Tunable Magnetism in Atomically Thin Itinerant Antiferromagnet with Room-Temperature Ferromagnetic Order.

Addressing the need for modulated spin configurations is crucial, as they serve as the foundational building blocks for next-generation spintronics, particularly in atomically thin structures and at room temperature. In this work, we realize intrinsic ferromagnetism in monolayer flakes and tunable ferro-/antiferromagnetism in (Fe0.56Co0.44)5GeTe2 antiferromagnets. Remarkably, the ferromagnetic ordering (≥1 L) and antiferromagnetic ordering (≥4 L) remain discernible up to room temperature. The TC (∼310 K) of the monolayer flakes sets a record high for known exfoliated monolayer van der Waals magnets. Within the framework of A-type antiferromagnetism, a notable odd-even layer-number effect at elevated temperatures (T = 150 K) is observed. Of particular interest is the strong ferromagnetic order in even-layer flakes at low temperatures. The intricate interplay among magnetic field strength, layer number, and temperature gives rise to a diverse array of phenomena, holding promise not only for new physics but also for practical applications.

A correlated ferromagnetic polar metal by design.

Polar metals have recently garnered increasing interest because of their promising functionalities. Here we report the experimental realization of an intrinsic coexisting ferromagnetism, polar distortion and metallicity in quasi-two-dimensional Ca3Co3O8. This material crystallizes with alternating stacking of oxygen tetrahedral CoO4 monolayers and octahedral CoO6 bilayers. The ferromagnetic metallic state is confined within the quasi-two-dimensional CoO6 layers, and the broken inversion symmetry arises simultaneously from the Co displacements. The breaking of both spatial-inversion and time-reversal symmetries, along with their strong coupling, gives rise to an intrinsic magnetochiral anisotropy with exotic magnetic field-free non-reciprocal electrical resistivity. An extraordinarily robust topological Hall effect persists over a broad temperature-magnetic field phase space, arising from dipole-induced Rashba spin-orbit coupling. Our work not only provides a rich platform to explore the coupling between polarity and magnetism in a metallic system, with extensive potential applications, but also defines a novel design strategy to access exotic correlated electronic states.

Pure ZrO2 Ferroelectric Thin Film for Nonvolatile Memory and Neural Network Computing.

The recent discovery of ferroelectricity in pure ZrO2 has drawn much attention, but the information storage and processing performances of ferroelectric ZrO2-based nonvolatile devices remain open for further exploration. Here, a ZrO2 (∼8 nm)-based ferroelectric capacitor using RuO2 oxide electrodes is fabricated, and the ferroelectric orthorhombic phase evolution under electric field cycling is studied. A ferroelectric remnant polarization (2Pr) of >30 µC/cm2, leakage current density of ∼2.79 × 10-8 A/cm2 at 1 MV/cm, and estimated polarization retention of >10 years are achieved. When the ferroelectric capacitor is connected with a transistor, a memory window of ∼0.8 V and eight distinct states can be obtained in such a ferroelectric field-effect transistor (FeFET). Through the conductance manipulation of the FeFET, a high object image recognition accuracy of ∼93.32% is achieved on the basis of the CIFAR-10 dataset in the convolutional neural network (CNN) simulation, which is close to the result of ∼94.20% obtained by floating-point-based CNN software. These results demonstrate the potential of ferroelectric ZrO2 devices for nonvolatile memory and artificial neural network computing.

Polyphenylene Oxide Film Sandwiched between SiO2 Layers for High-Temperature Dielectric Energy Storage.

The commercial capacitor using dielectric biaxially oriented polypropylene (BOPP) can work effectively only at low temperatures (less than 105 °C). Polyphenylene oxide (PPO), with better heat resistance and a higher dielectric constant, is promising for capacitors operating at elevated temperatures, but its charge-discharge efficiency (η) degrades greatly under high fields at 125 °C. Here, SiO2 layers are magnetron sputtered on both sides of the PPO film, forming a composite material of SiO2/PPO/SiO2. Due to the wide bandgap and high Young's modulus of SiO2, the breakdown strength (Eb) of this composite material reaches 552 MV/m at 125 °C (PPO: 534 MV/m), and the discharged energy density (Ue) under Eb improves to 3.5 J/cm3 (PPO: 2.5 J/cm3), with a significantly enhanced η of 89% (PPO: 70%). Furthermore, SiO2/PPO/SiO2 can discharge a Ue of 0.45 J/cm3 with an η of 97% at 125 °C under 200 MV/m (working condition in hybrid electric vehicles) for 20,000 cycles, and this value is higher than the energy density (∼0.39 J/cm3 under 200 MV/m) of BOPP at room temperature. Interestingly, the metalized SiO2/PPO/SiO2 film exhibits valuable self-healing behavior. These results make PPO-based dielectrics promising for high-temperature capacitor applications.

Silicon-Compatible Ferroelectric Tunnel Junctions with a SiO2/Hf0.5Zr0.5O2 Composite Barrier as Low-Voltage and Ultra-High-Speed Memristors.

The big data era requires ultrafast, low-power, and silicon-compatible materials and devices for information storage and processing. Here, ferroelectric tunnel junctions (FTJs) based on SiO2/Hf0.5Zr0.5O2 composite barrier and both conducting electrodes are designed and fabricated on Si substrates. The FTJ achieves the fastest write speed of 500 ps under 5 V (2 orders of magnitude faster than reported silicon-compatible FTJs) or 10 ns speed at a low voltage of 1.5 V (the lowest voltage among FTJs at similar speeds), low write current density of 1.3 × 104 A cm-2, 8 discrete states, good retention > 105 s at 85 °C, and endurance > 107. In addition, it provides a large read current (88 A cm-2) at 0.1 V, 2 orders of magnitude larger than reported FTJs. Interestingly, in FTJ-based synapses, gradually tunable conductance states (128 states) with high linearity (<1) are obtained by 10 ns pulses of <1.2 V, and a high accuracy of 91.8% in recognizing fashion product images is achieved by online neural network simulations. These results highlight that silicon-compatible HfO2-based FTJs are promising for high-performance nonvolatile memories and electrical synapses.

γ-Ray Irradiation Significantly Enhances Capacitive Energy Storage Performance of Polymer Dielectric Films.

Polymer dielectric capacitors are fundamental in advanced electronics and power grids but suffer from low energy density, hindering miniaturization of compact electrical systems. It is shown that high-energy and strong penetrating γ-irradiation significantly enhances capacitive energy storage performance of polymer dielectrics. γ-irradiated biaxially oriented polypropylene (BOPP) films exhibit an extraordinarily high energy density of 10.4 J cm-3 at 968 MV m-1 with an efficiency of 97.3%. In particular, an energy density of 4.06 J cm-3 with an ultrahigh efficiency of 98% is reliably maintained through 20 000 charge-discharge cycles under 600 MV m-1. At 125 °C, the γ-irradiated BOPP film still delivers a high discharged energy density of 5.88 J cm-3 with an efficiency of 90% at 770 MV m-1. Substantial improvements are also achieved for γ-irradiated cycloolefin copolymers at a high temperature of 150 °C, verifying the strategy generalizability. Experimental and theoretical analyses reveal that the excellent performance should be related to the γ-irradiation induced polar functional groups with high electron affinity in the molecular chain, which offer deep energy traps to impede charge transport. This work provides a simple and generally applicable strategy for developing high-performance polymer dielectrics.

Emergent zero-field anomalous Hall effect in a reconstructed rutile antiferromagnetic metal.

The anomalous Hall effect (AHE) that emerges in antiferromagnetic metals shows intriguing physics and offers numerous potential applications. Magnets with a rutile crystal structure have recently received attention as a possible platform for a collinear-antiferromagnetism-induced AHE. RuO2 is a prototypical candidate material, however the AHE is prohibited at zero field by symmetry because of the high-symmetry [001] direction of the Néel vector at the ground state. Here, we show AHE at zero field in Cr-doped rutile, Ru0.8Cr0.2O2. The magnetization, transport and density functional theory calculations indicate that appropriate doping of Cr at Ru sites reconstructs the collinear antiferromagnetism in RuO2, resulting in a rotation of the Néel vector from [001] to [110] while maintaining a collinear antiferromagnetic state. The AHE with vanishing net moment in the Ru0.8Cr0.2O2 exhibits an orientation dependence consistent with the [110]-oriented Hall vector. These results demonstrate that material engineering by doping is a useful approach to manipulate AHE in antiferromagnetic metals.

Ultralow Subthreshold Swing of a MOSFET Caused by Ferroelectric Polarization Reversal of Hf0.5Zr0.5O2 Thin Films.

The emergence of complementary metal-oxide semiconductor (CMOS)-compatible HfO2-based ferroelectric materials provides a promising way to achieve ferroelectric field-effect transistors (FeFETs) with a steep subthreshold swing (SS) reduced to below the Boltzmann thermodynamics limit (∼60 mV/dec at room temperature), which has important implications for lowering power consumption. In this work, a metal-oxide-semiconductor field-effect transistor (MOSFET) is connected with Hf0.5Zr0.5O2 (HZO)-based ferroelectric capacitors with different capacitances. By adjusting the capacitance of ferroelectric capacitors, an ultralow SS of ∼0.34 mV/dec in HfO2-based FeFETs can be achieved. More interestingly, by designing the sweeping voltage sequences, the SS can be adjusted to be 0 mV/dec with the drain current ranging over six orders of magnitude, and the threshold voltage for turning on the MOSFET can be further reduced. The manipulated SS could be attributed to the evolution of ferroelectric switching. Our work contributes to understanding the origin of ultralow SS in ferroelectric MOSFETs and the realization of low-power devices.

Artificially controlled nanoscale chemical reduction in VO2 through electron beam illumination.

Chemical reduction in oxides plays a crucial role in engineering the material properties through structural transformation and electron filling. Controlling the reduction at nanoscale forms a promising pathway to harvest functionalities, which however is of great challenge for conventional methods (e.g., thermal treatment and chemical reaction). Here, we demonstrate a convenient pathway to achieve nanoscale chemical reduction for vanadium dioxide through the electron-beam illumination. The electron beam induces both surface oxygen desorption through radiolytic process and positively charged background through secondary electrons, which contribute cooperatively to facilitate the vacancy migration from the surface toward the sample bulk. Consequently, the VO2 transforms into a reduced V2O3 phase, which is associated with a distinct insulator to metal transition at room temperature. Furthermore, this process shows an interesting facet-dependence with the pronounced transformation observed for the c-facet VO2 as compared with the a-facet, which is attributed to the intrinsically different oxygen vacancy formation energy between these facets. Remarkably, we readily achieve a lateral resolution of tens nanometer for the controlled structural transformation with a commercial scanning electron microscope. This work provides a feasible strategy to manipulate the nanoscale chemical reduction in complex oxides for exploiting functionalities.

Electric-field control of reversible electronic and magnetic transitions in two-dimensional oxide monolayer magnets.

Atomically thin oxide magnetic materials are highly desirable due to the promising potential to integrate two-dimensional (2D) magnets into next-generation spintronics. Therefore, 2D oxide magnetism is expected to be effectively tuned by the magnetic and electrical fields, holding prospective for future low-dissipation electronic devices. However, the electric-field control of 2D oxide monolayer magnetism has rarely been reported. Here, we present the realization of 2D monolayer magnetism in oxide (SrRuO3)1/(SrTiO3)N (N = 1, 3) superlattices that shows an efficient and reversible phase transition through electric-field controlled proton (H+) evolution. By using ionic liquid gating to modulate the proton concentration in (SrRuO3)1/(SrTiO3)1 superlattice, an electric-field induced metal-insulator transition was observed, along with gradually suppressed magnetic ordering and modulated magnetic anisotropy. Theoretical analysis reveals that proton intercalation plays a crucial role in both electronic and magnetic phase transitions. Strikingly, SrTiO3 layers can act as a proton sieve, which have a significant influence on proton evolution. Our work stimulates the tuning functionality of 2D oxide monolayer magnetism by voltage control, providing potential for future energy-efficient electronics.

Nanoscale multistate resistive switching in WO3 through scanning probe induced proton evolution.

Multistate resistive switching device emerges as a promising electronic unit for energy-efficient neuromorphic computing. Electric-field induced topotactic phase transition with ionic evolution represents an important pathway for this purpose, which, however, faces significant challenges in device scaling. This work demonstrates a convenient scanning-probe-induced proton evolution within WO3, driving a reversible insulator-to-metal transition (IMT) at nanoscale. Specifically, the Pt-coated scanning probe serves as an efficient hydrogen catalysis probe, leading to a hydrogen spillover across the nano junction between the probe and sample surface. A positively biased voltage drives protons into the sample, while a negative voltage extracts protons out, giving rise to a reversible manipulation on hydrogenation-induced electron doping, accompanied by a dramatic resistive switching. The precise control of the scanning probe offers the opportunity to manipulate the local conductivity at nanoscale, which is further visualized through a printed portrait encoded by local conductivity. Notably, multistate resistive switching is successfully demonstrated via successive set and reset processes. Our work highlights the probe-induced hydrogen evolution as a new direction to engineer memristor at nanoscale.

Coexistence of Both Localized Electronic States and Electron Gas at Rutile TiO2 Surfaces.

Localized electron polarons formed through the coupling of excess electrons and ionic vibrations play a key role in the functionalities of materials. However, the mechanism of the coexistence of delocalized electrons and localized polarons remains underexplored. Here, the discovery of high-mobility 2D electron gas at the rutile TiO2 surfaces through argon ion irradiation induced oxygen vacancies is reported. Strikingly, the electron gas forms localized electronic states at lower temperatures, resulting in an abrupt metal-insulator transition. Moreover, it is found that the low-temperature conductivity in the insulating state is dominated by excess free electrons with a high mobility of ≈103 cm2 V-1 s-1 , whereas the carrier density is dramatically suppressed with decreasing temperature. Remarkably, it reveals that the application of an electric field can lead to a collapse of the localized states, resulting in a metallic state. These results reveal the strongly correlated/coupled nature between the localized electrons and high-mobility electrons and offer a new pathway to probe and harvest the exotic electron states at the complex oxide surfaces.

Significantly enhanced high-temperature capacitive energy storage in cyclic olefin copolymer dielectric films via ultraviolet irradiation.

Polymer dielectrics with high operation temperature (∼150 °C) and excellent capacitive energy storage performance are vital for electric power systems and advanced electronic devices. Here, a very convenient and competitive strategy by preparing ultraviolet-irradiated cyclic olefin copolymer films is demonstrated to be effective in improving the energy storage performance at high temperatures. Compared with the unirradiated film, irradiated films exhibit a higher dielectric constant, higher breakdown strength and stronger mechanical properties as a result of the emergence of the carbonyl group and cross-linking network. Consequently, with a high efficiency above 95%, a superior discharged energy density of ∼3.34 J cm-3 is achieved at 150 °C, surpassing the current dielectric polymers and polymer nanocomposites. In particular, the energy storage performance remains highly reliable over 20 000 cycles under actual operating conditions (200 MV m-1 at 150 °C) in hybrid electric vehicles. This research offers a valuable pathway to build high-energy-density polymer-based capacitor devices working under harsh environments.

Manipulating the insulator-metal transition through tip-induced hydrogenation.

Manipulating the insulator-metal transition in strongly correlated materials has attracted a broad range of research activity due to its promising applications in, for example, memories, electrochromic windows and optical modulators1,2. Electric-field-controlled hydrogenation using ionic liquids3-6 and solid electrolytes7-9 is a useful strategy to obtain the insulator-metal transition with corresponding electron filling, but faces technical challenges for miniaturization due to the complicated device architecture. Here we demonstrate reversible electric-field control of nanoscale hydrogenation into VO2 with a tunable insulator-metal transition using a scanning probe. The Pt-coated probe serves as an efficient catalyst to split hydrogen molecules, while the positive-biased voltage accelerates hydrogen ions between the tip and sample surface to facilitate their incorporation, leading to non-volatile transformation from insulating VO2 into conducting HxVO2. Remarkably, a negative-biased voltage triggers dehydrogenation to restore the insulating VO2. This work demonstrates a local and reversible electric-field-controlled insulator-metal transition through hydrogen evolution and presents a versatile pathway to exploit multiple functional devices at the nanoscale.

Manipulating Berry curvature of SrRuO3 thin films via epitaxial strain.

Berry curvature plays a crucial role in exotic electronic states of quantum materials, such as the intrinsic anomalous Hall effect. As Berry curvature is highly sensitive to subtle changes of electronic band structures, it can be finely tuned via external stimulus. Here, we demonstrate in SrRuO3 thin films that both the magnitude and sign of anomalous Hall resistivity can be effectively controlled with epitaxial strain. Our first-principles calculations reveal that epitaxial strain induces an additional crystal field splitting and changes the order of Ru d orbital energies, which alters the Berry curvature and leads to the sign and magnitude change of anomalous Hall conductivity. Furthermore, we show that the rotation of the Ru magnetic moment in real space of a tensile-strained sample can result in an exotic nonmonotonic change of anomalous Hall resistivity with the sweeping of magnetic field, resembling the topological Hall effect observed in noncoplanar spin systems. These findings not only deepen our understanding of anomalous Hall effect in SrRuO3 systems but also provide an effective tuning knob to manipulate Berry curvature and related physical properties in a wide range of quantum materials.

Reversible manipulation of the magnetic state in SrRuO3 through electric-field controlled proton evolution.

Ionic substitution forms an essential pathway to manipulate the structural phase, carrier density and crystalline symmetry of materials via ion-electron-lattice coupling, leading to a rich spectrum of electronic states in strongly correlated systems. Using the ferromagnetic metal SrRuO3 as a model system, we demonstrate an efficient and reversible control of both structural and electronic phase transformations through the electric-field controlled proton evolution with ionic liquid gating. The insertion of protons results in a large structural expansion and increased carrier density, leading to an exotic ferromagnetic to paramagnetic phase transition. Importantly, we reveal a novel protonated compound of HSrRuO3 with paramagnetic metallic as ground state. We observe a topological Hall effect at the boundary of the phase transition due to the proton concentration gradient across the film-depth. We envision that electric-field controlled protonation opens up a pathway to explore novel electronic states and material functionalities in protonated material systems.

Enhancement of superconductivity in organic-inorganic hybrid topological materials.

Inducing or enhancing superconductivity in topological materials is an important route toward topological superconductivity. Reducing the thickness of transition metal dichalcogenides (e.g. WTe2 and MoTe2) has provided an important pathway to engineer superconductivity in topological matters. However, such monolayer sample is difficult to obtain, unstable in air, and with extremely low Tc. Here we report an experimentally convenient approach to control the interlayer coupling to achieve tailored topological properties, enhanced superconductivity and good sample stability through organic-cation intercalation of the Weyl semimetals MoTe2 and WTe2. The as-formed organic-inorganic hybrid crystals are weak topological insulators with enhanced Tc of 7.0 K for intercalated MoTe2 (0.25 K for pristine crystal) and 2.3 K for intercalated WTe2 (2.8 times compared to monolayer WTe2). Such organic-cation intercalation method can be readily applied to many other layered crystals, providing a new pathway for manipulating their electronic, topological and superconducting properties.

Manipulate the Electronic and Magnetic States in NiCo2 O4 Films through Electric-Field-Induced Protonation at Elevated Temperature.

Ionic-liquid-gating- (ILG-) induced proton evolution has emerged as a novel strategy to realize electron doping and manipulate the electronic and magnetic ground states in complex oxides. While the study of a wide range of systems (e.g., SrCoO2.5 , VO2 , WO3 , etc.) has demonstrated important opportunities to incorporate protons through ILG, protonation remains a big challenge for many others. Furthermore, the mechanism of proton intercalation from the ionic liquid/solid interface to whole film has not yet been revealed. Here, with a model system of inverse spinel NiCo2 O4 , an increase in system temperature during ILG forms a single but effective method to efficiently achieve protonation. Moreover, the ILG induces a novel phase transformation in NiCo2 O4 from ferrimagnetic metallic into antiferromagnetic insulating with protonation at elevated temperatures. This study shows that environmental temperature is an efficient tuning knob to manipulate ILG-induced ionic evolution.

Influence of In-Gap States on the Formation of Two-Dimensional Election Gas at ABO3/SrTiO3 Interfaces.

We explored in-gap states (IGSs) in perovskite oxide heterojunction films. We report that IGSs in these films play a crucial role in determining the formation and properties of interfacial two-dimensional electron gas (2DEG). We report that electron trapping by IGSs opposes charge transfer from the film to the interface. The IGS in films yielded insulating interfaces with polar discontinuity and explained low interface carrier density of conducting interfaces. An ion trapping model was proposed to explain the physics of the IGSs and some experimental findings, such as the unexpected formation of 2DEG at the initially insulating LaCrO3/SrTiO3 interface and the influence of substitution layers on 2DEG.

Electric-Field-Controlled Phase Transformation in WO3 Thin Films through Hydrogen Evolution.

Field-effect transistors with ionic-liquid gating (ILG) have been widely employed and have led to numerous intriguing phenomena in the last decade, due to the associated excellent carrier-density tunability. However, the role of the electrochemical effect during ILG has become a heavily debated topic recently. Herein, using ILG, a field-induced insulator-to-metal transition is achieved in WO3 thin films with the emergence of structural transformations of the whole films. The subsequent secondary-ion mass spectrometry study provides solid evidence that electrochemically driven hydrogen evolution dominates the discovered electrical and structural transformation through surface absorption and bulk intercalation.