Artificial Multiferroics and Enhanced Magnetoelectric Effect in van der Waals Heterostructures

Raman Fingerprints of Phase Transitions and Ferroic Couplings in van der Waals Multiferroic CuCrP2S6

CuCrP2S6 (CCPS), a type-II multiferroic material, exhibits unique phase transitions involving ferroelectric, antiferroelectric, and antiferromagnetic ordering. In this study, we conduct a comprehensive investigation on the intricate phase transitions and their multiferroic couplings in CCPS across a wide temperature range from 4 to 345 K through Raman spectroscopic measurements down to 5 cm-1. We first assign the observed Raman modes with the support of theoretical calculations and angle-resolved polarized Raman measurements. We further present clear signatures of phase transitions from the analyses of temperature-dependent Raman spectral parameters. Particularly, two low-frequency soft modes are observed at 36.1 cm-1 and 70.5 cm-1 below 145 K, indicating the antiferroelectric to quasi-antiferroelectric transition. Moreover, phonon mode hardening is observed when the temperature increases from 4 to 65 K, suggesting negative thermal expansion (NTE) and strong magnetoelastic coupling below 65 K. These findings advance the understanding of vdW multiferroic CCPS, paving the way for future design and engineering of multiferroicity in cutting-edge technologies, such as spintronics and quantum devices.

Emerging Physics in Magnetic Organic-Inorganic Hybrid Systems

The hybrid magnetic heterostructures and superlattices, composed of organic and inorganic materials, have shown great potential for quantum computing and next-generation information technology. Organic materials generally possess designable structural motifs and versatile optical, electronic, and magnetic properties, but are too delicate for robust integration into solid-state devices. In contrast, inorganic systems provide robust solid-state interface and excellent electronic properties but with limited customization space. Combining these two systems and taking respective advantages to exploit exotic physical properties has been a promising research direction but with tremendous challenges. Herein, we review the material preparation methods and discuss the emerging physical properties discovered in such magnetic organic-inorganic hybrid systems (MOIHSs), including recent progress on designable magnetic property modification, exchange bias effect, and the interplay of ferromagnetism and superconductivity, which provide a promising material platform for emerging magnetic memory and spintronic device applications.

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.

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.

Robust Electric-Field Control of Colossal Exchange Bias in CrI3 Homotrilayer

Controlling exchange bias (EB) by electric fields is crucial for next-generation magnetic random access memories and spintronics with ultralow energy consumption and ultrahigh speed. Multiferroic heterostructures have been traditionally used to electrically control EB and interfacial ferromagnetism through weak/indirect coupling between ferromagnetic and ferroelectric films. However, three major bottlenecks (lattice mismatch, interface defects, and weak/indirect coupling in multiferroic heterostructures) remain, resulting in only a few tens of milli-tesla EB field. Here, this study reports a robust electric-field control recipe to dynamically tailor the EB effect in a pure CrI3 homotrilayer on a ferroelectric Y-doped HfO2 (HYO) substrate, and demonstrate a colossal and tunable EB field (HE) from -0.15 to +0.33 T, giving rise to an EB modulation of 0.48 T. The charge doping due to ferroelectric HYO film divides a homo-configuration of CrI3 homotrilayer into one antiferromagnetic (AFM) bilayer CrI3 and one ferromagnetic (FM) monolayer CrI3, favoring direct exchange coupling. The synergies of charge doping and electric field induce a transition of magnetic orders from AFM to FM phase in bilayer CrI3, which is also supported by first-principles calculations, leading to the robust electric control of colossal EB effect. The results therefore open numerous opportunities for exploring 2D spintronics, memories, and braininspired in-memory computing.

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.

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.

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.

Electrically Controlled Anomalous Hall Effect and Orbital Magnetization in Topological Magnet MnBi_{2}Te_{4}

We propose an intrinsic mechanism to understand the even-odd effect, namely, opposite signs of anomalous Hall resistance and different shapes of hysteresis loops for even and odd septuple layers (SLs), of MBE-grown MnBi_{2}Te_{4} thin films with electron doping. The nonzero hysteresis loops in the anomalous Hall effect and magnetic circular dichroism for even-SLs MnBi_{2}Te_{4} films originate from two different antiferromagnetic (AFM) configurations with different zeroth Landau level energies of surface states. The complex form of the anomalous Hall hysteresis loop can be understood from two magnetic transitions, a transition between two AFM states followed by a second transition to the ferromagnetic state. Our model also clarifies the relationship and distinction between axion parameter and magnetoelectric coefficient, and shows an even-odd oscillation behavior of magnetoelectric coefficients in MnBi_{2}Te_{4} films.

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.

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.

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.

Spin-splitting and switchable half-metallicity in a van der Waals multiferroic CuBiP2Se6/GdClBr heterojunction

Multiferroic van der Waals (vdW) heterojunctions have a strong and nonvolatile magnetoelectric coupling effect, which is of great significance in spintronic devices. The electronic structure and magnetic properties of a GdClBr/CuBiP₂Se₆ vdW multiferroic heterojunction have been calculated using first-principles methods. Due to the spin-up charge transfer and Zeeman field, the ferroelectric CuBiP₂Se₆ exhibits spin splitting at the gamma point. It is found that the electronic structure and magnetic properties of the GdClBr/CuBiP₂Se₆ vdW multiferroic heterojunction have been significantly modulated by the electric polarization of CuBiP₂Se₆. During the reversal of the ferroelectric polarization of CuBiP₂Se₆, the ferromagnetic GdClBr monolayer transforms from a semiconductor to a half-metal. Meanwhile, in both upward and downward ferroelectric polarization, the GdClBr/CuBiP₂Se₆ heterojunction exhibits perpendicular magnetic anisotropy with a Curie temperature of 239 K. As the strain changes from -6% to 6%, the band structure of GdClBr shifts upward, and the band structure of CuBiP₂Se₆ shifts downward. Compressive strain can increase the Curie temperature of the GdClBr/CuBiP₂Se₆ heterojunction. The magnetic anisotropy of heterojunctions highly depends on biaxial strain, where the perpendicular (in-plane) magnetic anisotropy increases with the increased compressive (tensile) strain. The vdW multiferroic GdClBr/CuBiP₂Se₆ heterojunction has potential applications in spintronic devices.

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.

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.

First-principles prediction of two-dimensional MnOX (X = Cl, Br) monolayers: the half-metallic multiferroics with magnetoelastic coupling

Two-dimensional (2D) multiferroics have attracted extensive attention in recent years due to their potential applications in nano-electrical devices such as nonvolatile memory and magnetic sensors. However, 2D multiferroic materials with intrinsic ferromagnetism and ferroelasticity are very rare and most of them have low Curie temperatures. Herein, by performing the first-principles calculations, we systematically investigated the electronic structure and the magnetic properties of the MnOX (X = Cl, Br) monolayers. We demonstrated that the MnOX monolayers were intrinsic half-metallic multiferroics with the coexistence of ferromagnetism and ferroelasticity. The Curie temperatures evaluated from Monte Carlo simulations based on the Heisenberg model were about 220 K for MnOCl and 210 K for MnOBr, which could be further enhanced to 235 K and 230 K by 3% tensile strain. Moreover, their ground states exhibited significant big magnetic anisotropy energies of about 0.59 meV along the z-axis for MnOCl and 0.62 meV along the y-axis for MnOBr per unit cell. The in-plane magnetic easy axis of the MnOBr monolayer can be modulated by the ferroelastic switching due to the robust magnetoelastic coupling. These findings highlight that the MnOX monolayers (with 100% spin polarizability and high Curie temperature) are good candidates for next-generation multifunctional nanodevices.

The enhanced effect of magnetism on the thermoelectric performance of a CrI3 monolayer

The effect of magnetism on the thermoelectric (TE) transformation efficiency has recently attracted a lot of attention. A CrI₃ monolayer is a two-dimensional (2D) ferromagnetic (FM) semiconductor with a Curie temperature of 45 K. In this work, we employed first-principles calculations within the framework of density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) method and Landauer-Buttiker theory to study the effect of magnetism on the TE performance of a CrI₃ monolayer. The stability, electronic structures, density of states (DOS) and TE parameters of a CrI₃ monolayer are calculated. Our calculation results indicate that the TE performance of a CrI₃ monolayer in a FM state is superior to that in a non-magnetic (NM) state. Namely, magnetism is beneficial to improving the TE performance. To further investigate the physical mechanism, the phonon group velocity, the electronic and phonon transmission spectra and the effective mass of a CrI₃ monolayer in FM and NM states are analyzed in detail. For a CrI₃ monolayer in a NM state, the maximum ZT value at 40 K is 0.09 and 0.16 for p-type and n-type doping, respectively. Relative to that in a NM state, the maximum ZT of a CrI₃ monolayer in a FM state is largely improved, and can reach 0.23 and 1.58 for p-type and n-type doping. Our research provides a valuable reference by showing that magnetism is a possible factor for improving the TE efficiency.

Recent progress of two-dimensional heterostructures for thermoelectric applications

The rapid development of synthesis and fabrication techniques has opened up a research upsurge in two-dimensional (2D) material heterostructures, which have received extensive attention due to their superior physical and chemical properties. Currently, thermoelectric energy conversion is an effective means to deal with the energy crisis and increasingly serious environmental pollution. Therefore, an in-depth understanding of thermoelectric transport properties in 2D heterostructures is crucial for the development of micro-nano energy devices. In this review, the recent progress of 2D heterostructures for thermoelectric applications is summarized in detail. Firstly, we systematically introduce diverse theoretical simulations and experimental measurements of the thermoelectric properties of 2D heterostructures. Then, the thermoelectric applications and performance regulation of several common 2D materials, as well as in-plane heterostructures and van der Waals heterostructures, are also discussed. Finally, the challenges of improving the thermoelectric performance of 2D heterostructures materials are summarized, and related prospects are described.

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.

2D Multiferroicity with Ferroelectric Switching Induced Spin-Constrained Photoelectricity

Multiferroic materials with tunable magnetoelectric orders enable the integration of sensing, data storage, and processing into one single device. The scarcity of single-phase multiferroics spurs extensive research in pursuit of composite systems combining different types of ferroic materials. In this work, spin-constrained photoelectric memory is proposed in two-dimensional (2D) layered magnetic/ferroelectric heterostructures, holding the possibility of low-power electrical write operation and nondestructive optical read operation. The ground state of ferromagnetic (FM) and antiferromagnetic (AFM) orderings in the magnetic layer is altered by polarization direction of the ferroelectric layer. Specifically, the FM heterostructure exhibits a type-II band alignment. Due to the light-induced charge transfer, spin-polarized/unpolarized current arises from the FM/AFM state, which can be recorded as the "1"/"0" state and served for logic processing and memory applications. Our first-principles calculations demonstrate that the NiI₂/In₂Se₃ heterobilayer is an ideal candidate to realize such a spin-dependent photoelectric memory. The reversible FM state (easy-axis magnetic anisotropy) and AFM state (easy-plane magnetic orientation) in the NiI₂ layer originate from interfacial charge transfer and effective electric field due to the proximity effect. This work offers considerable potential in the integration of memory processing capability into one single device with 2D layered multiferroic heterostructures.

Tunable, Ferroelectricity-Inducing, Spin-Spiral Magnetic Ordering in Monolayer FeOCl

Spin spirals (SS) are a special case of noncollinear magnetism, where the magnetic-moment direction rotates along an axis. They have generated interest for novel phenomena, spintronics applications, and their potential formation in monolayers, but the search for monolayers exhibiting SS has not been particularly fruitful. Here, we employ density functional theory calculations to demonstrate that SS form in a recently synthesized monolayer, FeOCl. The SS wavelength and stability can be tuned by doping and uniaxial strain. The SS-state band gap is larger by 0.6 eV compared to the gap of both the ferromagnetic and antiferromagnetic state, enabling bandgap tuning and possibly an unusual formation of quantum wells in a single material via magnetic-field manipulation. The SS-induced out-of-plane ferroelectricity enables switching of the SS chirality by an electric field. Finally, forming heterostructures, for example, with graphene or boron nitride, maintains SS ordering and provides another method of modulation and a potential for magnetoelectric devices.

Magnetoelectric coupling effects on the band alignments of multiferroic In2Se3-CrI3 trilayer heterostructures

Due to unique magnetoelectric coupling effects, two-dimensional (2D) multiferroic van der Waals heterostructures (vdWHs) are promising for next-generation information processing and storage devices. Here, we design theoretically multiferroic In₂Se₃/CrI₃ trilayer vdWHs with different stacking patterns. For the CrI₃/In₂Se₃/CrI₃ trilayer vdWHs, whether ferroelectric upward or downward polarization, type-I and type-II band alignments are formed for spin-up and spin-down channels. However, for the CrI₃/In₂Se₃/In₂Se₃ trilayer vdWHs, downward polarization induces the type-III band alignment, which is typical for spin-tunnel transistors. Moreover, nonvolatile ferroelectric polarization and stacking patterns can induce the conversion between a unipolar semiconductor and a bipolar (unipolar) half-metal. These results provide a possible route to realize nanoscale multifunctional spintronic devices based on 2D multiferroic systems.

Tuning the Schottky barrier height in a multiferroic In2Se3/Fe3GeTe2 van der Waals heterojunction

The ferroelectric material In₂Se₃ is currently of significant interest due to its built-in polarisation characteristics that can significantly modulate its electronic properties. Here we employ density functional theory to determine the transport characteristics at the metal-semiconductor interface of the two-dimensional multiferroic In₂Se₃/Fe₃GeTe₂ heterojunction. We show a significant tuning of the Schottky barrier height as a result of the change in the intrinsic polarisation state of In₂Se₃: the switching in the electric polarisation of In₂Se₃ results in the switching of the nature of the Schottky barrier, from being n-type to p-type, and is accompanied by a change in the spin polarisation of the electrons. This switchable Schottky barrier structure can form an essential component in a two-dimensional field effect transistor that can be operated by switching the ferroelectric polarisation, rather than by the application of strain or electric field. The band structure and density of state calculations show that Fe₃GeTe₂ lends its magnetic and metallic characteristics to the In₂Se₃ layer, making the In₂Se₃/Fe₃GeTe₂ heterojunction a potentially viable multiferroic candidate in nanoelectronic devices like field-effect transistors. Moreover, our findings reveal a transfer of charge carriers from the In₂Se₃ layer to the Fe₃GeTe₂ layer, resulting in the formation of an in-built electric field at the metal-semiconductor interface. Our work can substantially broaden the device potential of the In₂Se₃/Fe₃GeTe₂ heterojunction in future low-energy electronic devices.

A multiferroic iron arsenide monolayer

Iron arsenide (FeAs) monolayers are known as a key component for building iron-based superconductors. Here, we predict by first-principles calculations that the FeAs monolayer is a highly stable and multiferroic material with coexisting ferroelasticity and antiferromagnetism. The ferroelasticity entails a reversible elastic strain of as large as 18% and an activation barrier of 20 meV per atom, attributed to a weak hybridization between Fe d and As p orbitals. The local moments of Fe atoms are oriented out-of-plane, so that the magnetic ordering is weakly coupled to the structural polarization. Interestingly, fluorination of the FeAs monolayer can align the local moments in parallel and reorient the easy axis along the in-plane direction. As such, the fluorinated FeAs monolayer is potentially a long-sought multiferroic material that enables a strong coupling between ferroelasticity and ferromagnetism.

Tunable magnetoelectric coupling and electrical features in an ultrathin Cr2Si2Te6/In2Se3 heterostructure

Two-dimensional (2D) van der Waals (vdW) heterostructures based on multiferroic materials have potential applications in novel low-dimensional spintronic devices. In this work, we have investigated a strong magnetoelectric coupling and electrical dependence between single layer (1L) Cr₂Si₂Te₆ and In₂Se₃. By switching the direction of ferroelectric polarization in In₂Se₃, we observed a significant magneto-crystalline anisotropy energy (MAE) enhancement of Cr₂Si₂Te₆. The analysis of the spin-resolved orbital-decomposed band structure shows stronger magnetoelectric coupling between the In₂Se₃ and Cr₂Si₂Te₆ layers. The modulation of the electrical features could also be achieved in the switching of the ferroelectric polarization. Furthermore, the switching of Ohmic-Schottky contacts in the heterojunction with different polarization states was successfully achieved under the effect of strain engineering. Based on these findings, we design a novel 2D ferroelectric-ferromagnetic heterojunction that exploits the controllability and nonvolatility of ferroelectrics to modulate the electrical properties of the device. These findings indicate the high application potential of Cr₂Si₂Te₆/In₂Se₃ multiferroic heterojunctions in spintronics.

First-principles study on multiferroicity in the Cr2Ge2Te6/In2Se3 heterostructure influenced by finite strains

Multiferroic materials that have more than two ferroicities at the same time have long been regarded as one of the strongest candidates to achieve technological breakthroughs in many kinds of nanodevice applications. Various types of multiferroic materials have been discovered and devised to date; however, related studies have been conducted without identifying a complete winner because each has a decisive disadvantage. The recently discovered multiferroicity in the 2D Cr₂Ge₂Te₆/In₂Se₃ van der Waals heterostructure represents an important opportunity to create a new turning point in multiferroic research. Through first-principles density functional theory calculations, we studied the preferential characteristics of the spin magnetic moment of 2D Cr₂Ge₂Te₆ induced by the ferroelectric switching of the In₂Se₃ monolayer in the presence of the strains that inevitably exist in any kind of heterostructure. From the results, we found that the multiferroicity in the Cr₂Ge₂Te₆/In₂Se₃ heterostructure reacts quite sensitively to the strain level, revealing the possibility of manipulating multiferroic properties in the structure.

2D Magnetic Heterostructures and Their Interface Modulated Magnetism

In recent years, two-dimensional (2D) magnetic heterostructures have captured widespread interest as they provide a fertile ground for exploring the novel properties induced by interfacial magnetic coupling, modulating the intrinsic magnetism of the 2D magnet, and exploiting new spintronic device applications. In this Spotlight on Applications, dominating synthetic strategies employed to fabricate 2D magnetic heterostructures are introduced first. Notably, we then concentrate on two different kinds of magnetic interfaces, namely, the magnetic-nonmagnetic interface and the magnetic-magnetic interface. Specifically, various interface modulated magnetisms such as valley splitting and the anomalous Hall effect as well as their related device applications such as magnetic tunnel junctions have been further reviewed and discussed. Finally, we briefly summarize the recent progress of 2D magnetic heterostructures and outline the future development direction of this booming field.

Nonvolatile magnetoelectric coupling in two-dimensional ferromagnetic-bilayer/ferroelectric van der Waals heterostructures

One of the promising research topics on two-dimensional (2D) van der Waals (vdW) material based devices is the nonvolatile electrical control of magnetism. Usually, it is very hard to tune ferromagnetic or antiferromagnetic ordering by ferroelectric polarization due to strong exchange coupling. The existence of vdW layer spacing, however, which is ubiquitous in 2D materials, makes interlayer magnetic exchange coupling much weaker than interlayer coupling. In this work, we design a multiferroic heterostructure composed of a CrOBr ferromagnetic bilayer and an In₂Se₃ ferroelectric monolayer. The weaker interlayer exchange coupling of the CrOBr bilayer makes it easier to be regulated by ferroelectric polarization, enabling reversible nonvolatile electric control of shifts between ferromagnetic and antiferromagnetic ordering. The unique electrically controlled interlayer magnetic coupling for tuning the overall magnetism may be available for the practical application of 2D vdW bilayer magnets in high-sensitivity sensors and high-density data storage.

Spin-constrained optoelectronic functionality in two-dimensional ferromagnetic semiconductor heterojunctions

Two-dimensional (2D) van der Waals (vdW) engineering has brought about many extraordinary and new physics concepts and potential applications. Herein, we propose a new type of spin-constrained optoelectronic device developed using 2D ferromagnetic semiconductor heterostructures (FMSs). It is based on a photoexcited double-band-edge transition model, involved coupling between the interlayer magnetic order and the spin-polarized band structure and can achieve the reversible switch of band alignment via reversal of magnetization. We demonstrate that such a unique magnetic optoelectronic device can be realized with a CrBr₃/CrCl₃ heterojunction and other 2D FMS heterojunctions that have the same direction as the easy magnetization axis and have a switchable band alignment that allows reconfiguration. This study opens a new application window for 2D vdW heterostructures and enables the possibility for fully vdW-based ultra-compact spintronics devices.

Imaging Vacancy Defects in Single-Layer Chromium Triiodide

As a van der Waals magnetic semiconductor, chromium triiodide (CrI₃) is widely considered for its high research value and potential applications. Defects in CrI₃ are inevitably present and significantly alter the material properties. However, experimental identification of defects of CrI₃ at the atomic level is still lacking. Here for the first time, we carried out a scanning tunneling microscopy (STM) study and density functional theory calculations to explore the intrinsic defects in monolayer CrI₃ grown by molecular beam epitaxy. The three most common types of intrinsic point defects, i.e., I vacancy (V_I), Cr vacancy (V_Cr), and multiatom CrI₃ vacancy (V_CrI3) with distinct spatial distributions of the localized defect states, are identified and characterized by high-resolution STM. Moreover, defect concentrations are estimated based on our experiments, which agree with the calculated formation energies. Our findings provide vital knowledge on the types, concentrations, electronic structures, and migration mechanism of the intrinsic point defects in monolayer CrI₃ for future defect engineering of this novel 2D magnet.

Interface-induced transition from Schottky-to-Ohmic contact in Sc2CO2-based multiferroic heterojunctions

In order to achieve a multiferroic heterojunction with a low resistance contact, we investigated a series of Sc2CO2-based van der Waals (vdW) multiferroic heterojunctions in which the ferromagnetics (1T-MnSe2, 1T-VSe2, and 1T-VTe2) were selected as the contact electrodes in terms of first-principles calculations. By reversing the polarization state of Sc2CO2 from Sc-P↑ to Sc-P↓, we found that the heterojunctions converted from Schottky-to-Ohmic contact. Moreover, this conversion, accompanied by an interface charge transfer is intrinsic and is not regulated by the interlayer spacing and biaxial strain. This work provides an avenue for the design of two-dimensional Sc2CO2-based multiferroic electronics in the future.

Nonvolatile electrical control of 2D Cr2Ge2Te6 and intrinsic half metallicity in multiferroic hetero-structures

The electrical control of two-dimensional (2D) van der Waals ferromagnets is a step forward for the realization of spintronic devices. However, using this approach for practical applications remains challenging due to its volatile memory. Herein, we adopt an alternative strategy, where the bistable ferroelectric switches (P↑ and P↓) of Sc₂CO₂ (SCO) assist the ferromagnetic states of Cr₂Ge₂Te₆ (CGT) in order to achieve non-volatile memories. Moreover, MXene SCO, being an aided layer in multiferroic CGT/SCO hetero-structures, also modifies the electronic properties of CGT to half metal by its polarized P↓ state. In contrast, the P↑ state does not change the semiconducting nature of CGT. Hence, non-volatile, electrical-controlled switching of ferromagnetic CGT can be engineered by the two opposite ferroelectric states of single layer SCO. Importantly, the magnetic easy axis of CGT switches from in-plane to out-of-plane when the direction of electric polarization of SCO is altered from P↓ to P↑.

Tuning Magnetism of Metal Porphyrazine Molecules by a Ferroelectric In2Se3 Monolayer

Electric field tuning of magnetism is highly desirable for nanoelectronics, but volatility in electron spin manipulation presents a major challenge that needs urgent resolution. Here, we show by first-principles calculations that magnetism of metal porphyrazine (MPz) molecules can be effectively tuned by switching ferroelectric polarization of an adjacent In₂Se₃ monolayer. The magnetic moments of TiPz and VPz (MnPz, FePz, and CoPz) decrease (increase) at one polarization but remain unchanged at reversed polarization. This intriguing phenomenon stems from distinct metal d-orbital occupation caused by electron transfer and energy-level shift associated with the polarization switch of the In₂Se₃ monolayer. Moreover, the ferroelectric switch also tunes the underlying electronic properties, producing a metallic, half-metallic, or semiconducting state depending on polarization. These findings of robust ferroelectric tuning of magnetism and related electronic properties in MPz-adsorbed In₂Se₃ hold great promise for innovative design and implementation in advanced magnetic memory storage, sensor, and spintronic devices.

Thermoelectric properties of two-dimensional magnet CrI3

The thermoelectric, phonon transport, and electronic transport properties of two-dimensional magnet CrI₃ are systematically investigated by combining density functional theory with Boltzmann transport theory. A low lattice thermal conductivity of 1.355 W m^-1K^-1 is presented at 300 K due to the low Debye temperature and phonon group velocity. The acoustic modes dominate the lattice thermal conductivity, and the longitudinal acoustic mode has the largest contribution of 42.31% on account of its relatively large phonon group velocity and phonon lifetime. The high band degeneracy and the peaky density of states near the conduction band minimum appear for the CrI₃ monolayer, which is beneficial for forming a significantly increased Seebeck coefficient (1561 μV K^-1). Furthermore, the thermoelectric figure of merit is calculated reasonably, and the value is 1.57 for the optimal n-type doping level at 900 K. N-type doping maintains a higher thermoelectric conversion efficiency than p-type doping throughout the temperature range, while the difference gradually increases as the temperature rises. Our investigation may provide some theoretical support for the application of the CrI₃ monolayer in the thermoelectric field.