Tunable Resistance Research Articles

Impact of Co doping on the magnetic and transport properties of FeRh

FeRh undergoes a first-order phase transition from the antiferromagnetic (AFM) to ferromagnetic (FM) state at ∼370 K, which is highly sensitive to strain and compositional changes. In this study, we investigate the magnetic and electronic properties of Co-doped FeRh films fabricated using a co-sputtering technique, to address how the magnetic transition behavior is influenced by the doping in FeRh films. By adjusting Co sputtering gun currents (=0, 5, 8, and 10 mA), we achieve Co doping levels from 1 to 2 at. %, where initial Co atoms (for 5 and 8 mA) substitute Rh sites, while doped Co levels (for 10 mA) begin to occupy Fe sites with unchanged Co doping level of 2 at. %. We find that Co substitution significantly lowers the transition temperature, attributed to an enhancement of the FM phase due to the contribution of magnetic Co doping. Furthermore, the Co doping leads to a remarkable increment in the magnetoresistance ratio during the transition, reaching up to 190% for only 2 at. % Co doping, while keeping the magnetization change. The Hall effect measurements indicate a slight reduction in carrier density with Co doping, maintaining changes in carrier type across the phase transition. These results highlight the tunable magnetic phase transition and resistance changes in Co-doped FeRh films. This study provides valuable insights into the complex physics underlying the Co doping in FeRh films, emphasizing their scientific value in understanding the mechanism of the AFM–FM transitions in achieving high magnetoresistance.

Electric-Field Control of the Local Thermal Conductivity in Charge Transfer Oxides.

Phonons, the collective excitations responsible for heat transport in crystalline insulating solids, lack electric charge or magnetic moment, which complicates their active control via external fields. This presents a significant challenge in designing thermal equivalents of basic electronic circuit elements, such as transistors or diodes. Achieving these goals requires precise and reversible modification of thermal conductivity in materials. In this work, the continuous tuning of local thermal conductivity in charge-transfer SrFeO3-x and La0.6Sr0.4CoO3-x oxides using a voltage-biased Atomic Force Microscopy (AFM)tip at room temperature is demonstrated. This method allows the creation of micron-sized domains with well-defined thermal conductivity, achieving reductions of up to 50%, measured by spatially resolved Frequency Domain Thermoreflectance (FDTR). By optimizing the oxide's chemical composition, the thermal states remain stable under normal atmospheric conditions but can be reverted to their original values through thermal annealing in air. A comparison between Mott-Hubbard and charge-transfer oxides reveals the critical role of redox-active lattice oxygen in ensuring full reversibility of the process. This approach marks a significant step toward fabricating oxide-based tunable microthermal resistances and other elements for thermal circuits.

Two-Dimensional Nanoarchitectonics of End-on Oligomers with Versatile Structures and Tunable Negative Differential Resistance.

Maximizing the molecular information density requires simultaneously functionalizing distinct monomers and their coupling connections. However, current synthesis generally focuses on distinct monomers rather than coupling reactions because the multistep reactions significantly escalate the synthetic complexity in an exponential increase. Here, we report the two-dimensional nanoarchitectures (2DNs) of end-on oligomers, with versatile molecular structures and negative differential resistance (NDR), synthesized by programmed and surface-initiated step electrosynthesis based on the simultaneous utilization of six reactions including cross- and homocouplings. The resulting vertically end-on oligomers, with similar values in thickness and molecular length, as crystalline 2DNs, exhibit subnanometer uniformity, ultrahigh compressive modulus of 40 GPa, and low-bias NDR at 0.13 V with an ultralow power consumption of down to 0.05 nW/μm2. This highly controlled electrosynthesis provides a unique dimension to enhance the structural diversity of molecular 2D nanomaterials for high-density and low-power consumption electronics.

Balancing stretchability and conductivity: Carbon nanotube layer-enhanced non-ionic conductive hydrogels with a sandwich structure

In non-ionic conductive hydrogels, conductivity often conflicts with mechanical properties, posing a challenge in creating hydrogels that strike a balance between conductivity and mechanical properties. Here, we addressed this issue by integrating carboxylated carbon nanotube (CNT) layers in a sandwich structure onto the surface of the cationic hydrogel (P(AM-co-DMDAAC), PAD), followed by dehydration for densification. This approach simultaneously improved the conductivity of the hydrogel while maintaining its stretchability. The stability of the composite structure was ensured through electrostatic interactions, hydrogen bonding, and physical entanglement between the PAD hydrogel and the CNT layer. Additionally, adjusting the water–solid ratio of the hydrogel allows fine-tuning of its conductivity and stretchability. Remarkably, at a 500 % water–solid ratio, the hydrogel achieved a conductivity of 2.3 S/m. By further reducing the water content, the conductivity could be increased to 25 S/m, significantly higher than similar hydrogels. The PAD-CNT hydrogel found applications in swelling response, tunable resistance wires, and as strain sensors. Notably, the sandwich structure enables the PAD-CNT hydrogel to output capacitive signals, exhibiting a more sensitive response in monitoring human movement. Overall, this study introduces a novel composite structure for conductive hydrogels, holding tremendous potential in flexible conductivity and smart response fields.

Cellulose nanofiber powered interface engineering strategy to manufacture mechanically stable, moldable, recyclable, and biodegradable cellulose foam

In this work, an ingenious and energy-efficient interface engineering strategy is developed to manufacture pulp cellulose foam via incorporating cellulose nanofiber/alkyl ketene dimer (CNF/AKD) Pickering emulsions. In this strategy, CNF/AKD emulsion can uniformly anchor onto pulp microfiber surface, not only achieving interfacial exoskeleton reinforcement, but also enabling low energy surface, which facilitates the direct and large-scale production of lightweight pulp/CNF/AKD (PCA) foam by oven drying without requiring any harmful crosslinking chemistries. By simply regulating the mass fractions of CNF and AKD in emulsion, the prepared PCA foams can achieve excellent and tunable 3D porous structure, mechanical performance, water resistance and wet stability. The dry compressive modulus and wet compressive modulus underwater of PCA foams with low density of 0.09 g/cm3 can reach 0.67 and 0.57 MPa, respectively. Even 1 week in water, the compressive modulus still retains 76.2 % of initial modulus. Notably, the proposed interface engineering strategy provides remarkable formability, moldability and structural designability, enabling cellulose foam to be customized into complex and delicate 3D macroscopic structures for different scenarios. Furthermore, the PCA foam displays economically feasible closed-loop recyclability by easy hot-water disintegration and natural biodegradability in soil. Therefore, this work proves a cost-effective and viable interface engineering strategy for scalable production of lightweight, mechanically stable, recyclable, biodegradable cellulose foams with high environmental benefits and low petrochemical consumption.

BSA regulated degradation performance and osteogenesis of CaCO3 coating on magnesium alloy for orthopedic applications

Rapid degradation of magnesium (Mg) alloys and slow osteogenesis surrounding implants greatly limit their clinical applications. In present study, influence of BSA on the characteristics of CaCO3 coating on MgZnCa alloy was investigated to adjust the degradation behaviors of Mg alloy and cell response to the coating. BSA can regulate the crystalline and the particle size of CaCO3 in the coating and deteriorate coating stability and thickness, thereby leading to a tunable corrosion resistance for CaCO3@BSA coatings. Moreover, the presence of BSA bestows more suitable composition, roughness and wettability on cells with a better osteogenic performance on the coating.

Natural Organic Honey-CNT Memristor Based Artificial Synaptic Devices for Sustainable Neuromorphic System

Neuromorphic computing is considered to have the potential to overcome the limitations of traditional von Neumann architecture due to its high efficiency, low energy consumption, and fault-tolerance. Hardware components that can emulate the synaptic plasticity of neurons, i.e. artificial synaptic devices, are required by neuromorphic systems. New devices have been examined for such components, such as phase-change artificial synapse, ferroelectric artificial synapse, and memristor synapses. Among them, memristor, a two-terminal metal-insulator-metal structure that are analogous to a biological synapse with presynaptic neuron (top electrode), postsynapticneuron (bottom electrode), and synaptic cleft (memristive film), is a promising device technology because of its tunable resistance, scalability, 3D integration compatibility, low power consumption, and relatively high speed. In contrary to inorganic materials such as metal oxides, natural organic materials have attracted interest to form the memristive layer because they are renewable, biodegradable, sustainable, biocompatible, and environmentally friendly. In this paper, honey solution embedded with carbon nanotubes (CNTs) was processed into the memristive layer by a low cost solution-based process, with synaptic plasticity of the final honey-CNT memristors characterized, including forget and relearn, spike-rate-dependent plasticity, spike-voltage-dependent plasticity, short-term to long-term memory transition, paired pulse facilitation, and spatial supra-linear summation behaviors. The successful emulation of these essential biological synaptic behaviors demonstrates the potential of honey-CNT memristors as a viable hardware component in neuromorphic computing systems.

Vanadium Oxides in Neuromorphic Computing: Unveiling Insights through First Principles Calculations

Neuromorphic computing represents a revolutionary approach to developing analog non-volatile memory (NVM) devices. This technology relies on electron-correlated materials capable of replicating the functioning of synapses and neurons in the brain, making it ideal for energy-efficient parallel computing applications. Among the myriad of innovative materials suitable for neuromorphic computing, such as conductive polymers, organic semiconductors, and nanowires, among others, electron-correlated transition metal oxides stand out for their ability to undergo metal-insulator transitions either by tuning their oxygen content or the stoichiometry of an intercalating metal ion. In this talk we discuss from a first principles perspective the pivotal role that vanadium oxide has the potential to play in the development neuromorphic computing devices. First, we examine the use of vanadium oxide as a Mott insulator capable of undergoing a stable phase separation process that is suitable for non-volatile electrochemically activated random access memory devices (ECRAM). A stable phase coexistence across multiple segregated vanadium oxides states provides the device the ability express non-volatile memory behavior after precise tunning of oxygen vacancies into VO2. The correlation between these oxides’ formation energies and their oxygen content provides clues on the stable formation of coexisting crystalline phases that provide tunable resistivity based on the relative fraction and distribution of each phase with unusual retention, with an estimated 1% loss over 14 years in ambient conditions. Second, we address the structural and electrical properties of β’-CuxV2O5 based on temperature and Cu content. Ab-initio molecular dynamics calculations provide a close-up view of the correlation between Cu content and mobility and its ability to induce structural distortions to the intercalating β’-CuxV2O5 host that spike close to the insulator-metal transition (IMT).

Pencil-drawn tunable electrical resistance and Joule heating demonstration using a smartphone-based thermal imaging camera

This paper delves into the educational potential of pencil circuits for teaching key electrical concepts. Using both practical experiments and theoretical examinations, this study explores the application of pencil circuits in demonstrating concepts related to series and parallel resistance, Ohm’s law, and Joule’s heating law. By leveraging the simplicity and accessibility of pencil-based setups, students can gain a practical understanding of these fundamental principles. Additionally, the integration of thermal imaging technology enhances the visualization of Joule’s heating law, further enriching the learning experience.

An Artificial Universal Tactile Nociceptor Based on 2D Polymer Film Memristor Arrays with Tunable Resistance Switching Behaviors.

Nociceptor is an important receptor in the organism's sensory system; it can perceive harmful stimuli and send signals to the brain in order to protect the body in time. The injury degree of nociceptor can be divided into three stages: self-healing injury, treatable injury, and permanent injury. However, the current studies on nociceptor simulation are limited to the self-healing stage due to the limitation of the untunable resistance switching behavior of memristors. In this study, we constructed Al/2DPTPAK+TAPB/Ag memristor arrays with adjustable memory behaviors to emulate the nociceptor of biological neural network of all three stages. For this purpose, a PDMS/AgNWs/ITO/PET pressure sensor was assembled to mimic the tactile perception of the skin. The memristor arrays can not only simulate all the response of nociceptor, i.e., the threshold, relaxation, no adaptation, and sensitization with the self-healing injury, but can also simulate the treatable injury and the permanent injury. These behaviors are both demonstrated with a single memristor and in the form of pattern mapping of the memristor array.

Adaptive impedance matching in microwave and terahertz metamaterial absorbers using PIN diodes and GaN HEMTs

Metamaterial absorbers allow electromagnetic waves to be converted into heat energy based on impedance matching. However, passive metamaterial absorbers exhibit fixed absorption characteristics, limiting their flexibility. This work demonstrates tunable microwave and terahertz absorbers by integrating adjustable resistors into the metamaterial units. First, a microwave absorber from 1 to 5 GHz was designed by embedding PIN diodes with voltage-controlled resistance. Calculations, simulations, and measurements verified two separate absorption peaks over 90% when optimized to a resistance of 250 Ω. The absorption frequencies shifted based on the resistor tuning. Building on this, a terahertz absorber was modeled by substituting gallium nitride high electron mobility transistors (GaN HEMTs) as the adjustable resistor component. The GaN HEMTs were controlled by an integrated gate electrode to modify the two-dimensional electron gas density, allowing resistance changes without external voltage terminals. Simulations revealed two absorption peaks exceeding 90% absorption at 0.34 THz and 1.06 THz by adjusting the equivalent resistance from 180 Ω to 380 Ω, and the tunable resistance is verified by DC measurement of single GaN HEMT in the unit. This work demonstrates how integrating adjustable resistors enables dynamic control over the absorption frequencies and bandwidths of metamaterial absorbers. The proposed geometries provide blueprints for tunable microwave and terahertz absorbers.

Structure and Properties of Na2S-SiS2-P2S5-NaPO3 Glassy Solid Electrolytes.

In the development of sodium all-solid-state batteries (ASSBs), research efforts have focused on synthesizing highly conducting and electrochemically stable solid-state electrolytes. Glassy solid electrolytes (GSEs) have been considered very promising due to their tunable chemistry and resistance to dendrite growth. For these reasons, we focus here on the atomic-level structures and properties of GSEs in the compositional series (0.6-0.08y)Na2S + (0.4 + 0.08y)[(1 - y)[(1 - x)SiS2 + xPS5/2] + yNaPO3] (NaPSiSO). The mechanical moduli, glass transition temperatures, and temperature-dependent conductivity were determined and related to their short-range order structures that were determined using Raman, Fourier transform infrared, and 31P and 29Si magic angle spinning nuclear magnetic resonance spectroscopies. In addition, the conductivity activation energies were modeled using the Christensen-Martin-Anderson-Stuart model. These GSEs appear to be highly crystallization-resistant in the supercooled liquid region where no measurable crystallization below 450 °C could be observed in differential scanning calorimetry studies. Additionally, these GSEs were found to be highly conducting, with conductivities on the order of 10-5 (Ω cm)-1 at room temperature, and processable in the supercooled state without crystallization. For all these reasons, these NaPSiSO GSEs are considered to be highly competitive and easily processable candidate GSEs for enabling sodium ASSBs.

Resistive Switching Mechanism in Ferroelectric Memristors with Thin Polycrystalline Barium Titanate Film

In a ferroelectric memristor, the manifestation of resistive effects is most often associated with the influence of polarization and dynamics of the domain structure on the charge transport. The role of point defects is either not taken into account or is reduced to the modulation of potential barriers at the interfaces with electrodes. However, the similarity of charge transport mechanisms in memristors based on thin ferroelectric and metal-oxide films suggests that the contribution of point defects in the anionic sublattice, namely, oxygen vacancies, to the resistive switching in ferroelectric memristors may be dominant. In order to identify the key factors responsible for resistive tuning in a ferroelectric memristor, a combined experimental and theoretical study of resistive switching effects in 10-nm polycrystalline barium titanate films was performed. X-ray photoelectron spectroscopy, piezoresponse force microscopy and scanning tunneling microscopy were employed to investigate the local resistive and ferroelectric properties and to estimate the stoichiometry of the films. A drift-diffusion numerical model of non-stationary processes in thin ferroelectric films was developed, including the Poisson equation and the continuity equation for each component of mobile charge carriers. Using the developed model as well as experimental I—V characteristics taken with scanning tunneling spectroscopy, the contribution of various mechanisms to the resistive switching in ferroelectric memristors with polycrystalline barium titanate has been studied. Non-stationary processes involving oxygen vacancies were found to govern the resistive switching.

The fluidic memristor as a collective phenomenon in elastohydrodynamic networks

Fluid flow networks are ubiquitous and can be found in a broad range of contexts, from human-made systems such as water supply networks to living systems like animal and plant vasculature. In many cases, the elements forming these networks exhibit a highly non-linear pressure-flow relationship. Although we understand how these elements work individually, their collective behavior remains poorly understood. In this work, we combine experiments, theory, and numerical simulations to understand the main mechanisms underlying the collective behavior of soft flow networks with elements that exhibit negative differential resistance. Strikingly, our theoretical analysis and experiments reveal that a minimal network of nonlinear resistors, which we have termed a ‘fluidic memristor’, displays history-dependent resistance. This new class of element can be understood as a collection of hysteresis loops that allows this fluidic system to store information, and it can be directly used as a tunable resistor in fluidic setups. Our results provide insights that can inform other applications of fluid flow networks in soft materials science, biomedical settings, and soft robotics, and may also motivate new understanding of the flow networks involved in animal and plant physiology.

Tunable Oxidized-Chitin Hydrogels with Customizable Mechanical Properties by Metal or Hydrogen Ion Exposure.

This study focuses on the optimization of chitin oxidation in C6 to carboxylic acid and its use to obtain a hydrogel with tunable resistance. After the optimization, water-soluble crystalline β-chitin fibrils (β-chitOx) with a degree of functionalization of 10% were obtained. Diverse reaction conditions were also tested for α-chitin, which showed a lower reactivity and a slower reaction kinetic. After that, a set of hydrogels was synthesized from β-chitOx 1 wt.% at pH 9, inducing the gelation by sonication. These hydrogels were exposed to different environments, such as different amounts of Ca2+, Na+ or Mg2+ solutions, buffered environments such as pH 9, PBS, pH 5, and pH 1, and pure water. These hydrogels were characterized using rheology, XRPD, SEM, and FT-IR. The notable feature of these hydrogels is their ability to be strengthened through cation chelation, being metal cations or hydrogen ions, with a five- to tenfold increase in their storage modulus (G'). The ions were theorized to alter the hydrogen-bonding network of the polymer and intercalate in chitin's crystal structure along the a-axis. On the other hand, the hydrogel dissolved at pH 9 and pure water. These bio-based tunable hydrogels represent an intriguing material suitable for biomedical applications.

Conversion of Layered WS2 Crystals into Mixed-Domain Electrochemical Catalysts by Plasma-Assisted Surface Reconstruction.

Electrocatalytic water splitting is crucial to generate clean hydrogen fuel, but implementation at an industrial scale remains limited due to dependence on expensive platinum (Pt)-based electrocatalysts. Here, an all-dry process to transform electrochemically inert bulk WS2 into a multidomain electrochemical catalyst that enables scalable and cost-effective implementation of the hydrogen evolution reaction (HER) in water electrolysis is reported. Direct dry transfer of WS2 flakes to a gold thin film deposited on a silicon substrate provides a general platform to produce the working electrodes for HER with tunable charge transfer resistance. By treating the mechanically exfoliated WS2 with sequential Ar-O2 plasma, mixed domains of WS2, WO3, and tungsten oxysulfide form on the surfaces of the flakes, which gives rise to a superior HER with much greater long-term stability and steady-state activity compared to Pt. Using density functional theory, ultraefficient atomic sites formed on the constituent nanodomains are identified, and the quantification of atomic-scale reactivities and resulting HER activities fully support the experimental observations.

Electrical Detection and Nucleation of a Magnetic Skyrmion in a Magnetic Tunnel Junction Observed via Operando Magnetic Microscopy.

Magnetic skyrmions are topological spin textures which are envisioned as nanometer scale information carriers in magnetic memory and logic devices. The recent demonstrations of room temperature skyrmions and their current induced manipulation in ultrathin films were first steps toward the realization of such devices. However, important challenges remain regarding the electrical detection and the low-power nucleation of skyrmions, which are required for the read and write operations. Here, we demonstrate, using operando magnetic microscopy experiments, the electrical detection of a single magnetic skyrmion in a magnetic tunnel junction (MTJ) and its nucleation and annihilation by gate voltage via voltage control of magnetic anisotropy. The nucleated skyrmion can be manipulated by both gate voltages and external magnetic fields, leading to tunable intermediate resistance states. Our results unambiguously demonstrate the readout and voltage controlled write operations in a single MTJ device, which is a major milestone for low power skyrmion based technologies.

Tunable anomalous resistance and large magnetoresistance in HfTe5 by atom doping

The Dirac layered material HfTe5 renews significant interest due to its exotic band structure, leading to abundant transport properties, such as the anomaly resistance peak and its large magnetoresistance. Here, we prepared single crystals HfTe5 and Cr-doped CrxHf1−xTe5 and carried out their electrical transport measurements to explore the underlying physical origin of the anomaly resistance behavior and the large magnetoresistance. An anomalous resistance peak was observed in both intrinsic HfTe5 and the Cr-doped ones. Specifically, the peak temperature in the doped ones experiences an obvious shift from 52 to 34 K as the doping concentration x increases from 0 to 0.15, as well as the magnitude of the peak resistance is significantly enhanced. Furthermore, the magnetoresistance of CrxHf1−xTe5 is reduced by more than one order of magnitude compared with the intrinsic one. The significant reduction in magnetoresistance after Cr doping is attributed to the breaking of the balance between electron and hole carriers, which is confirmed by Kohler's plots. Meanwhile, in the sample where the magnetoresistance was minimized, we observed Shubnikov–de Haas oscillations. These observations illustrate that the large magnetoresistance is primarily contributed by the compensation of electrons and holes rather than the high mobility. Our findings provide valuable insight into how to engineer HfTe5 to achieve large magnetoresistance and its further applications in magnetic sensors and spintronics.

Preparation and mechanism of black LiTaO3 wafer with controlled transmission efficiency of light and resistivity by reduction method

Tantalum lithium (LiTaO3) is known as a “universal” material in the field of functional materials. However, due to its high pyroelectric coefficient and transmission efficiency, practical applications are limited. In this study, black LiTaO3 wafer was prepared by a simple reduction process by using a new reductant composed of FeSiAl flaks, C nanoparticles, and Fe-C core–shell nanoparticles. It was found that the as-prepared wafers have tunable light transmission efficiency and resistance. Moreover, it was also found that the annealing temperature of 800 °C is the threshold temperature of the wafer in the reduction process. Furthermore, it was revealed that the underlying blacking mechanism could be attributed to the wafers' generation of TaC and O vacancies during the reduction process. This work presents a simple method to blackening, uncovers a novel mechanism, and provides tunable light and resistivity transmission efficiency of LiTaO3, which may serve as benchmarks for related studies and promote LiTaO3′s practical applicability.

An improved CMOS second‐generation current controlled conveyor with enhanced tunable range of RX and its applications

SummaryThis paper presents a new CMOS‐based second‐generation current controlled conveyor (CCCII), based on a mixed translinear cell composed of CMOS‐based translinear elements, which offers a wide tunable range of four decades for the X‐terminal input resistance RX. The current‐controlled grounded and floating tunable resistors and multifunction filter with tunable cut‐off frequency have been realized to show the practical utility of the proposed CCCII architecture. Various performance evaluations have been carried out through simulations conducted on Cadence Virtuoso software using 0.18 μm gpdk CMOS technology at a supply voltage of ±1.5 V. Simulation results show the effectiveness of the proposed CCCII in attaining a wide tunable Rx ranging from 130 MΩ to 13 kΩ, offering good linearity, nearly unity voltage and current transfer gains, high input impedance at port Y varying from 3.7 GΩ to 1.18 MΩ, high output impedance at port Z varying from 67.63 GΩ to 13.19 MΩ, with the power consumption in the range of 3.68 μW to 18.78 μW for variation in bias current (Io) from 100 pA to 1 μA.