Suppressing Dielectric Loss in MXene/Polymer Nanocomposites through Interfacial Interactions.

Although numerous polymer-based composites exhibit excellent dielectric permittivity, their dielectric performance in various applications is severely hampered by high dielectric loss induced by interfacial space charging and a leakage current. Herein, we demonstrate that embedding molten salt etched MXene into a poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE))/poly(methyl methacrylate) (PMMA) hybrid matrix induces strong interfacial interactions, forming a close-packed inner polymer layer and leading to significantly suppressed dielectric loss and markedly increased dielectric permittivity over a broad frequency range. The intensive molecular interaction caused by the dense electronegative functional terminations (-O and -Cl) in MXene results in restricted polymer chain movement and dense molecular arrangement, which reduce the transportation of the mobile charge carriers. Consequently, compared to the neat polymer, the dielectric constant of the composite with 2.8 wt % MXene filler increases from ∼52 to ∼180 and the dielectric loss remains at the same value (∼0.06) at 1 kHz. We demonstrate that the dielectric loss suppression is largely due to the formation of close-packed interfaces between the MXene and the polymer matrix.

Reconfigurable spin current transmission and magnon-magnon coupling in hybrid ferrimagnetic insulators.

Coherent spin waves possess immense potential in wave-based information computation, storage, and transmission with high fidelity and ultra-low energy consumption. However, despite their seminal importance for magnonic devices, there is a paucity of both structural prototypes and theoretical frameworks that regulate the spin current transmission and magnon hybridization mediated by coherent spin waves. Here, we demonstrate reconfigurable coherent spin current transmission, as well as magnon-magnon coupling, in a hybrid ferrimagnetic heterostructure comprising epitaxial Gd3Fe5O12 and Y3Fe5O12 insulators. By adjusting the compensated moment in Gd3Fe5O12, magnon-magnon coupling was achieved and engineered with pronounced anticrossings between two Kittel modes, accompanied by divergent dissipative coupling approaching the magnetic compensation temperature of Gd3Fe5O12 (TM,GdIG), which were modeled by coherent spin pumping. Remarkably, we further identified, both experimentally and theoretically, a drastic variation in the coherent spin wave-mediated spin current across TM,GdIG, which manifested as a strong dependence on the relative alignment of magnetic moments. Our findings provide significant fundamental insight into the reconfiguration of coherent spin waves and offer a new route towards constructing artificial magnonic architectures.

Magnetic Skyrmions above Room Temperature in a van der Waals Ferromagnet Fe3GaTe2.

2D van der Waals (vdW) ferromagnetic crystals are a promising platform for innovative spintronic devices based on magnetic skyrmions, thanks to their high flexibility and atomic thickness stability. However, room-temperature skyrmion-hosting vdW materials are scarce, which poses a challenge for practical applications. In this study, a chemical vapor transport (CVT) approach is employed to synthesize Fe3GaTe2 crystals and room-temperature Néel skyrmions are observed in Fe3GaTe2 nanoflakes above 58 nm in thickness through in situ Lorentz transmission electron microscopy (L-TEM). Upon an optimized field cooling procedure, zero-field hexagonal skyrmion lattices are successfully generated in nanoflakes with an extended thickness range (30-180 nm). Significantly, these skyrmion lattices remain stable up to 355 K, setting a new record for the highest temperature at which skyrmions can be hosted. The research establishes Fe3GaTe2 as an emerging above-room-temperature skyrmion-hosting vdW material, holding great promise for future spintronics.

Defective Biphenylene as High-Efficiency Hydrogen Evolution Catalysts.

Electrocatalysts play a pivotal role in advancing the application of water splitting for hydrogen production. This research unveils the potential of defective biphenylenes as high-efficiency catalysts for the hydrogen evolution reaction. Using first-principles simulations, we systematically investigated the structure, stability, and catalytic performance of defective biphenylenes. Our findings unveil that defect engineering significantly enhances the electrocatalytic activity for hydrogen evolution. Specifically, biphenylene with a double-vacancy defect exhibits an outstanding Gibbs free energy of -0.08 eV, surpassing that of Pt, accompanied by a remarkable exchange current density of -3.08 A cm-2, also surpassing that of Pt. Furthermore, we find the preference for the Volmer-Heyrovsky mechanism in the hydrogen evolution reaction, with a low energy barrier of 0.80 eV. This research provides a promising avenue for developing novel metal-free electrocatalysts for water splitting with earth-abundant carbon elements, making a significant step toward sustainable hydrogen production.

High-performance van der Waals antiferroelectric CuCrP2S6-based memristors.

Layered thio- and seleno-phosphate ferroelectrics, such as CuInP2S6, are promising building blocks for next-generation nonvolatile memory devices. However, because of the low Curie point, the CuInP2S6-based memory devices suffer from poor thermal stability (<42 °C). Here, exploiting the electric field-driven phase transition in the rarely studied antiferroelectric CuCrP2S6 crystals, we develop a nonvolatile memristor showing a sizable resistive-switching ratio of ~ 1000, high switching endurance up to 20,000 cycles, low cycle-to-cycle variation, and robust thermal stability up to 120 °C. The resistive switching is attributed to the ferroelectric polarization-modulated thermal emission accompanied by the Fowler-Nordheim tunneling across the interfaces. First-principles calculations reveal that the good device performances are associated with the exceptionally strong ferroelectric polarization in CuCrP2S6 crystal. Furthermore, the typical biological synaptic learning rules, such as long-term potentiation/depression and spike amplitude/spike time-dependent plasticity, are also demonstrated. The results highlight the great application potential of van der Waals antiferroelectrics in high-performance synaptic devices for neuromorphic computing.

Discovery of a three-proton insertion mechanism in α-molybdenum trioxide leading to enhanced charge storage capacity.

The α-molybdenum trioxide has attracted much attention for proton storage owing to its easily modified bilayer structure, fast proton insertion kinetics, and high theoretical specific capacity. However, the fundamental science of the proton insertion mechanism in α-molybdenum trioxide has not been fully understood. Herein, we uncover a three-proton intercalation mechanism in α-molybdenum trioxide using a specially designed phosphoric acid based liquid crystalline electrolyte. The semiconductor-to-metal transition behavior and the expansion of the lattice interlayers of α-molybdenum trioxide after trapping one mole of protons are verified experimentally and theoretically. Further investigation of the morphology of α-molybdenum trioxide indicates its fracture behavior upon the proton intercalation process, which creates diffusion channels for hydronium ions. Notably, the observation of an additional redox behavior at low potential endows α-molybdenum trioxide with an improved specific discharge capacity of 362 mAh g-1.

Controllable Skyrmionic Phase Transition between Néel Skyrmions and Bloch Skyrmionic Bubbles in van der Waals Ferromagnet Fe3-δ GeTe2.

The van der Waals (vdW) ferromagnet Fe3-δ GeTe2 has garnered significant research interest as a platform for skyrmionic spin configurations, that is, skyrmions and skyrmionic bubbles. However, despite extensive efforts, the origin of the Dzyaloshinskii-Moriya interaction (DMI) in Fe3-δ GeTe2 remains elusive, making it challenging to acquire these skyrmionic phases in a controlled manner. In this study, it is demonstrated that the Fe content in Fe3-δ GeTe2 has a profound effect on the crystal structure, DMI, and skyrmionic phase. For the first time, a marked increase in Fe atom displacement with decreasing Fe content is observed, transforming the original centrosymmetric crystal structure into a non-centrosymmetric symmetry, leading to a considerable DMI. Additionally, by varying the Fe content and sample thickness, a controllable transition between Néel-type skyrmions and Bloch-type skyrmionic bubbles is achieved, governed by a delicate interplay between dipole-dipole interaction and the DMI. The findings offer novel insights into the variable skyrmionic phases in Fe3-δ GeTe2 and provide the impetus for developing vdW ferromagnet-based spintronic devices.

Switching magnetic strip orientation using electric fields.

In spintronics, ordered magnetic domains are important for magnetic microdevices and controlling the orientation of ordered magnetic domains is important for applications such as domain wall resistance and spin wave propagation. Although a magnetic field or a current can reorient ordered magnetic domains, an energy-efficient electric-field-driven rotation of the ordered magnetic domains remains elusive. Here, using a nanotrenched polymeric layer, we obtain ordered magnetic strip domains in Ni films on a ferroelectric substrate. By applying electric fields to the ferroelectric substrate, we demonstrate that the ordered magnetic strip domains in Ni films are switched between the y- and x-axes driven by electric-fields. This switching of magnetic strip orientation is attributed to the electric-field-modulated in-plane magnetic anisotropies along the x- and y-axes of the Ni films, which are caused by the anisotropic biaxial strain of the ferroelectric substrate via strain-mediated magnetoelectric coupling. These results provide an energy-efficient approach for manipulating the ordered magnetic domains using electric fields.

Proton-mediated reversible switching of metastable ferroelectric phases with low operation voltages.

The exploration of ferroelectric phase transitions enables an in-depth understanding of ferroelectric switching and promising applications in information storage. However, controllably tuning the dynamics of ferroelectric phase transitions remains challenging owing to inaccessible hidden phases. Here, using protonic gating technology, we create a series of metastable ferroelectric phases and demonstrate their reversible transitions in layered ferroelectric α-In2Se3 transistors. By varying the gate bias, protons can be incrementally injected or extracted, achieving controllable tuning of the ferroelectric α-In2Se3 protonic dynamics across the channel and obtaining numerous intermediate phases. We unexpectedly discover that the gate tuning of α-In2Se3 protonation is volatile and the created phases remain polar. Their origin, revealed by first-principles calculations, is related to the formation of metastable hydrogen-stabilized α-In2Se3 phases. Furthermore, our approach enables ultralow gate voltage switching of different phases (below 0.4 volts). This work provides a possible avenue for accessing hidden phases in ferroelectric switching.

Fast water transport and molecular sieving through ultrathin ordered conjugated-polymer-framework membranes.

The development of membranes that block solutes while allowing rapid water transport is of great importance. The microstructure of the membrane needs to be rationally designed at the molecular level to achieve precise molecular sieving and high water flux simultaneously. We report the design and fabrication of ultrathin, ordered conjugated-polymer-framework (CPF) films with thicknesses down to 1 nm via chemical vapour deposition and their performance as separation membranes. Our CPF membranes inherently have regular rhombic sub-nanometre (10.3 × 3.7 Å) channels, unlike membranes made of carbon nanotubes or graphene, whose separation performance depends on the alignment or stacking of materials. The optimized membrane exhibited a high water/NaCl selectivity of ∼6,900 and water permeance of ∼112 mol m-2 h-1 bar-1, and salt rejection >99.5% in high-salinity mixed-ion separations driven by osmotic pressure. Molecular dynamics simulations revealed that water molecules quickly and collectively pass through the membrane by forming a continuous three-dimensional network within the hydrophobic channels. The advent of ordered CPF provides a route towards developing carbon-based membranes for precise molecular separation.

Magnetic Skyrmions with Unconventional Helicity Polarization in a Van Der Waals Ferromagnet.

Skyrmion helicity, which defines the spin swirling direction, is a fundamental parameter that may be utilized to encode data bits in future memory devices. Generally, in centrosymmetric ferromagnets, dipole skyrmions with helicity of -π/2 and π/2 are degenerate in energy, leading to equal populations of both helicities. On the other hand, in chiral materials where the Dzyaloshinskii-Moriya interaction (DMI) prevails and the dipolar interaction is negligible, only a preferred helicity is selected by the type of DMI. However, whether there is a rigid boundary between these two regimes remains an open question. Herein, the observation of dipole skyrmions with unconventional helicity polarization in a van der Waals ferromagnet, Fe5- δ GeTe2 , is reported. Combining magnetometry, Lorentz transmission electron microscopy, electrical transport measurements, and micromagnetic simulations, the short-range superstructures in Fe5- δ GeTe2 resulting in a localized DMI contribution, which breaks the degeneracy of the opposite helicities and leads to the helicity polarization, is demonstrated. Therefore, the helicity feature in Fe5- δ GeTe2 is controlled by both the dipolar interaction and DMI that the former leads to Bloch-type skyrmions with helicity of ±π/2 whereas the latter breaks the helicity degeneracy. This work provides new insights into the skyrmion topology in van der Waals materials.

Electrically and optically erasable non-volatile two-dimensional electron gas memory.

The high-mobility two-dimensional electron gas (2DEG) generated at the interface between two wide-band insulators, LaAlO3 (LAO) and SrTiO3 (STO), is an extensively researched topic. In this study, we have successfully realized reversible switching between metallic and insulating states of the 2DEG system via the application of optical illumination and positive pulse voltage induced by the introduction of oxygen vacancies as reservoirs for electrons. The positive pulse voltage irreversibly drives the electron to the defect energy level formed by the oxygen vacancies, which leads to the formation of the insulating state. Subsequently, the metallic state can be achieved via optical illumination, which excites the trapped electron back to the 2DEG potential well. The ON/OFF state is observed to be robust with a ratio exceeding 106; therefore, the interface can be used as an electrically and optically erasable non-volatile 2DEG memory.

High-Efficiency Magnon-Mediated Magnetization Switching in All-Oxide Heterostructures with Perpendicular Magnetic Anisotropy.

The search for efficient approaches to realize local switching of magnetic moments in spintronic devices has attracted extensive attention. One of the most promising approaches is the electrical manipulation of magnetization through electron-mediated spin torque. However, the Joule heat generated via electron motion unavoidably causes substantial energy dissipation and potential damage to spintronic devices. Here, all-oxide heterostructures of SrRuO3 /NiO/SrIrO3 are epitaxially grown on SrTiO3 single-crystal substrates following the order of the ferromagnetic transition metal oxide SrRuO3 with perpendicular magnetic anisotropy, insulating and antiferromagnetic NiO, and metallic transition metal oxide SrIrO3 with strong spin-orbit coupling. It is demonstrated that instead of the electron spin torques, the magnon torques present in the antiferromagnetic NiO layer can directly manipulate the perpendicular magnetization of the ferromagnetic layer. This magnon mechanism may significantly reduce the electron motion-related energy dissipation from electron-mediated spin currents. Interestingly, the threshold current density to generate a sufficient magnon current to manipulate the magnetization is one order of magnitude smaller than that in conventional metallic systems. These findings suggest a route for developing highly efficient all-oxide spintronic devices operated by magnon current.

Current-Induced Magnetization Switching Across a Nearly Room-Temperature Compensation Point in an Insulating Compensated Ferrimagnet.

Insulating compensated ferrimagnets, especially hosting room-temperature compensation points, are considered promising candidates for developing ultra-high-density and ultrafast magnonic devices owing to combining the characteristics of both ferromagnets and antiferromagnets. These intriguing features become outstanding close to their compensation points. However, their spin-orbit torque (SOT)-induced magnetization switching, particularly in the vicinity of the compensation points, remains unclear. Herein, we systematically investigated the SOT in insulating compensated ferrimagnetic Gd3Fe5O12/Pt heterostructures with perpendicular magnetic anisotropy. A nearly room-temperature compensation point (Tcomp ∼ 297 K) was consistently identified by the magnetization curves, spin Hall-induced anomalous Hall effect, and spin Hall magnetoresistance measurements. Moreover, using 100 ns duration pulsed current, deterministic current-induced magnetization switching below and above Tcomp, even at 294 and 301 K, was achieved with opposite switching polarity. It is found that a large current is required to switch the magnetization in the vicinity of Tcomp, although the effective SOT field increases close to Tcomp. Our finding provides alternative opportunities for exploring ultrafast room-temperature magnon-based devices.

Integrated Memory Devices Based on 2D Materials.

With the advent of the Internet of Things and big data, massive data must be rapidly processed and stored within a short timeframe. This imposes stringent requirements on memory hardware implementation in terms of operation speed, energy consumption, and integration density. To fulfill these demands, 2D materials, which are excellent electronic building blocks, provide numerous possibilities for developing advanced memory device arrays with high performance, smart computing architectures, and desirable downscaling. Over the past few years, 2D-material-based memory-device arrays with different working mechanisms, including defects, filaments, charges, ferroelectricity, and spins, have been increasingly developed. These arrays can be used to implement brain-inspired computing or sensing with extraordinary performance, architectures, and functionalities. Here, recent research into integrated, state-of-the-art memory devices made from 2D materials, as well as their implications for brain-inspired computing are surveyed. The existing challenges at the array level are discussed, and the scope for future research is presented.

Unconventional Spin Pumping and Magnetic Damping in an Insulating Compensated Ferrimagnet.

Recently, the interest in spin pumping (SP) has escalated from ferromagnets into antiferromagnetic systems, potentially enabling fundamental physics and magnonic applications. Compensated ferrimagnets are considered alternative platforms for bridging ferro- and antiferromagnets, but their SP and the associated magnetic damping have been largely overlooked so far despite their seminal importance for magnonics. Herein, an unconventional SP together with magnetic damping in an insulating compensated ferrimagnet Gd3 Fe5 O12 (GdIG) is reported. Remarkably, the divergence of the nonlocal effective magnetic damping induced by SP close to the compensation temperature in GdIG/Cu/Pt heterostructures is identified unambiguously. Furthermore, the coherent and incoherent spin currents, generated by SP and the spin Seebeck effect, respectively, undergo a distinct direction change with the variation of temperature. The physical mechanisms underlying these observations are self-consistently clarified by the ferrimagnetic counterpart of SP and the handedness-related spin-wave spectra. The findings broaden the conventional paradigm of the ferromagnetic SP model and open new opportunities for exploring the ferrimagnetic magnonic devices.

Controlled Deposition of Zinc-Metal Anodes via Selectively Polarized Ferroelectric Polymers.

Aqueous zinc-ion batteries are regarded as ideal candidates for stationary energy-storage systems due to their low cost and high safety. However, zinc can readily grow into dendrites, leading to limited cycling performance and quick failure of the batteries. Herein, a novel strategy is proposed to mitigate this dendrite problem, in which a selectively polarized ferroelectric polymer material (poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE))) is employed as a surface protective layer on zinc anodes. Such a polarized ferroelectric polymer layer can enable a locally concentrated zinc-ion distribution along the coated surface and thus enable the horizontal growth of zinc plates. As a result, symmetrical zinc batteries using such anodes exhibit long cycling lifespan at 0.2 mA cm-2 , 0.2 mAh cm-2 for 2000 h, and a high rate performance up to 15 mA cm-2 . Also, the full cell (including a Zn-MnO2 battery and a zinc-ion capacitor) based on this anode is demonstrated. This work provides a novel strategy to protect the zinc anode and even other metal anodes exploiting polymer ferroelectricity.

Unraveling the origin of ferroelectric resistance switching through the interfacial engineering of layered ferroelectric-metal junctions.

Ferroelectric memristors have found extensive applications as a type of nonvolatile resistance switching memories in information storage, neuromorphic computing, and image recognition. Their resistance switching mechanisms are phenomenally postulated as the modulation of carrier transport by polarization control over Schottky barriers. However, for over a decade, obtaining direct, comprehensive experimental evidence has remained scarce. Here, we report an approach to experimentally demonstrate the origin of ferroelectric resistance switching using planar van der Waals ferroelectric α-In2Se3 memristors. Through rational interfacial engineering, their initial Schottky barrier heights and polarization screening charges at both terminals can be delicately manipulated. This enables us to find that ferroelectric resistance switching is determined by three independent variables: ferroelectric polarization, Schottky barrier variation, and initial barrier height, as opposed to the generally reported explanation. Inspired by these findings, we demonstrate volatile and nonvolatile ferroelectric memristors with large on/off ratios above 104. Our work can be extended to other planar long-channel and vertical ultrashort-channel ferroelectric memristors to reveal their ferroelectric resistance switching regimes and improve their performances.

Using Dipole Interaction to Achieve Nonvolatile Voltage Control of Magnetism in Multiferroic Heterostructures.

Nonvolatile electrical control of magnetism is crucial for developing energy-efficient magnetic memory. Based on strain-mediated magnetoelectric coupling, a multiferroic heterostructure containing an isolated magnet requires nonvolatile strain to achieve this control. However, the magnetization response of an interacting magnet to strain remains elusive. Herein, Co/MgO/CoFeB magnetic tunnel junctions (MTJs) exhibiting dipole interaction on ferroelectric substrates are fabricated. Remarkably, nonvolatile voltage control of the resistance in the MTJs is demonstrated, which originates from the nonvolatile magnetization rotation of an interacting CoFeB magnet driven by volatile voltage-generated strain. Conversely, for an isolated CoFeB magnet, this volatile strain induces volatile control of magnetism. These results reveal that the magnetization response to volatile strain among interacting magnets is different from that among isolated magnets. The findings highlight the role of dipole interaction in multiferroic heterostructures and can stimulate future research on nonvolatile electrical control of magnetism with additional interactions.

Magnetic memory driven by topological insulators.

Giant spin-orbit torque (SOT) from topological insulators (TIs) provides an energy efficient writing method for magnetic memory, which, however, is still premature for practical applications due to the challenge of the integration with magnetic tunnel junctions (MTJs). Here, we demonstrate a functional TI-MTJ device that could become the core element of the future energy-efficient spintronic devices, such as SOT-based magnetic random-access memory (SOT-MRAM). The state-of-the-art tunneling magnetoresistance (TMR) ratio of 102% and the ultralow switching current density of 1.2 × 105 A cm-2 have been simultaneously achieved in the TI-MTJ device at room temperature, laying down the foundation for TI-driven SOT-MRAM. The charge-spin conversion efficiency θSH in TIs is quantified by both the SOT-induced shift of the magnetic switching field (θSH = 1.59) and the SOT-induced ferromagnetic resonance (ST-FMR) (θSH = 1.02), which is one order of magnitude larger than that in conventional heavy metals. These results inspire a revolution of SOT-MRAM from classical to quantum materials, with great potential to further reduce the energy consumption.