. Such morphology-dependent tunability in magnetoresistance properties over a wide temperature range is potentially interesting for developing oxide-based sensors and devices.

Effects of Li substitution on structural, magnetic properties and electroresistance behaviour in La0.8Na0.2-xLixMnO3 monovalent-doped manganite

The effects of Li substitution on structural, magnetic properties, and electroresistance (ER) behaviour in La0.8Na0.2-xLixMnO3 (x = 0 – 0.15) manganites prepared via the conventional solid-state method were reported. All samples exhibited a metal–insulator transition behaviour with a decrease in the metal–insulator transition temperature, TMI from 270 K (x = 0) to 255 K (x = 0.15) and reduction in Curie temperature, TC from 272 K (x = 0.10) to 235 K (x = 0.15). All samples exhibited a reduction in resistivity under higher applied current of 5 mA than that of 1 mA, which led to an ER effect. Large ER value of −59 % is achieved for sample × = 0.15 at 300 K, thereby indicating its potential application for spintronic-based devices. The competition between the occurrence of lattice distortion and the presence of magnetic inhomogeneity induced by A-site disorder (σ2) is suggested responsible for the observed ER effect.

Interface Effects of Strain-Energy Potentials on Phase Transition Characteristics of VO2 Thin-Films.

Metal-insulator-transition (MIT) of VO2 has attracted strong attention as a potential phenomenon to be utilized in nanostructured devices. Dynamics of MIT phase transition determines the feasibility of VO2 material properties in various applications, for example, photonic components, sensors, MEMS actuators, and neuromorphic computing. However, conventional interface strain model predicts the MIT effect accurately for bulk, but fairly for the thin films, and thus, a new model is needed. It was found that the VO2 thin film-substrate interface plays a crucial role in determining transition dynamics properties. In VO2 thin films on different substrates, coexistence of insulator-state polymorph phases, dislocations, and a few unit cell reconstruction layer form an interface structure minimizing strain energy by the increase of structural complexity. As a consequence, MIT temperature and hysteresis of structure increased as the transition enthalpy of the interface increased. Thus, the process does not obey the conventional Clausius-Clapeyron law anymore. A new model is proposed for residual strain energy potentials by implementing a modified Cauchy strain. Experimental results confirm that the MIT effect in constrained VO2 thin films is induced through the Peierls mechanism. The developed model provides tools for strain engineering in the atomic scale for crystal potential distortion effects in nanotechnology, such as topological quantum devices.

Magneto transport behavior of La0.67Pb0.33MnO3manganite system with various additives

The nanostructured manganites show low field magnetoresistance, where grain boundary effects influence the phenomenon. The current investigation aims to comprehend the impact of the grain boundaries on the magnetic and magneto transport characteristics of La0.67Pb0.33MnO3 manganite with 1% of additives ZrO2, Bi2O3, and Li2CO3. Two step synthesis processes was adopted to prepare the pure and additive samples. The samples were characterized with XRD, XPS and FESEM to establish the structure, phase purity, oxidation states, elemental analysis and morphology respectively. The prepared samples were formed in single-phase rhombohedral structure with the enhanced crystallite and grain size with additives when compared to the pure sample. The observed magnetic transition temperature, are well above room temperature (TC = 353 K) and a soft ferromagnetic nature with 70% of ferromagnetic fraction is noticed at T = 300 K for all the studied samples. The metal insulator transition temperature (TP) remains invariant for ZrO2, Bi2O3 additives, while it reduces for Li2CO3 additive. It is observed that 1% of additives are acting as conduction barriers at the grain boundaries as a consequence there is an increase in resistivity. The contribution to magnetoresistance is seen as intrinsic and extrinsic in pure and additive materials exhibiting high percentage both at low and room temperature. All the sample shows an appreciable temperature coefficient of resistance percentage at their respective transition temperatures which can be made useful for bolometric applications. Conduction mechanisms in the ferromagnetic and paramagnetic phases were examined with the electron-magnon scattering and small polaron hopping model respectively.

Coexistent VO2 (M) and VO2 (B) Polymorphous Thin Films with Multiphase-Driven Insulator-Metal Transition.

Reversible insulator-metal transition (IMT) and structure phase change in vanadium dioxide (VO2) remain vital and challenging with complex polymorphs. It is always essential to understand the polymorphs that coexist in desired VO2 materials and their IMT behaviors. Different electrical properties and lattice alignments in VO2 (M) and VO2 (B) phases have enabled the creation of versatile functional devices. Here, we present polymorphous VO2 thin films with coexistent VO2 (M) and VO2 (B) phases and phase-dependent IMT behaviors. The presence of VO2 (B) phases may induce lattice distortions in VO2 (M). The plane spacing of (011)M in the VO2 (M) phase becomes widened, and the V-V and V-O vibrations shift when more VO2 (B) phase exists in the VO2 (M) matrix. Significantly, the coexisting VO2 (B) phases promote the IMT temperature of the polymorphous VO2 thin films. We expect that such coexistent polymorphs and IMT variations would help us to understand the microstructures and IMT in the desired VO2 materials and contribute to advanced electronic transistors and optoelectronic devices.

Optimization of temperature coefficient of resistivity and magnetoresistance of La0.67Ca0.33Mn0.98Fe0.02O3: Agx ceramics by Ag adjustment

High-quality La0.67Ca0.33Mn0.98Fe0.02O3: Agx ceramics were prepared by sol-gel and solid-phase methods. Ag and Ag+ were found to be present in the system after Ag doping, exhibiting a significant effect on the crystal structure and the concentration of Mn3+/Mn4+. Ag provided the desired conductive pathway, promoted the connectivity between grains, and improved the electrical conductivity of the ceramic. With the increase of Ag doping, the resistivity gradually decreased, while the Curie and the metal-insulator transition temperatures gradually increased. The temperature coefficient of resistivity (TCR) and the magnetoresistance (MR) were greatly improved. At x = 0.15, the peak TCR reached 75.4%·K−1, and the peak MR reached 87.0%. In order to adjust the TCR and MR, the contents of Ag and Ag+ were optimized by controlling the amount of Ag doping. The results demonstrated that Ag doping can regulate the metal-insulator transition and ferromagnetic-paramagnetic transition of the system effectively.

Polaron-induced metal-to-insulator transition in vanadium oxides from density functional theory calculations

Vanadium oxides have been extensively studied as phase-change memory units in artificial synapses for neuromorphic computing due to their metal-insulator transitions (MIT) at or near room temperature. Recently, injection of charge carriers into vanadium oxides, e.g., via optically via a heterostructure, has been proposed as an alternative switching mechanism and also potentially as a means to tune the MIT temperature. In this study, we explore the formation of small polarons in the low temperature (LT) insulating phases for ${\mathrm{V}}_{3}{\mathrm{O}}_{5}, {\mathrm{VO}}_{2}$, and ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$, and the barriers to their migration using density functional theory calculations. We find that ${\mathrm{V}}_{3}{\mathrm{O}}_{5}$ exhibits very low hole and electron polaron migration barriers ($<100$ meV) compared to ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{VO}}_{2}$, leading to much higher estimated polaronic conductivity. We also link the relative migration barriers to the amount of distortion that has to travel when the polaron migrate from one site to another. Polarons in ${\mathrm{V}}_{3}{\mathrm{O}}_{5}$ also have smaller binding energies to vanadium and oxygen vacancy defects. These results suggest that the triggering of the MIT via injection of charge carriers are due to the formation of small polarons that can migrate rapidly through the crystal.

Temperature dependent photoresponse of MoS2/VO2 heterojunction

Van der Waals (VdW) heterojunctions composed of transition metal dichalcogenides and strongly correlated electronic materials have attracted great interest for feasible control of the optoelectrical properties. The MoS2/VO2 heterojunction is one of the typical structures whose optoelectrical property can be modulated by the metal insulator transition (MIT) of VO2. Here, we show a systematic investigation on the temperature dependent photoresponse across the MIT temperature of VO2. In-situ mapping on the junction surface potential confirms that both the MIT and depletion of MoS2 improve the quality of Schottky junction. As a result, photoresponse of the MoS2/VO2 heterojunction is significantly improved, especially, rapid improvement around MIT temperature and reduction of dark current are observed. The results clear reveal the temperature dependent energy structure of MoS2/VO2 heterojunction, and should be helpful to understand the optoelectrical behavior of similar vdW heterojunction.

Wide temperature range magnetoresistance enhancement of La0.67Ca0.33MnO3: NiO nanocomposites

This paper reports the addition of NiO nanoparticles into the LCMO nanocomposites through a simplified methodology in the heat treatment process compared to the previous work. Structural analysis showed that LCMO and NiO co-existed in the samples without the formation of other phases and confirmed their composite structures. The grain size distribution of samples was observed to become narrower and left-skewed as the concentration of NiO nanoparticles increased. This indicates that the NiO nanoparticles were distributed near the grain boundaries or on the grain surfaces. The presence of La, Ca, Mn, O, and Ni was confirmed by EDX analysis, and no additional impurities were noticeable, which revealed that the composites here are chemically pure in the composition. Temperature dependence magnetisation measurement (100–300 K) revealed the occurrence of ferromagnetic to paramagnetic transition. The Curie temperature, TC recorded at 260 K for all samples. However, the metal–insulator transition temperature, TMI was found to decrease from 246 to 120 K as the NiO content increased. Magnetoresistance of composites was observed to enhance over a wide temperature range at 10 kOe. The findings of LCMO: NiO composites in this work can set up a new perspective and unlock its potential to be utilised in magnetic field sensor element functioning at a wide range of temperatures.

Reversible Entropy-Driven Defect Migration and Insulator-Metal Transition Suppression in VO2 Nanostructures for Phase-Change Electronic Switching.

Oxygen defects are among essential issues and required to be manipulated in correlated electronic oxides with insulator-metal transition (IMT). Besides, surface and interface control are necessary but challenging in field-induced electronic switching towards advanced IMT-triggered transistors and optical modulators. Herein, we demonstrated reversible entropy-driven oxygen defect migrations and reversible IMT suppression in vanadium dioxide (VO2 ) phase-change electronic switching. The initial IMT was suppressed with oxygen defects, which is caused by the entropy change during reversed surface oxygen ionosorption on the VO2 nanostructures. This IMT suppression is reversible and reverts when the adsorbed oxygen extracts electrons from the surface and heals defects again. The reversible IMT suppression observed in the VO2 nanobeam with M2 phase is accompanied by large variations in the IMT temperature. We also achieved irreversible and stable IMT by exploiting an Al2 O3 partition layer prepared by atomic layer deposition (ALD) to disrupt the entropy-driven defect migration. We expected that such reversible modulations would be helpful for understanding the origin of surface-driven IMT in correlated vanadium oxides, and constructing functional phase-change electronic and optical devices.