Ivan K. Schuller

While the connection between colossal magnetoresistance and phase separation in praseodymium-doped rare-earth manganites has been well established, the underlying mechanisms enabling this connection still need to be fully understood. This paper thoroughly examines the phase separation in L a 5/8− x P r x C a 3/8 Mn O 3 (x= 0.40) using an integrated approach including magnetotransport, electron spin resonance, and magnetic susceptibility. We observed that the initially fluidlike magnetic phase separation at intermediate temperatures transforms into the solidlike phase separation via a glass transition at low temperatures. Magnetization dynamics measurements revealed that this transition is concurrent with the reentrance of the paramagnetic phase. Our findings offer a perspective on the freezing of phase separation fluidlike behavior resulting in the advent of the low-temperature solidlike state …

Y20. 00008: Full-field Structural Microscopy of the Voltage-Induced Transition in VO 2 Neuromorphic Devices*

Various materials exhibit resistive switching (RS), a useful feature which lends well to the development of novel bioinspired electronic devices, notably artificial neurons and synapses for neuromorphic computing. This effect often manifests itself through the percolation of conducting filaments or the formation of transverse barriers. The location and switching parameters of RS are often impacted by inherent material defects which pose a challenge for scalability. By selectively engineering defects in VO x and LSMO using a focused ion beam, we report a novel method for locally tuning the electronic properties (ie conductivity and metal-insulator transition temperature) of a material and by extension, controlling the location and geometry of RS. In addition to confining the conducting filament to the irradiated region, we observe a greater than 3 orders of magnitude reduction in RS power. Our work demonstrates that local …

Sounds of Science is the first movement of a symphony for many (scientific) instruments and voices, united in celebration of the frontiers of science and intended for a general audience. John Goodenough, the maestro who transformed energy usage and technology through the invention of the lithium-ion battery, opens the programme, reflecting on the ultimate limits of battery technology. This applied theme continues through the subsequent pieces on energy-related topics—the sodium-ion battery and artificial fuels, by Martin Månsson—and the ultimate challenge for 3D printing, the eventual production of life, by Anthony Atala. A passage by Gerianne Alexander follows, contemplating a related issue: How might an artificially produced human being behave? Next comes a consideration of consciousness and free will by Roland Allen and Suzy Lidström. Further voices and new instruments enter as Warwick Bowen …

Our research is dedicated to a novel type of thermal neuristors utilizing the quantum material VO 2. We demonstrated a wide array of reconfigurable electrical behaviors that mirror those of biological neurons, such as the all-or-nothing law, type-II neuronal rate coding law, spike-in and DC out effect, spike-in and spike-out effect, and stochastic leaky integrate-and-fire law. Remarkably, these behaviors are achieved solely through thermal interaction. Moreover, we achieved both inhibitory and excitatory functionalities within a single oxide device, enhancing its versatility. Information flow between consecutive neural layers is accomplished through thermal interactions, eliminating the necessity for traditional CMOS circuits. This groundbreaking method simplifies the incorporation of reconfigurable cascading neural layers with potential for scalable energy-efficient thermal neural networks for computing.* EQ and IKS were …

S62. 00003: Low temperature spin Seebeck effect in non-magnetic VO 2 thin film*

Neuromorphic functionalities in memristive devices are commonly associated with the ability to electrically create local conductive pathways by resistive switching. The archetypal correlated material, VO2, has been intensively studied for its complex electronic and structural phase transition as well as its filament formation under applied voltages. Local structural studies of the filament behavior are often limited due to time-consuming rastering which makes impractical many experiments aimed at investigating large spatial areas or temporal dynamics associated with the electrical triggering of the phase transition. Utilizing Dark Field X-ray Microscopy (DFXM), a novel full-field x-ray imaging technique, we study this complex filament formation process in-operando in VO2 devices from a structural perspective. We show that prior to filament formation, there is a significant gain of the metallic rutile phase beneath the metal electrodes that define the device. We observed that the filament formation follows a preferential path determined by the nucleation sites within the device. These nucleation sites are predisposed to the phase transition and can persistently maintain the high-temperature rutile phase even after returning to room temperature, which can enable a novel training/learning mechanism. Filament formation also appears to follow a preferential path determined by a nucleation site within the device which is predisposed to the rutile transition even after returning to room temperature. Finally, we found that small isolated low-temperature phase clusters can be present inside the high-temperature filaments indicating that the filament structure is not …

B21. 00008: Spin-Torque Ferromagnetic Resonance Investigation of a single La 0.7 Sr 0.3 MnO 3 layer*

MM03. 00004: Interfacial charge transfer mediated tunable electronic phase transition at complex oxide heterostructures*

Q42. 00004: First-principles calculations of the structural and electronic properties of cobaltites for neuromorphic applications*

In article number 2306818, Erbin Qiu and co-workers demonstrate the first instance of direct information flow and excitatory and inhibitory interactions between cascaded neural layers in hardware, exclusively implemented with thermally coupled neuristors, thus eliminating the need for complicated CMOS circuits. The emergent functionalities closely resemble biological neurons, fostering progress in brain-inspired computing.

Interfacial effects between antiferromagnetic (AFM) and ferromagnetic (FM) materials have long been a center of magnetism studies. Aside from the exchange bias occurring at the AFM/FM interface, controlling the coercivity is another significant topic in magnetic recordings. The coercivity of FM materials is often determined through varying grain size, alloy composition, density of defects, etc., which is set during material growth and offers limited room for modification after growth. Hematite (α-F e 2 O 3) is an AFM material that undergoes a temperature-controlled spin-flip transition, the so-called Morin transition. This transition gives an extra degree of freedom making hematite an intriguing component to study the exchange coupling when interfaced with an FM material. In this work, changes in the magnetic properties of soft magnetic permalloy (N i 81 F e 19, or Py) thin films grown on hematite were studied across the …

There is growing interest in strongly correlated insulator thin films because the intricate interplay of their intrinsic and extrinsic state variables causes memristive behavior that might be used for bio-mimetic devices in the emerging field of neuromorphic computing. In this study we find that laser irradiation tends to drive VO from supercooled/superheated metastable states towards thermodynamic equilibrium, most likely in a non-thermal way. We study thin films of the prototypical Mott-insulator VO, which show spontaneous phase separation into metal-insulator herringbone domains during the Mott transition. Here, we use low-temperature microscopy to investigate how these metal-insulator domains can be modified by scanning a focused laser beam across the thin film surface. We find that the response depends on the thermal history: When the thin film is heated from below the Mott transition temperature, the laser beam predominantly induces metallic domains. On the contrary, when the thin film is cooled from a temperature above the transition, the laser beam predominantly induces insulating domains. Very likely, the VO thin film is in a superheated or supercooled state, respectively, during the first-order phase transition, and the perturbation by a laser beam drives these metastable states into stable ones. This way, the thermal history is locally erased. Our findings are supported by a phenomenological model with a laser-induced lowering of the energy barrier between the metastable and equilibrium states.

Bulk V 2 O 3 features concomitant metal-insulator (MIT) and structural (SPT) phase transitions at T C∼ 160 K. In thin films, where the substrate clamping can impose geometrical restrictions on the SPT, the epitaxial relation between the V 2 O 3 film and substrate can have a profound effect on the MIT. Here, we present a detailed characterization of domain nucleation and growth across the MIT in (001)-oriented V 2 O 3 films grown on sapphire. By combining scanning electron transmission microscopy and photoelectron emission microscopy (PEEM), we imaged the MIT with planar and vertical resolution. We observed that upon cooling, insulating domains nucleate at the top of the film, where strain is lowest, and expand downwards and laterally. This growth is arrested at a critical thickness of 50 nm from the substrate interface, leaving a persistent bottom metallic layer. As a result, the MIT cannot take place in the …

Q04. 00011: Selective local modification of electronic transitions in Mott materials by Laser-induced oxygen doping*

Heat dissipation is a universal behavior commonly existing in our world, particularly within semiconductor chips. However, heat dissipation imposes significant limitations on the downscaling size of the semiconductor chips. In our research, we leverage the concept of heat dissipation between spiking oscillators for computational purposes. Instead of using electricity, information is transmitted through the medium of heat. Our investigation delves into the evolving synchronization patterns of spiking nano-oscillators arising from thermal interactions. Additionally, we showcase a wide range of reconfigurable electrical dynamics that mimic those of biological neurons, all achieved through heat dissipation. Notably, we achieve inhibitory functionality using a single oxide device, and the flow of information is exclusively driven by thermal interactions in a cascading manner. This study lays the groundwork for the development …

In the pursuit of scalable and energy-efficient neuromorphic devices, recent research has unveiled a novel category of spiking oscillators, termed ``thermal neuristors." These devices function via thermal interactions among neighboring vanadium dioxide resistive memories, closely mimicking the behavior of biological neurons. Here, we show that the collective dynamical behavior of networks of these neurons showcases a rich phase structure, tunable by adjusting the thermal coupling and input voltage. Notably, we have identified phases exhibiting long-range order that, however, does not arise from criticality, but rather from the time non-local response of the system. In addition, we show that these thermal neuristor arrays achieve high accuracy in image recognition tasks through reservoir computing, without taking advantage of this long-range order. Our findings highlight a crucial aspect of neuromorphic computing with possible implications on the functioning of the brain: criticality may not be necessary for the efficient performance of neuromorphic systems in certain computational tasks.

Materials displaying resistive switching have emerged as promising candidates for implementation as components for neuromorphic computing. Under an applied electric field, certain resistive switching materials undergo an insulator-to-metal transition through the formation of a percolating filament, resulting in large resistance changes. The location and shape of these filaments are strongly influenced by hard-to-control parameters, such as grain boundaries or intrinsic defects, making the switching process susceptible to cycle-to-cycle and device-to-device variation. Using focused Gaþ ion beam irradiation, we selectively engineer defects in VO2 and V2O3 thin films as a case study to control filament formation. Using defect pre-patterning, we can control the position and shape of metallic filaments and reduce the switching power significantly. A greater than three orders of magnitude reduction of switching power …

Mixed valence manganites often have complex phase diagrams where different phases can coexist. Proper stoichiometry allows the coexistence of ferromagnetic metal and antiferromagnetic charge ordered insulator phases in (La, Pr, Ca) MnO 3 (LPCMO). The phase separation in LPCMO has two behaviors: a fluid-like phase separation (FPS)'at intermediate temperatures' where the phase morphology evolution depends strongly on external stimuli, and a static phase separation (SPS)'at lower temperatures' that has a glass-like behavior. By performing ferromagnetic resonance and AC susceptibility measurements, we demonstrate that the magnetism dynamics of polycrystalline manganite La 5/8-x Pr x Ca 3/8 MnO 3 pellets are driven by the phase separation. Our results provide an insight into the mechanism behind the FPS-to-SPS transition in the LPCMO system.

We studied competing magnetic and superconducting interactions in Pr 0.5 La 0.2 Ca 0.3 MnO 3/YBa 2 Cu 3 O 7-δ/Pr 0.5 La 0.2 Ca 0.3 MnO 3 (PLCMO/YBCO/PLCMO) heterostructures. A highly sensitive and selective magnetic field-modulated microwave absorption spectroscopy (MFMMS) technique was used to compare the superconducting properties of single YBCO thin films to PLCMO/YBCO/PLCMO heterostructures. The MFMMS showed clear influence of the PLCMO magnetic state on the superconducting transition and vortex dynamics of the YBCO film. Surprisingly, magnetization measurements revealed that the magnetic anisotropy of PLCMO is strongly modified by the superconducting state of the YBCO layer. Our results, in combination with micromagnetic simulations, suggest that the superconductivity changes the temperature-dependent ratio of the ferromagnetic-metallic and antiferromagnetic …

We have developed a new optical microscopy technique able to track micron-sized surface clusters as temperature is varied. Potential candidates for study include phase separated metal-insulator materials, ferroelectrics, and porous structures. Several key techniques (including autofocus, step motor/cross correlation alignments, single-pixel thresholding, pair connectivity correlation length and image convolution) were implemented in order to obtain a time series of thresholded images. Here, we apply this new method to probe the archetypal phase separated insulator-metal transition in VO. A precise time and temperature series of the insulator-metal transition was achieved, allowing us to construct for the first time in this material spatial maps of the transition temperature T. These maps reveal multiple interesting features such as fractal electronic patterns on micron scales, regions of the sample with an extremely large or nearly absent local hysteresis, a positive correlation between the T value and the hysteresis width T, and high cycle-to-cycle reproducibility of the transition. These maps also allow for the identification of individual pixels with unique transition characteristics. This unprecedented knowledge of the local properties of each spot along with the behavior of the entire network paves the way to novel electronics applications enabled by, {\em e.g.}, addressing specific regions with desired memory and/or switching characteristics, as well as detailed explorations of open questions in the theory of hysteresis.

The capabilities of image probe experiments are rapidly expanding, providing new information about quantum materials on unprecedented length-and timescales. Many such materials feature inhomogeneous electronic properties with intricate pattern formation on the observable surface. This rich spatial structure contains information about interactions, dimensionality, and disorder—a spatial encoding of the Hamiltonian driving the pattern formation. Image recognition techniques from machine learning are an excellent tool for interpreting information encoded in the spatial relationships in such images. Here, we develop a deep learning framework for using the rich information available in these spatial correlations in order to discover the underlying Hamiltonian driving the patterns. We first vet the method on a known case, scanning near-field optical microscopy on a thin film of V O 2. We then apply our trained …

H04. 00003: Memory retention in substrate environment and its role in feedback mechanism in neuromorphic devices*

A highly anisotropic superconducting gap is found in single crystals of FeSe by studying the London penetration depth Δ λ measured down to 50 mK in samples before and after 2.5 MeV electron irradiation. The gap minimum increases with introduced pointlike disorder, indicating the absence of symmetry-imposed nodes. Surprisingly, the superconducting transition temperature T c increases by 0.4 K from T c 0≈ 8.8 K while the structural transition temperature T s decreases by 0.9 K from T s 0≈ 91.2 K after electron irradiation. We discuss several explanations for the T c enhancement and propose that local strengthening of the pair interaction by irradiation-induced Frenkel defects most likely explains the phenomenon.

We present a novel heterostructured approach to disentangle the mechanism of electrical transport of the strongly correlated PrNi O 3, by placing the nickelate under the photoconductor CdS. This enables the injection of carriers into PrNi O 3 in a controlled way, which can be used to interrogate its intrinsic transport mechanism. We find a nonvolatile resistance decrease when illuminating the system at temperatures below the PrNi O 3 metal-insulator transition. The photoinduced change becomes more volatile as the temperature increases. These data help understand the intrinsic transport properties of the nickelate-CdS bilayer. Together with data from a bare PrNi O 3 film, we find that the transport mechanism includes a combination of mechanisms, including both thermal activation and variable range hopping. At low temperatures without photoinduced carriers, the transport is governed by hopping, while at higher …

Numerous materials exhibit resistive switching, a useful property which lends well to implementation of bioinspired electronic devices, notably artificial neurons and synapses for neuromorphic computing. In many systems, this effect occurs through the percolation of conducting filaments across an insulating matrix. Often, the location and switching parameters are impacted by inherent material defects, which poses a serious challenge for scalability of neuromorphic circuits. By selectively engineering defects using a focused ion beam, we report a novel method of locally tuning a material's electronic properties (ie conductivity and metal-insulator transition temperature) and by extension, controlling the location and shape of the conducting filament. In addition to confining the filament to the irradiated region, we observe a greater than 3 orders of magnitude reduction in switching power. Our work demonstrates that local …

The complex physics of transition metal oxides (TMOs)-based heterostructures with atomically sharp interfaces leads the way to unveil several novel functionalities which cannot be realized in the constituent bulk counterparts. The bi-layer thin films made of strongly correlated 3d PrNiO 3 (PNO) and 5d SrIrO 3 (SIO) with strong spin-orbit coupling (SOC), are indeed exceptionally fascinating in which the tuning of interface properties as a function of correlations/SOC strength of the constituents can be achieved. PNO exhibits a temperature-dependent phase transition from Antiferromagnetic (AFM) Insulator to Paramagnetic Semimetal and SIO possess a Paramagnetic Semimetal ground state. PNO is well known to exhibit physical properties depending on the oxygen stoichiometry. On the other hand, SIO has shown nontrivial quantum states through the Dzyaloshinskii-Moriya interaction associated with the strong SOC …

The strongly correlated material La 0.7 Sr 0.3 Mn O 3 (LSMO) exhibits metal-to-insulator and magnetic transition near room temperature. Although the physical properties of LSMO can be manipulated by strain, chemical doping, temperature, or magnetic field, they often require large external stimuli. To include additional flexibility and tunability, we developed a hybrid optoelectronic heterostructure that uses photocarrier injection from cadmium sulfide (Cd S) to an LSMO layer to change its electrical conductivity. LSMO exhibits no significant optical response; however, the Cd S/LSMO heterostructures show an enhanced conductivity, with a resistance drop of about 37%, at the transition temperature under light stimuli. This enhanced conductivity in response to light is comparable to the effect of a 9 T magnetic field in pure LSMO. Surprisingly, the optical and magnetic responses of Cd S/LSMO heterostructures are …

H04. 00001: Filament localization and tuning of resistive switching characteristics in VO x via selective ion irradiation*

Machine learning has experienced unprecedented growth in recent years, often referred to as an “artificial intelligence revolution.” Biological systems inspire the fundamental approach for this new computing paradigm: using neural networks to classify large amounts of data into sorting categories. Current machine‐learning schemes implement simulated neurons and synapses on standard computers based on a von Neumann architecture. This approach is inefficient in energy consumption, and thermal management, motivating the search for hardware‐based systems that imitate the brain. Here, the present state of thermal management of neuromorphic computing technology and the challenges and opportunities of the energy‐efficient implementation of neuromorphic devices are considered. The main features of brain‐inspired computing and quantum materials for implementing neuromorphic devices are briefly …

Phase separation naturally occurs in a variety of magnetic materials and it often has a major impact on both electric and magnetotransport properties. In resistive switching systems, phase separation can be created on demand by inducing local switching, which provides an opportunity to tune the electronic and magnetic state of the device by applying voltage. Here we explore the magnetotransport properties in the ferromagnetic oxide La 0.7 Sr 0.3 MnO 3 (LSMO) during the electrical triggering of an intrinsic metal-insulator transition (MIT), which produces volatile resistive switching. This switching occurs in a characteristic spatial pattern, ie, the formation of a high-resistance barrier perpendicular to the current flow, enabling an electrically actuated ferromagnetic-paramagnetic-ferromagnetic phase separation. At the threshold voltage of the MIT triggering, both anisotropic and colossal magnetoresistances exhibit …

Recently a new ramp reversal memory effect was observed in VO 2, in which the resistivity was observed to increase by 20% upon applying a particular temperature sequence. One possible microscopic mechanism for the phenomenon is that''scars''accumulate wherever the metal-insulator domain walls reverse direction.[Vardi et al., Adv. Mater., 2017] By using optical microscopy to track the evolution of metal and insulator domains in VO 2 during the ramp reversal temperature protocol, we find that memory also accumulates deep inside the metal/insulator patches rather than only at phase boundaries, which was not foreseen by the''scar''model. We develop a new model based on defect motion during the ramp reversal protocol, using the diffusion-segregation equation. Our calculations show that memory can happen deep inside the metallic and insulating regions and that certain regions in the sample can have a …

Scalable neuromorphic architectures will require a single material as artificial neuron and synapse. The transition metal oxide VO 2 is known to spike like a neuron, and has recently shown synapse-like memory upon temperature cycling. In article number 2300085 Alexandre Zimmers and co-workers use new spatially resolved optical contrast measurements to show that surprisingly, synapse-like memory is encoded throughout the entire material.

The strongly correlated material, La 0.7 Sr 0.3 MnO 3 (LSMO) exhibits a metal-to-insulator (MIT) coincidental with a magnetic transition near room temperature. Although the physical properties of LSMO can be manipulated by strain, chemical doping, temperature, or magnetic field, they often require large external stimuli. In order to add additional flexibility and tunability we developed a hybrid optoelectronic heterostructure, that uses photocarrier injection from cadmium sulfide (CdS) to an LSMO layer to change its electrical conductivity. Although LSMO exhibits no significant optical response, our ultra-thin CdS/LSMO heterostructures show an enhanced conductivity, with~ 37% resistance drop, at the transition temperature under light stimuli. This enhanced conductivity in response to light is comparable to the effect a 9 T magnetic field in pure LSMO. Surprisingly, the optical and magnetic responses of our …

Synchronization of electrical oscillators is a crucial step toward practical implementation of oscillator-based and bio-inspired computing. Here, we report the emergence of an unusual stochastic pattern in coupled spiking Mott nanodevices. Although a moderate capacitive coupling results in a deterministic alternating spiking, increasing the coupling strength leads counterintuitively to stochastic disruptions of the alternating spiking sequence. The disruptions of the deterministic spiking sequence are a direct consequence of the small intrinsic stochasticity in electrical triggering of the insulator–metal transition. Although the stochasticity is subtle in individual nanodevices, it becomes dramatically enhanced just in a single pair of coupled oscillators and, thus, dominates the synchronization. This is different from the stochasticity and multimodal coupling, appearing due to collective effects in large oscillator networks. The …

In-memory computing with emerging non-volatile memory devices (eNVMs) has shown promising results in accelerating matrix-vector multiplications. However, activation function calculations are still being implemented with general processors or large and complex neuron peripheral circuits. Here, we present the integration of Ag-based conductive bridge random access memory (Ag-CBRAM) crossbar arrays with Mott rectified linear unit (ReLU) activation neurons for scalable, energy and area-efficient hardware (HW) implementation of deep neural networks. We develop Ag-CBRAM devices that can achieve a high ON/OFF ratio and multi-level programmability. Compact and energy-efficient Mott ReLU neuron devices implementing ReLU activation function are directly connected to the columns of Ag-CBRAM crossbars to compute the output from the weighted sum current. We implement convolution filters and …

Manipulation of antiferromagnetic (AFM) materials as active elements provides a crucial combination of electrical, thermal, and magnetic properties for spintronics. This study shows how the spin current generated in heavy metal is induced by spin‐orbit torque into an adjacent AFM insulator. The bulk unpinned spins of the AFM layer drive a spin current that is transmitted to the top ferromagnet. This mechanism allows the electrical control of the exchange bias, coercive field, and blocking temperature of the system. Further support is provided by a model calculation that quantitatively describes the effect of the spin current injection into the AFM.

Voltage induced phase transitions in VO2 and LSMO neuromorphic devices are one pathway to next generation computing. Barrier formation in LMSO has been limited to MOKE measurements, while filament formation in VO2 devices have been observed using optical reflectivity measurements. We present a pathway to observing the structural effects of a voltage induced phase transition in these devices using dark field x-ray microscopy. In addition, preliminary observations of the barrier formation in LSMO are presented and the difficulties with performing such measurements are discussed.

Ramp‐reversal memory has recently been discovered in several insulator‐to‐metal transition materials where a non‐volatile resistance change can be set by repeatedly driving the material partway through the transition. This study uses optical microscopy to track the location and internal structure of accumulated memory as a thin film of VO2 is temperature cycled through multiple training subloops. These measurements reveal that the gain of insulator phase fraction between consecutive subloops occurs primarily through front propagation at the insulator‐metal boundaries. By analyzing transition temperature maps, it is found, surprisingly, that the memory is also stored deep inside both insulating and metallic clusters throughout the entire sample, making the metal‐insulator coexistence landscape more rugged. This non‐volatile memory is reset after heating the sample to higher temperatures, as expected …

Enabling electrical control of antiferromagnetic (AFM) properties is one of the key requirements for the development of novel spintronic technologies. We present a device concept in which the spin configuration of an AFM insulator (FeF 2) can be modified taking advantage of the spin-orbit-coupling existing in heavy metals (HM), such as W or Pt. We consider a trilayer, HM| AFM| FM, where the top ferromagnetic (FM) layer is used to monitor the changes in the AFM spin configuration. We performed magneto-optical Kerr effect measurements to probe the FM hysteresis loops as a function of temperature (T) and applied current (I). We found that the exchange bias (EB) and coercivity (H c) produced at the top AFM| FM interface can be strongly modified by the I passing at bottom HM layer. We attribute this effect to an active spin-orbit torque generated at the HM| AFM interface. that reaches the AFM| FM top interface …

Recently, evidence for a conducting surface state (CSS) below 19 K was reported for the correlated d-electron small gap semiconductor FeSi. In the work reported herein, the CSS and the bulk phase of FeSi were probed via electrical resistivity ρ measurements as a function of temperature T, magnetic field B to 60 T, and pressure P to 7.6 GPa, and by means of a magnetic field-modulated microwave spectroscopy (MFMMS) technique. The properties of FeSi were also compared with those of the Kondo insulator SmB6 to address the question of whether FeSi is a d-electron analogue of an f-electron Kondo insulator and, in addition, a “topological Kondo insulator” (TKI). The overall behavior of the magnetoresistance of FeSi at temperatures above and below the onset temperature TS = 19 K of the CSS is similar to that of SmB6. The two energy gaps, inferred from the ρ(T) data in the semiconducting regime, increase with …

The metal‐insulator phase transitions in V2O3 are considered archetypal manifestations of Mott physics. Despite decades of research, the effects of doping, pressure, and anisotropic strains on the transitions are still debated. To understand how these parameters control the transitions, anisotropically strained pure V2O3 films are explored with nearly the same contraction along the c‐axis, but different degrees of ab‐plane expansion. With small ab‐plane expansion, the films behave similar to bulk V2O3 under hydrostatic pressure. However, with large ab‐plane expansion, the films are driven into the “negative pressure” regime, similar to that of Cr‐doped V2O3, exhibiting clear coexistence of paramagnetic insulator and paramagnetic metal phases between 180–500 K. This shows that c‐axis contraction alone, or an increase in c/a ratio is insufficient for inducing “negative pressure” effects. Actually, c‐axis contraction …

Electrical triggering of a metal-insulator transition (MIT) often results in the formation of characteristic spatial patterns such as a metallic filament percolating through an insulating matrix or an insulating barrier splitting a conducting matrix. When the MIT triggering is driven by electrothermal effects, the temperature of the filament or barrier can be substantially higher than the rest of material. Using x-ray microdiffraction and dark-field x-ray microscopy, we show that electrothermal MIT triggering leads to the development of an inhomogeneous strain profile across the switching device, even when the material does not undergo a 1st order structural phase transition coinciding with the MIT. Diffraction measurements further reveal evidence of lattice distortions and twinning occurring within the MIT switching device, highlighting a qualitative distinction between the electrothermal process and equilibrium thermal lattice expansion in nonlinear electrical systems. Electrically induced strain development, lattice distortions, and twinning could have important contributions in the MIT triggering process and could drive the material into non-equilibrium states, providing an unconventional pathway to explore the phase space of strongly correlated electronic systems.

This work reports that synchronization of Mott material-based nanoscale coupled spiking oscillators can be drastically different from that in conventional harmonic oscillators. We investigated the synchronization of spiking nanooscillators mediated by thermal interactions due to the close physical proximity of the devices. Controlling the driving voltage enables in-phase 1:1 and 2:1 integer synchronization modes between neighboring oscillators. Transition between these two integer modes occurs through an unusual stochastic synchronization regime instead of the loss of spiking coherence. In the stochastic synchronization regime, random length spiking sequences belonging to the 1:1 and 2:1 integer modes are intermixed. The occurrence of this stochasticity is an important factor that must be taken into account in the design of large-scale spiking networks for hardware-level implementation of novel computational …

Vanadium Dioxide (VO2) exhibits unique pattern formation while it undergoes temperature-driven Metal-Insulator (MI) phase transition. We use optical microscopy techniques to take videos of the surface of thin film VO2, simultaneously measuring resistance. We observed patches of metal and insulator form while undergoing MI phase transition and showed hysteretic effects. To get the per-pixel resistance corresponding to a specific greyscale value, we feed the bulk resistance obtained from the experiment to a 2-D Random Field Ising Model. By converting each site of the RFIM into an insulating (-1) or a metallic (1) node into four resistors, we generated a 2D resistor grid. To reduce the resistor network into a single effective resistance, we developed an efficient Bond Propagation Algorithm mediated by Y-Delta/Delta-Y transformations in JAVA. Now that we have a correspondence between greyscale values and …

The spin Seebeck effect (SSE) involves the generation of spin current in the presence of a temperature gradient across a magnetically active material. Although much work has been done in ferrimagnetic and antiferromagnetic materials, SSE in paramagnets is considerably less studied, and the mechanism of paramagnetic SSE is still unclear. Here we present a systematic study of the longitudinal SSE (LSSE) in paramagnetic insulating thin film VO 2 at low temperatures. The LSSE grows with increasing field and saturates at high field. With the increasing temperature, the magnitude of LSSE voltage at fixed heater power decreases, and varies as T-2 relationship between 15 K and 50 K. The LSSE shows the expected angular dependence with the in-plane field orientation and is linear in the heating power. The sign of the LSSE response is not consistent with mobile triplet excitations as the carriers of spin. We will …

Fast and sensitive phase transition detection is one of the most important requirements for new material synthesis and characterization. For solid-state samples, microwave absorption techniques can be employed for detecting phase transitions because it simultaneously monitors changes in electronic and magnetic properties. However, microwave absorption techniques require expensive high-frequency microwave equipment and bulky hollow cavities. Due to size limitations in conventional instruments, it is challenging to implement these cavities inside a laboratory cryostat. In this work, we designed and built a susceptometer that consists of a small helical cavity embedded into a custom insert of a commercial cryostat. This cavity resonator operated at sub-GHz frequencies is extremely sensitive to changes in material parameters, such as electrical conductivity, magnetization, and electric and magnetic …

The discovery of new mechanisms of controlling magnetic properties by electric fields or currents furthers the fundamental understanding of magnetism and has important implications for practical use. Here, we present an approach of utilizing resistive switching to control magnetic anisotropy. We study a ferromagnetic oxide that exhibits an electrically triggered metal-to-insulator phase transition producing a volatile resistive switching. The switching occurs in a characteristic spatial pattern, the formation of an insulating barrier perpendicular to the current flow, which results in an unusual ferromagnetic/paramagnetic/ferromagnetic configuration. The formation of this voltage-driven paramagnetic insulating barrier is accompanied by the emergence of a strong uniaxial magnetic anisotropy that overpowers the intrinsic material anisotropy. Our results demonstrate that resistive switching is an effective tool for manipulating …

Rare-earth nickelates host a complex entanglement of various degrees of freedoms which offers a high tunability over several emerging noble phases, realizing a perfect playground for developing hybridized functionalities. Among these nickelates, the PrNiO 3 (PNO) possess a sharp metal to insulator transition (MIT) at~ 120K concomitant with paramagnetic (PM) to antiferromagnetic (AFM) phase transition. These transitions are highly tunable upon application of substrate induced strain and growth symmetry. Here we study PNO thin films grown on NdGaO 3 substrate with two different orthorhombic crystal plane orientation, ie,(101) and (110); PNO (101) and PNO (110), respectively. The optical functionalities of these films are realized with engineering photoconductor (CdS)/PNO hybrid Bi-layers. Depending on the growth symmetry CdS/PNO (101) and CdS/PNO (110) films exhibit two very different low temperature …

The Metal-Insulator Transition (MIT) exhibited by a number of transition metal oxides has garnered much interest for various technological applications, including hardware-level implementation of neuromorphic computing architectures, as it can induce several orders of magnitude change in a material's resistivity. Various electronic degrees of freedom can influence this transition, including the presence of defects and doping. In our study, we couple two forms of doping: selectively engineered defects via ion irradiation and photoinduced carrier injection, to tune and suppress the MIT. We fabricated ion-irradiated VOx thin film devices with a layer of an archetypal photoconductor, Cadmium Sulfide, to study the interplay between photodoping and material defects. We show that ion irradiation is a controllable mode of selectively engineering defects within vanadium oxides and can be used for tuning transport properties …

Mott materials such as vanadium oxides, when subject to a strong applied voltage, present an inhomogeneous insulator-to-metal transition with formation of metallic filaments within the insulating bulk. This property is enabling the development of compact and power-efficient neuromorphic devices known as Mott neurons. However, the nature of the transition has not been fully understood yet, as it may be attributed to different effects, including Joule self-heating and hot-carrier injection. Moreover, the experimental determination of the threshold voltage needed to induce the transition has proven to be challenging, as the transition becomes increasingly unpredictable when the threshold is approached. The physical understanding of these issues would not only deepen our understanding of Mott insulators, but would also be an important step toward the realization of neuromorphic devices based on such materials. In …

OBJECTIVES: Wilson’s disease (WD) is a rare autosomal recessive disease caused by ATPB7 gene mutations that alter copper metabolism. Despite high prevalence, few studies have addressed the epidemiology and clinical characteristics of WD in Turkey. We analyzed data of 868 WD patients (Ferenci score>= 3) diagnosed between 1993-2021 in 43 hepatology centers in Turkey.MATERIALS & METHODS: A total of 868 patients (57.9% Male and 42.1% Female) were analyzed. 40 patients were adults. The mean diagnostic age was 118.78±65.734 months. The prevalence of consanguineous marriage was 59%(74.8% 1 st degree, 19.4% 2 nd degree and 5.8%>= 3 rd degree). 45.8% of the patients had family history for WD (80.7% 1 st degree, 8.5% 2 nd degree and 10.8%>= 3 rd degree). 22.9% of the patients were diagnosed with family screening. Clinical manifestations, biochemical, genetic and histological findings are summarized in Tables 1, 2.RESULTS: Zinc monotherapy was initiated in 11 patients (1.2%). 113 patients (13%) were treated with D-penicillamine (DP) alone. 530 patients (61.1%) received combination of DP and zinc treatments. 23 patients (2.6%) only received Trientine treatment while 73 patients were treated with Trientine in combination with zinc (8.4%). DP side effects were reported in 8.5% of the patients. Main side effects were urticarial rash (2.3%), proteinuria (1.4%) and deterioration of neurological symptoms (0.7%). 87 patients (% 10) underwent liver transplantation (LT)(63.1% living donor, 36.9% cadaveric donor). 31/70 (% 44) of patients presented with acute liver failure (ALF) underwent LT. Mortality rate after LT …

Big data processing and large-scale computational needs will soon be limited by local on-chip power dissipation of available semiconductor technologies. Moreover, the increasing demand for information processing will likely create an unsustainable exponential growth in global energy consumption. This will slow down Moore's law and will require the holistic rethinking of computation all the way from unique materials, unconventional devices, and functionalities to novel systems and architectures. Neuromorphic systems based on conventional CMOS technologies have demonstrated promise in providing brain-like functionalities such as pattern recognition, adaptive learning, and complex sensing. However, their future potential is limited by unsurmountable issues, intrinsic to the materials properties of conventional semiconductors. Quantum materials and devices are likely to serve as a revolutionary, energy …

Neuromorphic computing approaches become increasingly important as we address future needs for efficiently processing massive amounts of data. The unique attributes of quantum materials can help address these needs by enabling new energy-efficient device concepts that implement neuromorphic ideas at the hardware level. In particular, strong correlations give rise to highly non-linear responses, such as conductive phase transitions that can be harnessed for short-and long-term plasticity. Similarly, magnetization dynamics are strongly non-linear and can be utilized for data classification. This Perspective discusses select examples of these approaches and provides an outlook on the current opportunities and challenges for assembling quantum-material-based devices for neuromorphic functionalities into larger emergent complex network systems.

The manipulation of the insulating to metal transition (IMT) of strongly correlated oxides is of major interest due to its potential applications in optoelectronics and neuromorphic computing. We have recently showed that the IMT of VO 2, V 2 O 3 and V 3 O 5 can largely be manipulated through the use of photodoping. We discovered recently large effects in heterostructures which incorporate a photoconducting material (CdS) and a strongly correlated Mott oxide. In this work, we have extended it to a heterostructures containing manganites, eg, La0. 3 Sr0. 7 MnO 3 (LSMO), whose insulating state resistance drops notably when exposed to light. We will discuss the possible physical origin of this interesting room temperature, volatile effect.

We take advantage of the threshold resistive switching and self-oscillation of the Mott insulator VO 2, to implement stochastic spiking oscillators, which resemble jittering behavior of biological neurons. Interestingly, we observe that the intrinsic spiking stochasticity has a strong impact on capacitively coupled oscillators. A deterministic anti-phase synchronization can be achieved when two oscillators are coupled with a small capacitor. However, as the capacitive coupling strength increases, the deterministic alternating spiking gives way to stochastic spiking patterns in which an oscillator may have counterintuitive stochastic disruptive events. The stochastic disruptions of the alternating sequence of coupled spiking oscillators leads to a multimodal inter spike interval (ISI) distribution which resembles the multimodal spiking behavior in biological sensory neurons. This may have potential applications in Spiking Neural …

Resistive switching is of the key phenomena for applications such as nonvolatile memories or neuromorphic computing. V 3 O 5, a compound of the vanadium oxide Magnéli series, is one of the rare materials to exhibit an insulator-metal transition above room temperature (T c∼ 415 K). The switching mechanisms in this material are still not clear. Here, we demonstrate both static dc volatile resistive switching and fast oscillatory spiking regimes in V 3 O 5 devices at room temperature (120 K below the phase transition temperature) by applying an electric field. We couple electrical measurements, operando optical imaging, and infrared measurements to track the reflectivity change and the temperature of a device during the resistive switching. We find that the resistive switching starts via thermal runaway deep in the insulating state and triggers the phase transition with the formation of a filament of the high-temperature …

The low temperature monoclinic, insulating phase of vanadium dioxide is ordinarily considered nonmagnetic, with dimerized vanadium atoms forming spin singlets, though paramagnetic response is seen at low temperatures. We find a nonlocal spin Seebeck signal in VO 2 films that appears below 30 K and that increases with a decrease in temperature. The spin Seebeck response has a nonhysteretic dependence on the in-plane external magnetic field. This paramagnetic spin Seebeck response is discussed in terms of prior findings on paramagnetic spin Seebeck effects and expected magnetic excitations of the monoclinic ground state.

The emergence of spin‐orbit torques as a promising approach to energy‐efficient magnetic switching has generated large interest in material systems with easily and fully tunable spin‐orbit torques. Here, current‐induced spin‐orbit torques in VO2/NiFe heterostructures are investigated using spin‐torque ferromagnetic resonance, where the VO2 layer undergoes a prominent insulator‐metal transition. A roughly twofold increase in the Gilbert damping parameter, α, with temperature is attributed to the change in the VO2/NiFe interface spin absorption across the VO2 phase transition. More remarkably, a large modulation (±100%) and a sign change of the current‐induced spin‐orbit torque across the VO2 phase transition suggest two competing spin‐orbit torque generating mechanisms. The bulk spin Hall effect in metallic VO2, corroborated by the first‐principles calculation of the spin Hall conductivity σSH≈−104ℏeΩ …

Controlling the magnetic ground states at the nanoscale is a long-standing basic research problem and an important issue in magnetic storage technologies. Here, we designed a nanostructured material that exhibits very unusual hysteresis loops due to a transition between vortex and double pole states. Arrays of 700 nm diamond-shape nanodots consisting of Py (30 nm)/Ru (tRu)/Py (30 nm)(Py, permalloy (Ni80Fe20)) trilayers were fabricated by interference lithography and e-beam evaporation. We show that varying the Ru interlayer spacer thickness (tRu) governs the interaction between the Py layers. We found this interaction mainly mediated by two mechanisms: magnetostatic interaction that favors antiparallel (antiferromagnetic, AFM) alignment of the Py layers and exchange interaction that oscillates between ferromagnetic (FM) and AFM couplings. For a certain range of Ru thicknesses, FM coupling dominates and forms magnetic vortices in the upper and lower Py layers. For Ru thicknesses at which AFM coupling dominates, the magnetic state in remanence is a double pole structure. Our results showed that the interlayer exchange coupling interaction remains finite even at 4 nm Ru thickness. The magnetic states in remanence, observed by Magnetic Force Microscopy (MFM), are in good agreement with corresponding hysteresis loops obtained by Magneto-Optic Kerr Effect (MOKE) and micromagnetic simulations.

We report on a temperature-driven reversible change of the in-plane magnetic anisotropy of V 2 O 3/Ni bilayers. This is caused by the rhombohedral to monoclinic structural phase transition of V 2 O 3 at T C= 160 K. The in-plane magnetic anisotropy is uniaxial above T C, but as the bilayer is cooled through the structural phase transition, a secondary magnetic easy axis emerges. Ferromagnetic resonance measurements show that this change in magnetic anisotropy is reversible with temperature. We identify two structural properties of the V 2 O 3/Ni bilayers affecting the in-plane magnetic anisotropy:(1) a growth-induced uniaxial magnetic anisotropy associated with steplike terraces in the bilayer microstructure and (2) a low-temperature strain-induced biaxial anisotropy associated with the V 2 O 3 structural phase transition. Magnetoresistance measurements corroborate the change in magnetic anisotropy across the …

Electricity is pivotal to modern human civilization, powering different devices used in houses, business and industry. However, the most reliable source of electricity generation has been fossil fuel, which generates greenhouse gases leading to global warming. Indoor temperature regulation alone accounts for half the electricity use in US, making it the largest expense in the country. Therefore, a photonic platform that can regulate household temperature autonomously depending on ambient temperature without the use of electricity is the need of the hour. In this work, we present a passive nanophotonic thermostat based on VO2 thin films, capable of self-adjusting its absorptivity and emissivity to maintain a set temperature approximately locked within the phase transition regions. The experimental results demonstrate that the structured metasurface sample reflects most of the incident IR radiation during high …

Over recent years, phase separation has been revealed from the nanometer to micron scales in various transition metal oxides. These maps have offered the possibility to extract and analyze cluster size, critical exponents etc... at snapshot temperatures across the transition. Tracking fine changes in the cluster dynamics at slow temperature sweep has, however, not yet been possible. To do so we have developed a 77K-600K variable temperature autofocus microscope operating in the visible range. Up to 1000 images can be typically measured crossing T c. We will first review the experimental setup and image analysis developed to complete this study. We will then review key results obtained on Vanadium Dioxide, VO 2: critical temperature maps (T c maps), ΔT c hysteretic maps, memory maps, switching maps, resistor network and pattern recognition of clusters using machine learning.

Using the extensively studied V2O3 as a prototype system, we investigate the role of percolation in metal-insulator transition (MIT). We apply scanning microwave impedance microscopy to directly determine the metallic phase fraction p and relate it to the macroscopic conductance G, which shows a sudden jump when p reaches the percolation threshold. Interestingly, the conductance G exhibits a hysteretic behavior against suggesting two different percolating processes upon cooling and warming. Based on our image analysis and model simulation, we ascribe such hysteretic behavior to different domain nucleation and growth processes between cooling and warming, which is likely caused by the decoupled structural and electronic transitions in V2O3 during MIT. Our work provides a microscopic view of how the interplay of structural and electronic degrees of freedom affects MIT in strongly correlated systems.

Probabilistic computing is a paradigm in which data are not represented by stable bits, but rather by the probability of a metastable bit to be in a particular state. The development of this technology has been hindered by the availability of hardware capable of generating stochastic and tunable sequences of “1s” and “0s”. The options are currently limited to complex CMOS circuitry and, recently, magnetic tunnel junctions. Here, we demonstrate that metal–insulator transitions can also be used for this purpose. We use an electrical pump/probe protocol and take advantage of the stochastic relaxation dynamics in VO2 to induce random metallization events. A simple latch circuit converts the metallization sequence into a random stream of 1s and 0s. The resetting pulse in between probes decorrelates successive events, providing a true stochastic digital sequence.

In article number 2101826 by Rafael Morales, Gabriela Romero, and co-workers, magnetic microdiscs fabricated by litography techniques are utilized to magnetically induce mechanotransduction in primary neural cultures. The red and blue colors in the microdiscs indicate a magnetic component of a special spin configuration known as vortex state.

Controlling the magnetic ground states at the nanoscale is a longstanding basic research problem and an important issue in magnetic storage technologies. Here, we designed a nanostructured material that exhibits very unusual hysteresis loops due to a transition between vortex and double pole states. Arrays of 700 nm diamond-shaped nanodots consisting of Py(30 nm)/Ru(tRu)/Py(30 nm) (Py, permalloy (Ni80Fe20)) trilayers were fabricated by interference lithography and e-beam evaporation. We show that varying the Ru interlayer spacer thickness (tRu) governs the interaction between the Py layers. We found this interaction mainly mediated by two mechanisms: magnetostatic interaction that favors antiparallel (antiferromagnetic, AFM) alignment of the Py layers and exchange interaction that oscillates between ferromagnetic (FM) and AFM couplings. For a certain range of Ru thicknesses, FM coupling …

Several highly effective Covid-19 vaccines are in emergency use, although more-infectious coronavirus strains, could delay the end of the pandemic even further. Because of this, it is highly desirable to develop fast antiviral drug treatments to accelerate the lasting immunity against the virus. From a theoretical perspective, computational approaches are useful tools for antiviral drug development based on the data analysis of gene expression, chemical structure, molecular pathway, and protein interaction mapping. This work studies the structural stability of virus–host interactome networks based on the graphical representation of virus–host protein interactions as vertices or nodes connected by commonly shared proteins. These graphical network visualization methods are analogous to those use in the design of artificial neural networks in neuromorphic computing. In standard protein-node-based network …

Low-field microwave absorption techniques are ultrasensitive, nondestructive methods for probing electric and magnetic properties of solids. Nonresonant low-field microwave absorption techniques such as magnetic field modulated microwave spectroscopy (MFMMS) can easily detect electromagnetic phase transitions in minute and inhomogeneous samples. While this technique can easily and almost selectively identify superconducting transitions, magnetic phase transitions produce more varied responses. Here, we present a technique to investigate the electric and magnetic properties of a sample with complex electromagnetic responses. This technique involves taking a series of magnetic hysteresis loops and magnetoresistance measurements. These can be compared to MFMMS data to identify features having electric or magnetic origin. This approach is applied to magnetite (Fe 3 O 4), which possesses an …

Application of a strong electric stimulus, voltage or current, to the ferromagnetic oxide (La, Sr) MnO 3 (LSMO) triggers the intrinsic metal-insulator transition producing a volatile switching from a low-to high-resistance state. This resistive switching occurs in a characteristic spatial pattern, the formation of a paramagnetic insulating barrier perpendicular to the current flow, in contrast to the conventional filamentary percolation parallel to the current. We explored the evolution of anisotropic and colossal magnetoresistance in LSMO devices as they undergo resistive switching. We found that the magnetoresistance magnitude can be increased severalfold by initiating the switching and inducing the formation of a paramagnetic barrier inside the device. Moreover, by driving the LSMO device through the resistive switching the sign of magnetoresistance can be flipped from positive to negative and vice versa. Our results …

We use optical microscopy to image spatial structure of metal and insulator patches as a thin film of VO 2 is repeatedly driven partway through its temperature-driven insulator-to-metal transition. The location and shape of accumulated memory was tracked after each subloop, revealing for the first time the internal structure of the memory effect. A combination of insulator domains appearing at cluster boundaries, as well as large insulator nucleation sites, was identified. Two large temperature sweeps were subsequently completed to reset the ramp reversal memory. Transition temperature maps reveal that memory is surprisingly also stored deep in the insulating and metallic puddles throughout the entire sample surface. We discuss two possible diffusion models to explain these 2D surface memory maps. These results pave the way to enhancing, controlling and manipulating the memory in this material in the near …

Memristors based on oxide tunnel junctions are promising candidates for energy efficient neuromorphic computing. However, the low power sensing of the nonvolatile resistive state is an important challenge. We report the optically induced sensing of the resistive state of a memristor based on a La0. 7Sr0. 3MnO3/BaTiO3/In2O3: SnO2 (90: 10) heterostructure with a 3nm thick BaTiO3 ferroelectric barrier. The nonvolatile memristive response originates from the modulation of an interfacial Schottky barrier at the La0. 7Sr0. 3MnO3/BaTiO3 interface, yielding robust intermediate memristive states. The Schottky barrier produces a photovoltaic response when illuminated with a 3.3 eV UV LED, which depends on the state. The open circuit voltage Voc correlates linearly with the resistance of each state, enabling active sensing of the memristive state at light power densities as low as 20 mW/cm2 and temperatures up to 100 …

In Mott insulators electrical currents can change resistance by orders-of-magnitude due to an insulator-metal phase transition. The volatility of switching in Mott insulators can be tuned, enabling both memory devices and neuron-like functionalities. In this talk I will present our recent advances in defect-and strain-engineering of Mott insulators, with the aim of developing highly energy-efficient resistive switching devices and novel functionalities.I will first discuss how defects induced by ion irradiation in V 2 O 3 and VO 2 nanowire devices can dramatically reduce the electric field and energy required for switching by inducing non-thermal insulator-metal transitions. I will then discuss strain engineering in V 2 O 3 which allows to tune the trajectory of the phase transition, allowing access to hitherto inaccessible regions of the phase diagram. We first report on a thermally-induced transition into the paramagnetic insulating …

Important effects appear in low and high temperature magnetometry of multiferroic BiFeO3 (BFO) nanoparticles as sizes are reduced from 105 nm to 18 nm. In these particles, a magnetic transition appears close to 200 K probably associated with a spin reorientation which is absent in bigger particles. At higher temperatures and up to the bulk ferroelectric Curie temperature (1000 K), two additional magnetic transitions can be identified. Around TN 600 K, a size-dependent magnetic transition appears, associated with an antiferromagnetic transition. A model based on atomic vibration instability can describe the variation of TN with nanoparticle size. Furthermore, for certain sizes, the properties are dominated by the glassy state arising from the disorder produced by the formation of a core-shell structure. This implies that the magnetic behavior of nanoparticles is strongly affected by their crystal structure close to the …

We demonstrate a VO 2 based phase-change metasurface that self regulates its emissivity in response to the ambient temperature. The mid-infrared emissivity increases to near unity when operating above the insulator-to-metal phase transition temperature of VO 2.

Nonvolatile resistive switching is one of the key phenomena for emerging applications in optoelectronics and neuromorphic computing. In most of the cases, an electric field is applied to a two terminal dielectric material device and leads to the formation of a low resistance filament due to ion migration. However, the stochastic nature of the ion migration can be an impediment for the device robustness and controllability, with uncontrolled variations of high and low resistance states or threshold voltages. Here, we report an optically induced resistive switching based on a CdS/V 3 O 5 heterostructure which can overcome this issue. V 3 O 5 is known to have a second order insulator to metal transition around T c≈ 415 K, with an electrically induced threshold switching at room temperature. Upon illumination, the direct transfer of the photoinduced carriers from the CdS into V 3 O 5 produces a nonvolatile resistive …

The spin Seebeck effect (SSE) involves the generation of spin current in the presence of a temperature gradient across the material. Here we present a systematic study of the transverse SSE in the paramagnetic insulating phase of VO 2 at low temperatures. In the nonlocal geometry, the spin current is thermally generated by an AC current flowing in one Pt wire and the SSE is detected by another Pt wire using a lock-in amplifier at the second harmonic via the inverse spin Hall effect. The pronounced SSE at 3 K, decreases sharply as the temperature increases to 10K, and suddenly changes sign at 15 K, before vanishing above about 40 K. The SSE shows the expected angle dependence with the in-plane field; however, when the field is out-of-plane, the signal is accompanied by a Nernst effect contribution. We will also discuss the dependence of the observed SSE on heater-detector distance.

Vanadium Dioxide (VO 2) exhibits multiscale pattern formation while it undergoes a temperature-driven Metal-Insulator (MI) phase transition. We use optical microscopy techniques to image the entire surface of a two-terminal etched VO 2 microbridge, simultaneously measuring the macroscopic resistance of the device. Patches of metal and insulator form while undergoing the MI phase transition and display hysteresis. We employ a random field Ising model to predict sub-pixel spatial structure below optical resolution, mapping the reflectivity of each pixel to the Ising pseudomagnetization, in order to predict the effective microscopic resistance of each pixel. We use the (exact) bond propagation algorithm to reduce the 2D resistor grid into a single equivalent resistance. These studies pave the way toward a deeper understanding of resistance avalanches, memory effects, and spiking behavior in VO 2.

Transition metal oxides (TMOs) are promising materials to realize low-power neuromorphic devices. Their physical properties critically depend on their oxygen vacancy concentration, whose experimental determination remains a challenging task. Here, we focus on cobaltites, in particular La1–xSrxCoO3−δ (LSCO), and present a strategy to identify fingerprints of oxygen vacancies in X-ray absorption (XA) spectra. Using a combination of experiments and theory, we show that the variation of the oxygen vacancy concentration in the perovskite phase of LSCO is correlated with the change in the relative peak positions of the O K-edge XA spectra. We also identify an additional geometrical fingerprint that captures both the changes in the Co–O bond length and Co–O–Co bond angle in the material due to the presence of oxygen vacancies. Finally, we predict the oxygen vacancy concentration of experimental samples …

The recently discovered ramp reversal memory (RRM) is a nonvolatile memory effect observed in correlated oxides with temperature-driven insulator–metal transitions (IMT). It appears as a resistance increase at predefined temperatures that are set or erased by simple heating–cooling (ie, ramp reversal) protocols. Until now RRM was measured for two materials: VO 2 and NdNiO 3. A heuristic model suggests that the RRM is caused by a local transition temperature increase at boundaries of spatially separated metallic and insulating domains during ramp reversal. However, there is no experimental measure of the magnitude of the effect, which is crucial for the development of a theoretical account of the RRM. Here we show that V 2 O 3 also shows RRM, including all related features, highlighting the generality of the effect. Moreover, an analysis of the RRM as an effective (average) increase of the critical …

We have studied the influence of controlled defects on the magnetic properties of La0.67Sr0.33MnO3 (LSMO) thin films. We used 100 nm thick epitaxial LSMO films, which were grown on SrTiO3 substrates and introduced stoichiometry changes and defects using oxygen ion irradiation through nanoporous alumina masks. Oxygen irradiation through the mask creates cascades of defects in spatially-modulated regions in the plane of the LSMO film. The magnetic properties of the samples were compared before and after the irradiation. We have found that irradiation reduces the ferromagnetic ordering temperature, decreases the total magnetization, enhances the coercivity, and induces exchange bias below 50 K. The coercivity enhancement is dramatically higher below 50 K and can be associated with exchange bias. These results can be explained by the formation of Mn-rich antiferromagnetic or ferrimagnetic …

In article number 2200331, José Manuel Díez, José Luis F. Cuñado, Julio Camarero, and co-workers demonstrate a new general mechanism controlling the exchange bias phenomena in antiferromagnetic/ferromagnetic (AFM/FM) heterostructures: the magnetic texture of the FM layer during reversal determines and controls the exchange bias, its temperature dependence, and other related phenomena. It allows to enhance the existing devices relying on exchange bias or to design innovative ones that were not possible before.

Resistive switching—the current-and voltage-induced change of electrical resistance—is at the core of memristive devices, which play an essential role in the emerging field of neuromorphic computing. This study is about resistive switching in a Mott insulator, which undergoes a thermally driven metal-to-insulator transition. Two distinct switching mechanisms are reported for such a system: electric-field-driven resistive switching and electrothermal resistive switching. The latter results from an instability caused by Joule heating. Here, we present the visualization of the reversible resistive switching in a planar V 2 O 3 thin-film device using high-resolution wide-field microscopy in combination with electric transport measurements. We investigate the interaction of the electrothermal instability with the strain-induced spontaneous phase separation in the V 2 O 3 thin film at the Mott transition. The photomicrographs show …

In solids, strong repulsion between electrons can inhibit their movement and result in a “Mott” metal-to-insulator transition (MIT), a fundamental phenomenon whose understanding has remained a challenge for over 50 years. A key issue is how the wave-like itinerant electrons change into a localized-like state due to increased interactions. However, observing the MIT in terms of the energy- and momentum-resolved electronic structure of the system, the only direct way to probe both itinerant and localized states, has been elusive. Here we show, using angle-resolved photoemission spectroscopy (ARPES), that in V2O3, the temperature-induced MIT is characterized by the progressive disappearance of its itinerant conduction band, without any change in its energy-momentum dispersion, and the simultaneous shift to larger binding energies of a quasi-localized state initially located near the Fermi level.

Application of an electric stimulus to a material with a metal-insulator transition can trigger a large resistance change. Resistive switching from an insulating into a metallic phase, which typically occurs by the formation of a conducting filament parallel to the current flow, is a highly active research topic. Using the magneto-optical Kerr imaging, we found that the opposite type of resistive switching, from a metal into an insulator, occurs in a reciprocal characteristic spatial pattern: the formation of an insulating barrier perpendicular to the driving current. This barrier formation leads to an unusual N-type negative differential resistance in the current-voltage characteristics. We further demonstrate that electrically inducing a transverse barrier enables a unique approach to voltage-controlled magnetism. By triggering the metal-to-insulator resistive switching in a magnetic material, local on/off control of ferromagnetism is …

Vanadium dioxide (VO2), which exhibits a near-room-temperature insulator–metal transition, has great potential in applications of neuromorphic computing devices. Although its volatile switching property, which could emulate neuron spiking, has been studied widely, nanoscale studies of the structural stochasticity across the phase transition are still lacking. In this study, using in situ transmission electron microscopy and ex situ resistive switching measurement, we successfully characterized the structural phase transition between monoclinic and rutile VO2 at local areas in planar VO2/TiO2 device configuration under external biasing. After each resistive switching, different VO2 monoclinic crystal orientations are observed, forming different equilibrium states. We have evaluated a statistical cycle-to-cycle variation, demonstrated a stochastic nature of the volatile resistive switching, and presented an approach to study …

We performed ferromagnetic resonance measurements of a (La 1− x Pr x) 1− y Ca y Mn O 3− δ with x= 0.52±0.05, y= 0.23±0.04, and δ= 0.14±0.10 thin single crystalline film which, in combination with micromagnetic simulations, reveal three temperature regions consistent with (i) a ferromagnetic-paramagnetic transition in which ferromagnetic domains nucleate and grow,(ii) followed by a filamentary fluidlike percolation of magnetic domains exhibiting dynamic processes and finally, iii) the existence of a blocking temperature below which the magnetism is a metastable glassy-like state with strong decoherence of the uniform resonance mode. Our results suggest a strain-liquid to strain-glass spin order transition in which the magnetism and fluidlike dynamics of the separated phases freeze at low temperatures. We show the magnetism dynamics depend strongly on the phase-separated state and morphology of the …

Many correlated systems feature an insulator-to-metal transition that can be triggered by an electric field. Although it is known that metallization takes place through filament formation, the details of how this process initiates and evolves remain elusive. We use in-operando optical reflectivity to capture the growth dynamics of the metallic phase with space and time resolution. We demonstrate that filament formation is triggered by nucleation at hotspots, with a subsequent expansion over several decades in time. By comparing three case studies (VO2, V3O5, and V2O3), we identify the resistivity change across the transition as the crucial parameter governing this process. Our results provide a spatiotemporal characterization of volatile resistive switching in Mott insulators, which is important for emerging technologies, such as optoelectronics and neuromorphic computing.