Multiferroicity in atomic van der Waals heterostructures

Tunable magnetoelectricity and polarity in van der Waals antiferromagnetic CuCr1-xFexP2S6

The coexistence of electric and magnetic orders with intrinsic coupling, referred to as magnetoelectric coupling in multiferroics, has been extensively studied in oxide materials but remains relatively unexplored in van der Waals materials. Among these, CuCrP2S6 (CCPS) is notable for its emergent antiferromagnetic (AFM) and antiferroelectric (AFE) characteristics. However, investigations into magnetoelectric coupling in CCPS are limited, and the effects of dopants on its magnetic properties have yet to be fully addressed. In this study, we synthesized CuCr1-xFexP2S6 (CCFPS) samples using the chemical vapor transport (CVT) method to investigate the influence of iron doping on the magnetic and nonlinear optical properties of the CCFPS system. Our results indicate that the AFM state is preserved, while the Néel temperature (TN) varies with the doping concentration. First-principles calculations were employed to assess the exchange interactions among magnetic atoms. Notably, for samples with doping concentrations x < 0.5, we observed both magnetic-dielectric coupling and second harmonic generation (SHG) effects. However, these effects were absent at higher doping levels. Furthermore, our analysis revealed a distinct odd-even dependence of SHG, suggesting the presence of interlayer symmetry-breaking coupling. These findings advance our understanding of two-dimensional (2D) multiferroic materials and lay the groundwork for designing and optimizing magnetoelectric coupling materials with enhanced performance.

Imaging Domain Walls in van der Waals Ferroelectrics Using Tip-Enhanced Second Harmonic Generation

van der Waals ferroelectrics have captured significant interest for applications in novel memory and neuromorphic computing devices. Within these materials, interfaces between regions of different ferroelectric polarizations (i.e., domain walls) exhibit distinctive behaviors. Characterization of these behaviors remains a challenge, as domain wall thickness resides at the nanoscale. Using tip-enhanced second harmonic generation (TESHG), we demonstrate nanoscale imaging of domain walls in 2D ferroelectric α-In2Se3. With a narrow-band near-IR laser, we take advantage of high-wavelength, off-resonant signal enhancement to generate robust and reproducible TESHG. A 16 nm spatial resolution is achieved, and we identify spectral features suggesting that nonlocal effects from domain walls persist nanometers into adjacent domains.

Designing Chiral Organometallic Nanosheets with Room-Temperature Multiferroicity and Topological Nodes

Two-dimensional (2D) room-temperature chiral multiferroic and magnetic topological materials are essential for constructing functional spintronic devices, yet their number is extremely limited. Here, by using the chiral and polar HPP (HPP = 4-(3-hydroxypyridin-4-yl)pyridin-3-ol) as an organic linker and transition metals (TM = Cr, Mo, W) as nodes, we predict a class of 2D TM(HPP)2 organometallic nanosheets that incorporate homochirality, room-temperature magnetism, ferroelectricity, and topological nodes. The homochirality is introduced by chiral HPP linkers, and the change in structural chirality induces a topological phase transition of Weyl phonons. The room-temperature magnetism arises from the strong d-p spin coupling between TM cations and HPP doublet anions. The ferroelectricity is attributed to the breaking of spatial inversion symmetry in the lattice structure. Additionally, by adjusting the type of TMs, these nanosheets show rich and tunable band structures. Notably, all predicted materials are topologically nontrivial, featuring a quadratic nodal point around the Fermi level.

Realizing Intralayer Magnetoelectric Coupling in Two-Dimensional Frustrated Multiferroic Heterostructures

Recent studies have demonstrated the ability to switch weakly coupled interlayer magnetic orders by using electric polarization in insulating van der Waals heterostructures. However, controlling strongly coupled intralayer magnetic orders remains a significant challenge. In this work, we propose that frustrated multiferroic heterostructures can exhibit enhanced intralayer magnetoelectric coupling. Through first-principles calculations, we have investigated a heterostructure composed of MnBr2 and Nb3I8, wherein there is a competition between frustrated intralayer magnetic orders within the MnBr2 and interlayer magnetic coupling via a unique spin-local field effect. As a result, manipulating the vertical electric polarization of the Nb3I8 layer successfully controls the ground-state intralayer magnetic order in the frustrated MnBr2 layer, inducing transitions between zigzag antiferromagnetic and ferromagnetic orders. Our findings offer a novel approach to controlling intralayer spin structures, paving the way for advancements in spintronic applications in a single atomic layer, which cannot be achieved by interlayer magnetoelectric coupling.

First-principles investigation of half-metallic CaTGe2O6 (T = Mn, Fe, Co) clinopyroxenes: Potential for spintronics and optoelectronics applications

Clinopyroxene is a subgroup of pyroxene that is found in a variety of igneous and metamorphic rocks. This study investigates the structural, mechanical, electronic, optical, and thermal characteristics of CaTGe2O6 (T = Mn, Fe, Co) using density functional theory. These structures' lattice parameters have been optimized using GGA-PBE, GGA-PBESOL, and LDA exchange-correlation functionals, where all these structures are found to be stable in monoclinic symmetry having a little variation with experimental results. All the structures are mechanically stable and ductile in nature. The CaFeGe2O6 has the highest melting point and Debye temperature among the three structures. The electronic band diagram and spin-polarized PDOS of these structures confirm the half-metallic nature of all three structures. The half-metallic band gaps of CaTGe2O6 (T = Mn, Fe, Co) are 3.05, 1.69, 1.99 eV in GGA + U and 2.88, 1.52, 1.78 eV in the LDA + U method, respectively. CaMnGe2O6 is metallic in the spin-up state, whereas both CaFeGe2O6 and CaMnGe2O6 are metallic in the spin-down state. Refractive indices and dielectric functions of these structures ensure the transparency of CaTGe2O6 (T = Mn, Fe, Co) clinopyroxenes around 28.5 eV photon energy. These structures possess the highest reflectivity and absorption coefficients in the UV region. These salient features of these structures suggest that the spintronics and optoelectronics industries may benefit from these clinopyroxene structures in the future.

Recent Advances in Two-Dimensional Ferromagnetic Materials-Based van der Waals Heterostructures

Two-dimensional (2D) ferromagnetic materials are subjects of intense research owing to their intriguing physicochemical properties, which hold great potential for fundamental research and spintronic applications. Specifically, 2D van der Waals (vdW) ferromagnetic materials retain both structural integrity and chemical stability even at the monolayer level. Moreover, due to their atomic thickness, these materials can be easily manipulated by stacking them with other 2D vdW ferroic and nonferroic materials, enabling precise control over their physical properties and expanding their functional applications. Consequently, 2D vdW ferromagnetic materials-based heterostructures offer a platform to tailor magnetic properties and explore advanced spintronic devices. This review aims to provide an overview of recent developments in emerging 2D vdW ferromagnetic materials-based heterostructures and devices. The fabrication approaches for 2D ferromagnetic vdW heterostructures are primarily summarized, followed by a review of two categories of heterostructures: ferromagnetic/ferroic and ferromagnetic/nonferroic vdW heterostructures. Subsequently, the progress made in modulating magnetic properties and emergence of various phenomena in these heterostructures is highlighted. Furthermore, the applications of such heterostructures in spintronic devices are discussed along with their future perspectives and potential directions in this exciting field.

Suppressed Nonreciprocal Second-Harmonic Generation of Antiferromagnet MnPSe3 in the MnPSe3/Graphene Heterostructure Due to Interfacial Magnon-Plasmon Coupling

Interfacial coupling is one of the keys to manipulating magnetic/nonmagnetic two-dimensional (2D) heterostructures for novel functionalities. The MnPSe3/graphene heterostructure is a prospective platform for quantum information and metrology. However, how graphene affects MnPSe3 through interfacial coupling is still poorly understood. Herein, second-harmonic generation (SHG) of antiferromagnet MnPSe3 in the MnPSe3/graphene heterostructure is revealed. Surprisingly, it was found that below TN, nonreciprocal or c-type SHG of MnPSe3 disappeared when interfacing with the graphene, suggesting the existence of interfacial couplings and/or interactions. Most interestingly, different from the short-range interfacial proximity interaction, this interfacial interaction could be contactless and long-range and varied from 2D metallic/semiconducting to insulating underlayers. Interfacial magnon-plasmon coupling probably played an important role in suppressing the c-type SHG signals of MnPSe3 in the MnPSe3/graphene heterostructure. This study will improve our understanding of the manipulation of nonlinear optical properties in 2D heterostructures.

Engineering Two-Dimensional Magnetic Heterostructures: A Theoretical Perspective

Two-dimensional (2D) magnetic materials have attracted great attention due to their promise for applications in future high-speed, low-energy quantum computing and memory devices. By integrating 2D magnetic materials with other magnetic or nonmagnetic materials to form heterostructures, the synergistic effects of interlayer orbital hybridization, spin-orbit coupling, and symmetry breaking can surpass the performance of single-layer materials and lead to novel physical phenomena. This review provides a comprehensive theoretical analysis of engineering 2D magnetic heterostructures, emphasizing the fundamental physics of interlayer interactions and the resulting enhancements and novel properties. It reviews the mechanisms and progress in tuning the magnetic ordering, enhancing the Curie temperature (Tc) and modulating properties such as topological magnetic structures, spin polarization, electronic band topology, valley polarization, and magnetoelectric coupling through the construction of 2D magnetic heterostructures. Additionally, this review discusses the current challenges faced by 2D magnetic heterostructures, aiming to guide the future design of higher-performance magnetic heterostructures.

Magnetoelectric Tuning of 2D Ferromagnetism in 1T-CrTe2 through In2Se3 Substrate

Electric field control of two-dimensional (2D) materials with optimized magnetic properties is not only of scientific interest but also of technological importance in terms of the functionality of various nanoscale devices. Here, we report the multiferroic control of the 2D ferromagnetism in 1T-CrTe2 monolayer through a ferroelectric In2Se3 sublayer. Our results reveal the effect of polarization switching on the electronic structures and magnetic properties of 1T-CrTe2/In2Se3 heterostructures, enabling effective manipulation of their magnetic anisotropy energy (MAE) and magnetization orientation. Additionally, we also demonstrate the strong dependence of their MAE and switching effect on the external strain and surface hydrogenation. Notably, polarization switching exhibits a reversal modification in the hydrogenated multiferroic structures. These tunable behaviors are primarily attributed to the alteration of p-orbitals near the Fermi level of the interfacial Te atoms due to magnetoelectric coupling. Our findings suggest the potential of 1T-CrTe2/In2Se3 heterojunctions for the practical application of 2D multiferroic spintronic devices.

Photoresponsive Two-Dimensional Magnetic Junctions for Reconfigurable In-Memory Sensing

Magnetic tunneling junctions (MTJs) lie in the core of magnetic random access memory, holding promise in integrating memory and computing to reduce hardware complexity, transition latency, and power consumption. However, traditional MTJs are insensitive to light, limiting their functionality in in-memory sensing─a crucial component for machine vision systems in artificial intelligence applications. Herein, the convergence of magnetic memory with optical sensing capabilities is achieved in the all-two-dimensional (2D) magnetic junction Fe3GaTe2/WSe2/Fe3GaTe2, which combines 2D magnetism and optoelectronic properties. The clean intrinsic band gap and prominent photoresponse of interlayer WSe2 endow the tunneling barrier with optical tunability. The on-off states of junctions and the magnetoresistance can be flexibly controlled by the intensity of the optical signal at room temperature. Based on the optical-tunable magnetoresistance in all-2D magnetic junctions, a machine vision system with the architecture of in-memory sensing and computing is constructed, which possesses high performance in image recognition. Our work exhibits the advantages of 2D magneto-electronic devices and extends the application scenarios of magnetic memory devices in artificial intelligence.

Altermagnetism Induced by Sliding Ferroelectricity via Lattice Symmetry-Mediated Magnetoelectric Coupling

Altermagnets, distinct from conventional ferromagnets or antiferromagnets, have recently attracted attention as the third category of collinear magnets, which exhibit the coexistence of zero net magnetization and spin polarization due to their unique lattice symmetries. Meanwhile, the additional layer degrees of freedom in multilayer sliding ferroelectrics offer opportunities for coupling with lattice symmetries, paving the way for an innovative approach to constructing multiferroic lattices. In this study, altermagnetic tuning in SnS2/MnPSe3/SnS2 heterostructures is achieved by breaking and restoration of lattice inversion symmetry through sliding ferroelectric switching. First-principles calculations reveal that the spin density and corresponding time-reversal symmetry of MnPSe3 can be manipulated by lattice symmetry, triggering phase transitions between antiferromagnetism and altermagnetism. This research establishes a novel form of magnetoelectric coupling mediated by lattice symmetry and provides a theoretical basis for the design of miniature information processing and memory devices based on altermagnetism.

Ferroelectric Modulation of Quantum Emitters in Monolayer WS2

Quantum photonics promises significant advances in secure communications, metrology, sensing, and information processing/computation. Single-photon sources are fundamental to this endeavor. However, the lack of high-quality single photon sources remains a significant obstacle. We present here a paradigm for the control of single photon emitters (SPEs) and single photon purity by integrating monolayer WS2 with the organic ferroelectric polymer poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)). We demonstrate that the ferroelectric domains in the P(VDF-TrFE) film control the purity of single photon emission from the adjacent WS2. By switching the ferroelectric polarization, we reversibly tune the single photon purity between the semiclassical and quantum light regimes, with single photon purities as high as 94%. This demonstrates a method for modulating and encoding quantum photonic information, complementing more complex approaches. This multidimensional heterostructure introduces an approach for control of quantum emitters by combining the nonvolatile ferroic properties of a ferroelectric with the radiative properties of the zero-dimensional atomic-scale emitters embedded in the two-dimensional WS2 semiconductor monolayer.

Nonvolatile Electric Control of Rashba Spin Splitting in Sb/In2Se3 Heterostructure

Ferroelectric Rashba semiconductors (FRS) are highly demanded for their potential capability for nonvolatile electric control of electron spins. An ideal FRS is characterized by a combination of room temperature ferroelectricity and a strong Rashba effect, which has, however, been rarely reported. Herein, we designed a room-temperature FRS by vertically stacking a Sb monolayer on a room-temperature ferroelectric In2Se3 monolayer. Our first-principles calculations reveal that the Sb/In2Se3 heterostructure exhibits a clean Rashba splitting band near the Fermi level and a strong Rashba effect coupled to the ferroelectric order. Switching the electric polarization direction enhances the Rashba effect, and the flipping is feasible with a low energy barrier of 22 meV. This Rashba-ferroelectricity coupling effect is robust against changes of the heterostructure interfacial distance and external electric fields. Such a nonvolatile electrically tunable Rashba effect at room temperature enables potential applications in next-generation data storage and logic devices operated under small electrical currents.

Ferroelectrically controlled electromagnetic and transport properties of VN2H2/Al2O3 van der Waals multiferroic heterostructures

The vertical integration of a ferromagnetic monolayer and a ferroelectric monolayer into van der Waals heterostructures offers a promising route to achieve two-dimensional multiferroic semiconductors owing to the lack of intrinsic single-phase multiferroic materials in nature. In this study, we propose a VN2H2/Al2O3 van der Waals magnetoelectric multiferroic heterostructure and investigate its electronic, magnetic, and transport properties using density functional theory combined with the Boltzmann transport theory. The VN2H2 monolayer is a room-temperature ferromagnetic semiconductor with a band gap of 0.24 eV and a Curie temperature of 411 K, while the Al2O3 monolayer is a ferroelectric semiconductor with a polarization value of 0.11 C m-2. In the VN2H2/Al2O3 van der Waals heterostructures, the conversion between the metal and the semiconductor can be controlled by altering the polarization of the Al2O3 layer. The VN2H2/Al2O3 van der Waals heterostructure retains room-temperature ferromagnetism, and the reverse of polarization is accompanied with a change in the direction of the easy magnetization axis. In addition, electrostatic doping can significantly improve the conductivity of the downward polarization state and transform the upward polarization state from a metal to a half-metal, achieving 100% spin polarization. Our results thus pave the way for achieving highly tunable electromagnetic and transport properties in van der Waals magnetoelectric heterostructures, which have potential applications in next-generation low-power logic and memory devices.

Spin-Phonon Coupling and Magnetic Transition in an Organic Molecule Intercalated Cr2Ge2Te6

The manipulation of spin-phonon coupling in both formations and explorations of magnetism in two-dimensional van der Waals ferromagnetic semiconductors facilitates unprecedented prospects for spintronic devices. The interlayer engineering with spin-phonon coupling promises controllable magnetism via organic cation intercalation. Here, spectroscopic evidence reveals the intercalation effect on the intrinsic magnetic and electronic transitions in quasi-two-dimensional Cr2Ge2Te6 using tetrabutyl ammonium (TBA+) as the intercalant. The temperature evolution of Raman modes, Eg3 and Ag1, along with the magnetization measurements, unambiguously captures the enhancement of the ferromagnetic Curie temperature in the intercalated heterostructure. Moreover, the Eg4 mode highlights the increased effect of spin-phonon interaction in magnetic-order-induced lattice distortion. Combined with the first-principle calculations, we observed a substantial number of electrons transferred from TBA+ to Cr through the interface. The interplay between spin-phonon coupling and magnetic ordering in van der Waals magnets appeals for further understanding of the manipulation of magnetism in layered heterostructures.

Engineering interfacial polarization switching in van der Waals multilayers

In conventional ferroelectric materials, polarization is an intrinsic property limited by bulk crystallographic structure and symmetry. Recently, it has been demonstrated that polar order can also be accessed using inherently non-polar van der Waals materials through layer-by-layer assembly into heterostructures, wherein interfacial interactions can generate spontaneous, switchable polarization. Here we show that deliberate interlayer rotations in multilayer van der Waals heterostructures modulate both the spatial ordering and switching dynamics of polar domains. The engendered tunability is unparalleled in conventional bulk ferroelectrics or polar bilayers. By means of operando transmission electron microscopy we show how alterations of the relative rotations of three WSe2 layers produce structural polytypes with distinct arrangements of polar domains with either a global or localized switching response. Furthermore, the presence of uniaxial strain generates structural anisotropy that yields a range of switching behaviours, coercivities and even tunable biased responses. We also provide evidence of mechanical coupling between the two interfaces of the trilayer, a key consideration for the control of switching dynamics in polar multilayer structures more broadly.

Nonreciprocal and Nonvolatile Electric-Field Switching of Magnetism in van der Waals Heterostructure Multiferroics

Multiferroic materials provide robust and efficient routes for the control of magnetism by electric fields, which have been diligently sought after for a long time. Construction of two-dimensional (2D) vdW multiferroics is a more exciting endeavor. To date, the nonvolatile manipulation of magnetism through ferroelectric polarization still remains challenging in a 2D vdW heterostructure multiferroic. Here, we report a van der Waals (vdW) heterostructure multiferroic comprising the atomically thin layered antiferromagnet (AFM) CrI3 and ferroelectric (FE) α-In2Se3. We demonstrate anomalously nonreciprocal and nonvolatile electric-field control of magnetization by ferroelectric polarization. The nonreciprocal electric control originates from an intriguing antisymmetric enhancement of interlayer ferromagnetic coupling in the opposite ferroelectric polarization configurations of α-In2Se3. Our work provides numerous possibilities for creating diverse heterostructure multiferroics at the limit of a few atomic layers for multistage magnetic memories and brain-inspired in-memory computing.

Enhancing Single Photon Emission Purity via Design of van der Waals Heterostructures

Quantum emitters are essential components of quantum photonic circuitry envisioned beyond the current optoelectronic state-of-the-art. Two dimensional materials are attractive hosts for such emitters. However, the high single photon purity required is rarely realized due to the presence of spectrally degenerate classical light originating from defects. Here, we show that design of a van der Waals heterostructure effectively eliminates this spurious light, resulting in purities suitable for a variety of quantum technological applications. Single photon purity from emitters in monolayer WSe2 increases from 60% to 92% by incorporating this monolayer in a simple graphite/WSe2 heterostructure. Fast interlayer charge transfer quenches a broad photoluminescence background by preventing radiative recombination through long-lived defect bound exciton states. This approach is generally applicable to other 2D emitter materials, circumvents issues of material quality, and offers a path forward to achieve the ultrahigh single photon purities ultimately required for photon-based quantum technologies.

RNA velocity prediction via neural ordinary differential equation

RNA velocity is a crucial tool for unraveling the trajectory of cellular responses. Several approaches, including ordinary differential equations and machine learning models, have been proposed to interpret velocity. However, the practicality of these methods is constrained by underlying assumptions. In this study, we introduce SymVelo, a dual-path framework that effectively integrates high- and low-dimensional information. Rigorous benchmarking and extensive studies demonstrate that SymVelo is capable of inferring differentiation trajectories in developing organs, analyzing gene responses to stimulation, and uncovering transcription dynamics. Moreover, the adaptable architecture of SymVelo enables customization to accommodate intricate data and diverse modalities in forthcoming research, thereby providing a promising avenue for advancing our understanding of cellular behavior.

Switching Intrinsic Magnetic Skyrmions with Controllable Magnetic Anisotropy in van der Waals Multiferroic Heterostructures

Magnetic skyrmions, topologically nontrivial whirling spin textures at nanometer scales, have emerged as potential information carriers for spintronic devices. The ability to efficiently create and erase magnetic skyrmions is vital yet challenging for such applications. Based on first-principles studies, we find that switching between intrinsic magnetic skyrmion and high-temperature ferromagnetic states can be achieved in the two-dimensional van der Waals (vdW) multiferroic heterostructure CrSeI/In2Te3 by reversing the ferroelectric polarization of In2Te3. The core mechanism of this switching is traced to the controllable magnetic anisotropy of CrSeI influenced by the ferroelectric polarization of In2Te3. We propose a useful descriptor linking the presence of magnetic skyrmions to magnetic parameters and validate this connection through studies of a variety of similar vdW multiferroic heterostructures. Our work demonstrates that manipulating magnetic skyrmions via tunable magnetic anisotropies in vdW multiferroic heterostructures represents a highly promising and energy-efficient strategy for the future development of spintronics.

A Route to Two-Dimensional Room-Temperature Organometallic Multiferroics: The Marriage of d-p Spin Coupling and Structural Inversion Symmetry Breaking

Two-dimensional (2D) room-temperature multiferroic materials are highly desirable but still very limited. Herein, we propose a potential strategy to obtain such materials in 2D metal-organic frameworks (MOFs) by utilizing the d-p direct spin coupling in conjunction with center-symmetry-breaking six-membered heterocyclic rings. Based on this strategy, a screening of 128 2D MOFs results in the identification of three multiferroics, that is, Cr(1,2-oxazine)2, Cr(1,2,4-triazine)2, and Cr(1,2,3,4-trazine)2, simultaneously exhibiting room-temperature ferrimagnetism and ferroelectricity/antiferroelectricity. The room-temperature ferrimagnetic order (306-495 K) in these MOFs originates from the strong d-p direct magnetic exchange interaction between Cr cations and ligand anions. Specifically, Cr(1,2-oxazine)2 exhibits ferroelectric behavior with an out-of-plane polarization of 4.24 pC/m, whereas the other two manifest antiferroelectric characteristics. Notably, all three materials present suitable polarization switching barriers (0.18-0.31 eV). Furthermore, these MOFs are all bipolar magnetic semiconductors with moderate band gaps, in which the spin direction of carriers can be manipulated by electrical gating.

Coexisting Magnetism, Ferroelectric, and Ferrovalley Multiferroic in Stacking-Dependent Two-Dimensional Materials

Two-dimensional (2D) multiferroic materials have widespread application prospects in facilitating the integration and miniaturization of nanodevices. However, the magnetic, ferroelectric, and ferrovalley properties in one 2D material are rarely coupled. Here, we propose a mechanism for manipulating magnetism, ferroelectric, and valley polarization by interlayer sliding in a 2D bilayer material. Monolayer GdI2 is a ferromagnetic semiconductor with a valley polarization of up to 155.5 meV. More interestingly, the magnetism and valley polarization of bilayer GdI2 can be strongly coupled by sliding ferroelectricity, making these tunable and reversible. In addition, we uncover the microscopic mechanism of the magnetic phase transition by a spin Hamiltonian and electron hopping between layers. Our findings offer a new direction for investigating 2D multiferroic devices with implications for next-generation electronic, valleytronic, and spintronic devices.

Robust Magnetoelectric Coupling in FeTiO3/Ga2O3 Non-van der Waals Heterostructures

Magnetoelectric coupling represents a significant breakthrough for next-generation electronics, offering the ability to achieve nonvolatile magnetic control via electrical means. In this comprehensive investigation, leveraging first-principles calculations, we unveil a robust magnetoelectric coupling within multiferroic heterostructures (HSs) by ingeniously integrating a non-van der Waals (non-vdW) magnetic FeTiO3 monolayer with the ferroelectric (FE) Ga2O3. Diverging from conventional van der Waals (vdW) multiferroic HSs, the magnetic states of the FeTiO3 monolayer can be efficiently toggled between ferromagnetic (FM) and antiferromagnetic (AFM) configurations by reversing the polarization of the Ga2O3 monolayer. This intriguing phenomenon arises from polarization-dependent substantial interlayer electron transfers and the interplay between superexchange and direct-exchange magnetic couplings of the iron atoms. The carrier-mediated interfacial interactions induce crucial shifts in Fermi level positions, decisively imparting distinct electronic characteristics near the Fermi level of composite systems. These novel findings offer exciting prospects for the future of magnetoelectric technology.

Two-Dimensional Asymmetric Multiferroics: Unique Way toward Strong Magnetoelectric Coupling and Multistate Memory

Two-dimensional (2D) materials have provided a fascinating platform for exploring novel multiferroics and emergent magnetoelectric coupling mechanisms. Here, a novel 2D asymmetric multiferroic based on Janus 2D multiferroic MXene-analogous oxynitrides (InTlNO2) is presented by using first-principles calculations. We find three inequivalent phases for InTlNO2, including two metallic phases (p1 and p2) and one semiconducting phase (p3) with a band gap of 0.88 eV. All phases are room-temperature multiferroics with different Curie temperatures, leading to tunability by phase transitions. We show that there is a 90° rotation of the magnetic anisotropy easy axis between p1 and p2, where p1 favors the in-plane and p2 the out-of-plane easy axis. Therefore, the magnetic anisotropy can be tuned by reversing the out-of-plane polarization. Our strategy provides a unique way toward strong magnetoelectric coupling and multistate memory.

Nonvolatile Multistate Manipulation of Topological Magnetism in Monolayer CrI3 through Quadruple-Well Ferroelectric Materials

Nonvolatile multistate manipulation of two-dimensional (2D) magnetic materials holds promise for low dissipation, highly integrated, and versatile spintronic devices. Here, utilizing density functional theory calculations and Monte Carlo simulations, we report the realization of nonvolatile and multistate control of topological magnetism in monolayer CrI3 by constructing multiferroic heterojunctions with quadruple-well ferroelectric (FE) materials. The Pt2Sn2Te6/CrI3 heterojunction exhibits multiple magnetic phases upon modulating FE polarization states of FE layers and interlayer sliding. These magnetic phases include Bloch-type skyrmions and ferromagnetism, as well as a newly discovered topological magnetic structure. We reveal that the Dzyaloshinskii-Moriya interaction (DMI) induced by interfacial coupling plays a crucial role in magnetic skyrmion manipulation, which aligns with the Fert-Levy mechanism. Moreover, a regular magnetic skyrmion lattice survives when removing a magnetic field, demonstrating its robustness. The work sheds light on an effective approach to nonvolatile and multistate control of 2D magnetic materials.

Nonvolatile switchable half-metallicity and magnetism in the MXene Hf2MnC2O2/Sc2CO2 multiferroic heterostructure

Nonvolatile electrical control of two-dimensional (2D) van der Waals (vdW) magnetism is important for spintronic devices. Here, using first-principles calculations, we systematically investigated the magnetic properties of the MXene Hf2MnC2O2 combined with the ferroelectric MXene Sc2CO2. When flipping the electric polarization of Sc2CO2, a transition between a semiconductor and a half-metal occurs in the Hf2MnC2O2 monolayer. Moreover, the ferromagnetic exchange parameter J1 can be enhanced to 9.67 meV under polarized P↑ of Sc2CO2, much larger than those of the pristine Hf2MnC2O2 monolayer and Hf2MnC2O2/Sc2CO2-P↓. In addition, the easy magnetization axis of the Hf2MnC2O2 monolayer is also dependent on the polarization orientation of Sc2CO2. Our results indicate a multiferroic heterostructure based on MXenes, offering an effective way for obtaining nonvolatile electrical control of electronic and magnetic properties.

Computational Discovery of High-Temperature Ferromagnetic Semiconductor Monolayer H-MnN2

In the past few years, two-dimensional (2D) high-temperature ferromagnetic semiconductor (FMS) materials with novelty and excellent properties have attracted much attention due to their potential in spintronics applications. In this work, using first-principles calculations, we predict that the H-MnN2 monolayer with the H-MoS2-type structure is a stable intrinsic FMS with an indirect band gap of 0.79 eV and a high Curie temperature (Tc) of 380 K. The monolayer also has a considerable in-plane magnetic anisotropy energy (IMAE) of 1005.70 μeV/atom, including a magnetic shape anisotropy energy induced by the dipole-dipole interaction (shape-MAE) of 168.37 μeV/atom and a magnetic crystalline anisotropy energy resulting from spin-orbit coupling (SOC-MAE) of 837.33 μeV/atom. Further, based on the second-order perturbation theory, its in-plane SOC-MAE of 837.33 μeV/atom is revealed to mainly derive from the couplings of Mn-dxz,dyz and Mn-dx2-y2,dxy orbitals through Lz in the same spin channel. In addition, the biaxial strain and carrier doping can effectively tune the monolayer's magnetic and electronic properties. Such as, under the hole and few electrons doping, the transition from semiconductor to half-metal can be realized, and its Tc can go up to 520 and 620 K under 5% tensile strain and 0.3 hole doping, respectively. Therefore, our research will provide a new, promising 2D FMS for spintronics devices.

Anthraquinodimethane-Based Molecular Switches Tethered by Four-Arm Star-like Polymers

Molecular switches that reversibly change their structures and physical properties are important for applications such as sensing and information processing at molecular scales. In order to avoid the intermolecular aggregation that is often detrimental to the stimuli-responses of molecular switches, previous studies of molecular switches have been often conducted in dilute solutions which are difficult for applications in solid-state devices. Here we report molecular design and synthesis that integrates anthraquinodimethane as molecular switching units into polymers with amenable processibility in solid states. Optical and electron spin resonance characterizations indicate that the four-arm polymers of poly(ϵ-caprolactone) or poly(D,L-lactide) tethered from anthraquinodimethane slow down the dynamics of the conformational switching between the folded and the twisted conformations, enhance the photoluminescence in solid states and impart materials with a small energy gap from singlet ground state to thermally accessible triplet state.

Fully Electrically Controlled van der Waals Multiferroic Tunnel Junctions

The fully electrical control of the magnetic states in magnetic tunnel junctions is highly pursued for the development of the next generation of low-power and high-density information technology. However, achieving this functionality remains a formidable challenge at present. Here we propose an effective strategy by constructing a trilayer van der Waals multiferroic structure, consisting of CrI3-AgBiPSe6 and Cr2Ge2Te6-In2Se3, to achieve full-electrical control of multiferroic tunnel junctions. Within this structure, two different magnetic states of the magnetic bilayers (CrI3/Cr2Ge2Te6) can be modulated and switched in response to the polarization direction of the adjacent ferroelectric materials (AgBiPSe6/In2Se3). The intriguing magnetization reversal is mainly attributed to the polarization-field-induced band structure shift and interfacial charge transfer. On this basis, we further design two multiferroic tunnel junction devices, namely, graphene/CrI3-AgBiPSe6/graphene and graphene/Cr2Ge2Te6-In2Se3/graphene. In these devices, good interfacial Ohmic contacts are successfully obtained between the graphene electrode and the heterojunction, leading to an ultimate tunneling magnetoresistance of 9.3 × 106%. This study not only proposes a feasible strategy and identifies a promising candidate for achieving fully electrically controlled multiferroic tunnel junctions but also provides insights for designing other advanced spintronic devices.

Spin-Driven Ferroelectricity in Two-Dimensional Magnetic Heterostructures

Achieving magnetic control of ferroelectricity or electric control of magnetism is usually challenging in material systems as their magnetism and ferroelectricity have distinct fundamental origins and are subject to different symmetry constraints. However, such control has significant promise for a wide range of device applications. In this work, we employ first-principles density functional theory calculations to demonstrate the emergence of spin-driven ferroelectricity in a vertically stacked two-dimensional (2D) van der Waals magnetic heterostructure, formed by two ferromagnetic (FM) CrBr3 layers separated by an antiferromagnetic (AFM) MnPSe3 layer, delicately designed to be structurally inversion symmetric but magnetically asymmetric. The spin-induced out-of-plane electric polarization of the entire heterostructure can be reversibly controlled by an external magnetic field. We further validate the effectiveness of this design strategy in several other lattice-matched FM/AFM/FM heterostructures, thereby providing a novel family of multiferroic systems based on 2D materials.

Nonvolatile magnetoelectric coupling in two-dimensional van der Waals sandwich heterostructure CuInP2S6/MnCl3/CuInP2S6

Electrical control of magnetism is of great interest for low-energy-consumption spintronic applications. Due to the recent experimental breakthrough in two-dimensional materials, with the absence of hanging bonds on the surface and strong tolerance for lattice mismatch, heterogeneous integration of different two-dimensional materials provides a new opportunity for coupling between different physical properties. Here, we report the realization of nonvolatile magnetoelectric coupling in vdW sandwich heterostructure CuInP2S6/MnCl3/CuInP2S6. Using first-principles calculations, we reveal that when interfacing with ferroelectric CuInP2S6, the Dirac half-metallic state of monolayer MnCl3 will be destroyed. Moreover, depending on the electrically polarized direction of CuInP2S6, MnCl3 can be a half-metal or a ferromagnetic semiconductor. We unveil that the obtained ferromagnetic semiconductor in MnCl3 can be attributed to the different gain and loss of electrons on the two adjacent Mn atoms due to the sublattice symmetry broken by interlayer coupling. The effects of interfacial magnetoelectric coupling on magnetic anisotropy and ferromagnetic Curie temperature of MnCl3 are also investigated, and a multiferroic memory based on this model is designed. Our work not only provides a promising way to design nonvolatile electrical control of magnetism but also renders monolayer MnCl3 an appealing platform for developing low-dimensional memory devices.

Full electrical control of multiple resistance states in van der Waals sliding multiferroic tunnel junctions

The recent development of two-dimensional magnetic and sliding-ferroelectric van der Waals (vdW) materials opens a new way to realize vdW sliding multiferroic tunnel junctions (MFTJs) for low-power nonvolatile memory applications. Here, we propose and investigate full electrical control of four nonvolatile resistance states in sliding MFTJs, Au/CrI3/bilayer h-BN/CrI3-MnBi2Te4/Au, via first principles. We found four stable states associated with different polarization orientations in bilayer h-BN and magnetization alignment in two CrI3 magnetic layers, which can be controlled purely by electrical voltage and current, respectively. The MFTJ has a giant tunneling magnetoresistance (TMR) of ∼10 000% (2000% in the presence of SOC) and a sizeable tunneling electroresistance (TER) of ∼70%. The write performance is explored by spin-transfer-torque calculations which show an impressive low critical current (∼1.5 × 1010 A m-2) to switch the magnetization of the free layer of CrI3, while antiferromagnetic MnBi2Te4 pins the reference layer with a large interfacial exchange coupling.

Ferroic Phases in Two-Dimensional Materials

Two-dimensional (2D) ferroics, namely ferroelectric, ferromagnetic, and ferroelastic materials, are attracting rising interest due to their fascinating physical properties and promising functional applications. A variety of 2D ferroic phases, as well as 2D multiferroics and the novel 2D ferrovalleytronics/ferrotoroidics, have been recently predicted by theory, even down to the single atomic layers. Meanwhile, some of them have already been experimentally verified. In addition to the intrinsic 2D ferroics, appropriate stacking, doping, and defects can also artificially regulate the ferroic phases of 2D materials. Correspondingly, ferroic ordering in 2D materials exhibits enormous potential for future high density memory devices, energy conversion devices, and sensing devices, among other applications. In this paper, the recent research progresses on 2D ferroic phases are comprehensively reviewed, with emphasis on chemistry and structural origin of the ferroic properties. In addition, the promising applications of the 2D ferroics for information storage, optoelectronics, and sensing are also briefly discussed. Finally, we envisioned a few possible pathways for the future 2D ferroics research and development. This comprehensive overview on the 2D ferroic phases can provide an atlas for this field and facilitate further exploration of the intriguing new materials and physical phenomena, which will generate tremendous impact on future functional materials and devices.

First-principles investigations of metal-semiconductor MoSH@MoS2 van der Waals heterostructures

Two-dimensional (2D) metal-semiconductor heterostructures play a critical role in the development of modern electronics technology, offering a platform for tailored electronic behavior and enhanced device performance. Herein, we construct a novel 2D metal-semiconductor MoSH@MoS2 heterostructure and investigate its structures, electronic properties and contact characteristics using first-principles investigations. We find that the MoSH@MoS2 heterostructure exhibits a p-type Schottky contact, where the specific Schottky barrier height varies depending on the stacking configurations employed. Furthermore, the MoSH@MoS2 heterostructures possess low tunneling probabilities, indicating a relatively low electron transparency across all the patterns of the MoSH@MoS2 heterostructures. Interestingly, by modulating the electric field, it is possible to modify the Schottky barriers and achieve a transformation from a p-type Schottky contact into an n-type Schottky contact. Our findings pave the way for the development of advanced electronics technology based on metal-semiconductor MoSH@MoS2 heterostructures with enhanced tunability and versatility.

Voltage control of magnetism in Fe3-xGeTe2/In2Se3 van der Waals ferromagnetic/ferroelectric heterostructures

We investigate the voltage control of magnetism in a van der Waals (vdW) heterostructure device consisting of two distinct vdW materials, the ferromagnetic Fe3-xGeTe2 and the ferroelectric In2Se3. It is observed that gate voltages applied to the Fe3-xGeTe2/In2Se3 heterostructure device modulate the magnetic properties of Fe3-xGeTe2 with significant decrease in coercive field for both positive and negative voltages. Raman spectroscopy on the heterostructure device shows voltage-dependent increase in the in-plane In2Se3 and Fe3-xGeTe2 lattice constants for both voltage polarities. Thus, the voltage-dependent decrease in the Fe3-xGeTe2 coercive field, regardless of the gate voltage polarity, can be attributed to the presence of in-plane tensile strain. This is supported by density functional theory calculations showing tensile-strain-induced reduction of the magnetocrystalline anisotropy, which in turn decreases the coercive field. Our results demonstrate an effective method to realize low-power voltage-controlled vdW spintronic devices utilizing the magnetoelectric effect in vdW ferromagnetic/ferroelectric heterostructures.

Manipulating two-dimensional magnetic states via electric field and pressure

Topological spin configurations have been an intriguing topic due to the exotic transport properties and promising applications in spintronic devices. The discovery of two-dimensional (2D) magnetic materials such as CrI3 provides new platforms for manipulating magnetic structures. Here, by first-principles calculations and Monte Carlo methods, we investigated the exchange interaction and magnetic states of 2D van der Waals ferromagnetic/ferroelectric heterostructure CrI3/In2Se3. By switching the polarization in the ferroelectric In2Se3 layer under an electric field and changing the interlayer distance between CrI3 and In2Se3 under pressure, four spin configurations, ferromagnetic states, topological domain wall skyrmions, topological bimerons, and stripe domains can be realized. These striking tunable magnetic states can be understood from the Dzyaloshinskii-Moriya interaction and single-ion anisotropy parameters being modified by switching the polarization and changing the interlayer distance. Our results of controllable topological/non-topological spin states broaden the spin phenomena and potential of spintronic applications in van der Waals heterostructures.

2D Ferroic Materials for Nonvolatile Memory Applications

The emerging nonvolatile memory technologies based on ferroic materials are promising for producing high-speed, low-power, and high-density memory in the field of integrated circuits. Long-range ferroic orders observed in 2D materials have triggered extensive research interest in 2D magnets, 2D ferroelectrics, 2D multiferroics, and their device applications. Devices based on 2D ferroic materials and heterostructures with an atomically smooth interface and ultrathin thickness have exhibited impressive properties and significant potential for developing advanced nonvolatile memory. In this context, a systematic review of emergent 2D ferroic materials is conducted here, emphasizing their recent research on nonvolatile memory applications, with a view to proposing brighter prospects for 2D magnetic materials, 2D ferroelectric materials, 2D multiferroic materials, and their relevant devices.

Novel germanene-arsenene and germanene-antimonene lateral heterostructures: interline-dependent electronic and magnetic properties

Seamlessly stitching two-dimensional (2D) materials may lead to the emergence of novel properties triggered by the interactions at the interface. In this work, a series of 2D lateral heterostructures (LHSs), namely germanene-arsenene (Gem-As8-m) and germanene-antimonene (Gem-Sb8-m), are investigated using first-principles calculations. The results demonstrate a strong interline-dependence of the electronic and magnetic properties. Specifically, the LHS formation along an armchair line preserves the non-magnetic nature of the original materials. However, this is an efficient approach to open the electronic band gap of the germanene monolayer, where band gaps as large as 0.74 and 0.76 eV are induced for Ge2-As6 and Ge2-Sb6 LHSs, respectively. Meanwhile, magnetism may appear in the zigzag-LHSs depending on the chemical composition (m = 3, 4, 5, and 6 for germanene-arsenene and m = 2, 3, 4, 5, and 6 for germanene-antimonene), where total magnetic moments between 0.13 and 0.50 μB are obtained. Herein, magnetic properties are produced mainly by the spin-up state of Ge atoms at the interface, where a small contribution comes from As(Sb) atoms. Spin-resolved band structures show a multivalley profile in both the valence band and the conduction band with a topological insulator-like behavior, where the interface states are derived mainly from the interface Ge-pz state. The results introduce new 2D lateral heterostructures with novel electronic and magnetic properties to allow new functionalities, which could be further explored for optoelectronic and spintronic applications.

Spin-gapless van der Waals heterostructure for spin gating through magnetic injection devices

Spin-gapless semiconductors (SGSs) are new magnetic zero-bandgap materials whose band structure is extremely sensitive to external influences (pressure or magnetic fields) and have great potential for high-speed and low-energy spintronics applications. The first-principles method was used to systematically study the heterostructures constructed of an asymmetric surface-functionalized Janus MXene material, Cr₂NOF, and a two-dimensional hexagonal lattice (2DH) semiconductor material and to study the effects of the electronic structure, Curie temperature, magnetism, and the design of unusual band structures and magnetic injection in the bilayer to obtain an SGS structure. Through the design and construction of Cr₂NOF/2DH van der Waals heterojunction spintronic devices, the spin-filtering effect of the devices can reach 100%, especially, realizing spin gating through magnetic injection. We report the transport mechanism of the heterojunction spintronic devices to achieve the goal of a controllable optimization of the device functions, which provides a theoretical basis for the design of MXene van der Waals heterojunctions for high-efficiency and low-power-consumption spintronic devices.

Database Construction of Two-Dimensional Charged Building Blocks for Functional-Oriented Material Design

Databases for charge-neutral two-dimensional (2D) building blocks (BBs), i.e., 2D materials, have been built for years due to their applications in nanoelectronics. Though lots of solids are constructed from charged 2DBBs, a database for them is still missing. Here, we identify 1028 charged 2DBBs from Materials Project database using a topological-scaling algorithm. These BBs host versatile functionalities including superconductivity, magnetism, and topological properties. We construct layered materials by assembling these BBs considering valence state and lattice mismatch and predict 353 stable layered materials by high-throughput density functional theory calculations. These materials can not only inherit their functionalities but also show enhanced/emergent properties compared with their parent materials: CaAlSiF displays superconducting transition temperature higher than NaAlSi; Na₂CuIO₆ shows bipolar ferromagnetic semiconductivity and anomalous valley Hall effect that are absent in KCuIO₆; LaRhGeO possesses nontrivial band topology. This database expands the design space of functional materials for fundamental research and potential applications.

Recent progress in the theoretical design of two-dimensional ferroelectric materials

Two-dimensional (2D) ferroelectrics (FEs), which maintain stable electric polarization in ultrathin films, are a promising class of materials for the development of various miniature functional devices. In recent years, several 2D FEs with unique properties have been successfully fabricated through experiments. They have been found to exhibit some unique properties either by themselves or when they are coupled with other functional materials (e.g., ferromagnetic materials, materials with 5d electrons, etc.). As a result, several new types of 2D FE functional devices have been developed, exhibiting excellent performance. As a type of newly discovered 2D functional material, the number of 2D FEs and the exploration of their properties are still limited and this calls for further theoretical predictions. This review summarizes recent progress in the theoretical predictions of 2D FE materials and provides strategies for the rational design of 2D FE materials. The aim of this review is to provide guidelines for the design of 2D FE materials and related functional devices.

Polaritons in Van der Waals Heterostructures

2D monolayers supporting a wide variety of highly confined plasmons, phonon polaritons, and exciton polaritons can be vertically stacked in van der Waals heterostructures (vdWHs) with controlled constituent layers, stacking sequence, and even twist angles. vdWHs combine advantages of 2D material polaritons, rich optical structure design, and atomic scale integration, which have greatly extended the performance and functions of polaritons, such as wide frequency range, long lifetime, ultrafast all-optical modulation, and photonic crystals for nanoscale light. Here, the state of the art of 2D material polaritons in vdWHs from the perspective of design principles and potential applications is reviewed. Some fundamental properties of polaritons in vdWHs are initially discussed, followed by recent discoveries of plasmons, phonon polaritons, exciton polaritons, and their hybrid modes in vdWHs. The review concludes with a perspective discussion on potential applications of these polaritons such as nanophotonic integrated circuits, which will benefit from the intersection between nanophotonics and materials science.

Nonvolatile Electrical Control and Reversible Gas Capture by Ferroelectric Polarization Switching in 2D FeI2/In2S3 van der Waals Heterostructures

Nonvolatile electrical control is the core of future magnetoelectric nanodevices. In this work, we systematically explore both the electronic structures and transport properties of multiferroic van der Waals (vdW) heterostructures consisting of a ferromagnetic FeI₂ monolayer and a ferroelectric In₂S₃ monolayer using density functional theory and the nonequilibrium Green's function method. The results reveal that the FeI₂ monolayer can be reversibly switched between semiconducting and half-metallic properties by nonvolatile control of the In₂S₃ ferroelectric polarization states. Correspondingly, the proof-of-concept two-probe nanodevice based on the FeI₂/In₂S₃ vdW heterostructure exhibits a significant valving effect by modulating the ferroelectric switching. Moreover, it is also found that the preference of nitrogen-containing gases such as NH₃, NO, and NO₂ for adsorption on the surface of FeI₂/In₂S₃ vdW heterostructures strongly depends on the polarization direction of the ferroelectric layer. In particular, the FeI₂/In₂S₃ heterostructure shows reversible capture behavior for NH₃. As a result, the FeI₂/In₂S₃ vdW heterostructure-based gas sensor demonstrates high selectivity and sensitivity. These findings may open up a new route for the application of multiferroic heterostructures to spintronics, nonvolatile memories, and gas sensors.

Multiferroicity in a Two-Dimensional Non-van der Waals Crystal of AgCr2X4 (X = S or Se)

Two-dimensional (2D) intrinsic multiferroics have long been pursued not only for their potential technological applications but also as model systems for studying emergent quantum phenomena and coupling mechanisms between various order parameters in low-dimensional space. However, the realization of 2D multiferroics is still a challenge. In this paper, we reveal that 2D AgCr₂X₄ (X = S or Se) crystals, which have been synthesized from the non-van der Waals (non-vdW) AgCrX₂ bulk phase, are type I half-metallic/metallic multiferroics in which ferroelectricity and ferromagnetism coexist. The off-centering displacement of the Ag ion introduces out-of-plane polarization, and the magnetism originates from the interactions between Cr atoms. Remarkably, AgCr₂Se₄ shows topologically nontrivial spin textures, such as Meron pairs and Néel-type skyrmions, under suitable temperatures and magnetic fields. Our findings demonstrate that 2D multiferroics can be achieved from non-vdW materials and in turn open a new avenue for 2D multiferroics.

Giant ferroelectric polarization in a bilayer graphene heterostructure

At the interface of van der Waals heterostructures, the crystal symmetry and the electronic structure can be reconstructed, giving rise to physical properties superior to or absent in parent materials. Here by studying a Bernal bilayer graphene moiré superlattice encapsulated by 30°-twisted boron nitride flakes, we report an unprecedented ferroelectric polarization with the areal charge density up to 10¹³cm⁻², which is far beyond the capacity of a moiré band. The translated polarization ~5 pC m⁻¹ is among the highest interfacial ferroelectrics engineered by artificially stacking van der Waals crystals. The gate-specific ferroelectricity and co-occurring anomalous screening are further visualized via Landau levels, and remain robust for Fermi surfaces outside moiré bands, confirming their independence on correlated electrons. We also find that the gate-specific resistance hysteresis loops could be turned off by the other gate, providing an additional control knob. Furthermore, the ferroelectric switching can be applied to intrinsic properties such as topological valley current. Overall, the gate-specific ferroelectricity with strongly enhanced charge polarization may encourage more explorations to optimize and enrich this novel class of ferroelectricity, and promote device applications for ferroelectric switching of various quantum phenomena.

Interfacial ferroelectricity may emerge in moiré superlattices. Here, the authors find that the polarized charge is much larger than the capacity of the moiré miniband and the associated anomalous screening exists outside the band.

Integrating ferromagnetism and ferroelectricity in an iron chalcogenide monolayer: a first-principles study

Two-dimensional (2D) ferro-type materials have received great attention owing to the remarkable polarization effect in optoelectronics and spintronics. Using the first-principles method, the coupling between ferromagnetism and ferroelectricity is investigated in a multiferroic Janus 1T-FeSSe monolayer, which has a strong Stoner ferromagnetic ground state. The magnetic anisotropy energy (MAE) is apparently impacted by the out-of-plane asymmetry donated ferroelectricity, which is reflected by the asymmetry of the Z-MAE image. The easy magnetization axis of Janus FeSSe is the +y axis with a large MAE of 0.59 meV, rooting in unpaired d electrons of Fe atoms. The transformation of band splitting and Fermi surface can be effectively engineered by different magnetic polarization directions. The ferromagnetic (FM) coupling of the FeSSe monolayer is very robust under external strain within the range of -6% to 6%, while the strength of magnetic moment of Fe atoms and polarization are easily strain-engineered, the intrinsic mechanism of which can be elaborated by the GKA rules that depend on angles and distances. This multiferroic FeSSe monolayer provides a new platform for exploring the coupling of 2D ferromagnetism and ferroelectricity and designing low-dimensional multiferroic electronics.

Tuning of the Valley Structures in Monolayer In2Se3/WSe2 Heterostructures via Ferroelectricity

Recent findings of two-dimensional ferroelectric (FE) materials have enabled the integration of nonvolatile FE functions into device applications based on van der Waals (vdW) heterojunctions (HJs), resulting in versatile technological advances. In this paper, we report the results of direct probing of the electronic structures of In₂Se₃/WSe₂ heterostructures at the single-layer limit, where monolayer (ML)-In₂Se₃ was found to be either antiferroelectric (AFE, β') or ferroelectric (β*) at sufficiently low temperatures. A general type-II band alignment was revealed for this heterostructure. Moreover, we observed significant modulations of the valley structures of WSe₂, and in situ transformations between the FE and AFE In₂Se₃ phases demonstrated the dominant role of the polarizations in the top ML-In₂Se₃ layer. The observed phenomena can be attributed to the combination of both the linear and quadratic Stark shifts from the out-of-plane electric field, which has only been previously theoretically explored for ML-transition metal dichalcogenides (TMDs).

Ferroelectric control of band alignments and magnetic properties in the two-dimensional multiferroic VSe2/In2Se3

Our first-principles evidence shows that the two-dimensional (2D) multiferroic VSe₂/In₂Se₃experiences continuous change of electronic structures, i.e. with the change of the ferroelectric (FE) polarization of In₂Se₃, the heterostructure can possess type-I, -II, and -III band alignments. When the FE polarization points from In₂Se₃to VSe₂, the heterostructure has a type-III band alignment, and the charge transfer from In₂Se₃into VSe₂induces half-metallicity. With reversal of the FE polarization, the heterostructure enters the type-I band alignment, and the spin-polarized current is turned off. When the In₂Se₃is depolarized, the heterostructure has a type-II band alignment. In addition, influence of the FE polarization on magnetism and magnetic anisotropy energy of VSe₂was also analyzed, through which we reveal the interfacial magnetoelectric coupling effects. Our investigation about VSe₂/In₂Se₃predicts its wide applications in the fields of both 2D spintronics and multiferroics.

Remarkable ferroelectricity-modulated electronic and magnetic properties in a 2H-VS2/BiAlO3(0001) hybrid system

In the present work, a 2H-VS₂/BiAlO₃(0001) hybrid system is constructed to perform first-principles density functional theory (DFT) calculations. The results reveal that, in addition to the ionic-vdW interface coupling, the ferromagnetic semiconductive 2H-VS₂ monolayer on the ferroelectric BiAlO₃(0001) substrate exhibits n-type or p-type doping behavior and even half-metal characteristics. Furthermore, the magnetoelectric coefficient (α_S) for the 2H-VS₂/BiAlO₃(0001) structures can reach a value of 10^-10 G cm² V^-1 with ferroelectric polarization reversal. The estimated Curie temperatures (T_c) of the 2H-VS₂ monolayer on the BiAlO₃(0001) Z+ (positive), Z+↓ (polarization-reversed Z+), Z- (negative), and Z-↑ (polarization-reversed Z-) polar surfaces were found to be 176, 276, 266, and 87 K, respectively. This indicates that the magnetic properties of the 2H-VS₂ monolayer are remarkably tunable using a ferroelectric BiAlO₃(0001) knob. These important findings provide a distinctive treatment option for controllable and adjustable nanoelectronic, photoelectronic, and spintronic devices based on a 2H-VS₂ monolayer.