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Two-dimensional (2D) ferroelectricity has attracted great interest for its potential to develop various flexible and stretchable ultra-thin smart devices. The ultra-thin nature of 2D materials makes domain control very challenging, as an external electric field inevitably leads to leakage currents and even material breakdown. Therefore, it is highly desirable to explore more practical and feasible methods to control ferroelectric (FE) domains. In this work, based on the coupling between the ferroelasticity and ferroelectricity in 2D multiferroic materials, we propose a strategy to control the FE polarization direction and domain through the strain engineering. Taking β′-In2Se3 as an example, we revealed the regulation mechanism of the uniaxial strain and shear strain on the ferroelasticity and ferroelectricity. We found that the polarization direction of FE β′-In2Se3 is tunable by manipulating the strain, which demonstrates the feasibility to tailor the FE single domain as well as domain wall (DW) patterns. In addition, we also found that the angle between the stretching direction and the DW plays a crucial role in regulating the DW type, which provides an important reference for controlling DW. Therefore, the strain engineering not only provides an alternative solution for forming large-sized single domain FE materials, but also enable customized FE domain structures for DW electronics by ingeniously designing strain patterns. [ABSTRACT FROM AUTHOR]

Discovery and design of materials with the coexistence and coupling of different functional properties is extremely attractive for both fundamental physics and technical applications. In this work, we proposed a strategy to realize the coexistence of ferroelectricity, magnetism and topological order in a two-dimensional van der Waals heterostructure composed of a ferroelectric material α-In2Se3 and a topological hydrogenated germanene. More importantly, we found the strong coupling of the topological electronic structure and ferroelectric order. Transition from a normal semiconductor to a quantum anomalous Hall insulator can be switched by reversing the polarization direction of α-In2Se3. The topological phase transition mechanism is revealed by the interfacial charge transfer and energy band alignment. The difference in the energy band alignment stems from the difference of interfacial potential energy for two opposite ferroelectric phases. So, ferroelectric heterostructure engineering is an effective way to realize band inversion and topological phase transition by the interface effect. Our work is not only of great scientific significance for studying the coupling mechanism of various degrees of freedom through the interface effect, but also provides an effective strategy for realizing nonvolatile topological switches and storage devices in practical applications. [ABSTRACT FROM AUTHOR]

Magnetoelectric coupling is of great interest recently to both understand the fundamental physics and device applications. Materials with strong magnetoelectric coupling, high Curie temperature, and large electric polarization are still rare. We suggest a heterostructure that combines the known memory effect of the switchable ferroelectric In2Se3 [Adv. Funct. Mater. 2019, 29, 1808606] with a van der Waals bonded two-dimensional (2D) metal-organic framework (MOF) film. The magnetic ground state of this MOF can be changed from an antiferromagnetic state to a ferromagnetic through hole-doping. We use first-principles calculations to show that in such a heterostructure, adequate doping differences to cause this phase transition are expected from the changes in the interfacial charge transfer between the MOF and In2Se3 when the polarization direction of the In2Se3 is reversed. This and similar 2D heterostructures may, therefore, provide both a fascinating material platform for understanding the fundamental physics of magnetoelectric coupling and a strategy for designing spin-current-based nonvolatile memory structures. [ABSTRACT FROM AUTHOR]

The coupling between ferroelectric and magnetic orders in multiferroic materials is significant for both fundamental science and technological applications. However, it has been a major challenge to discover and design two-dimensional multiferroic materials with large electric polarization and magnetoelectric coupling. Here, we propose a strategy to realize electric control of magnetism by constructing a new class of two-dimensional van der Waals heterostructures CuInP2S6/InSe based on monolayer ferroelectric CuInP2S6 and monolayer InSe. We find that at a small hole doping concentration, different magnetic states can be realized by controlling the polarization direction of the ferroelectric CuInP2S6. The physical origin of this purely electric-controlled magnetism can be well understood by considering carrier confinement effects and the Stoner mechanism. The predicted magnetoelectric coupling not only enriches our understanding of the coupling between ferroelectric and magnetic orders in ferroelectric heterostructures, but also has important significance for potential magnetoelectric device applications. [ABSTRACT FROM AUTHOR]

Identifying intrinsic low-dimensional ferromagnets with high magnetic transition temperature and electrically tunable magnetism is crucial for the development of miniaturized spintronics and magnetoelectrics. Recently, long-range 2D ferromagnetism was observed in van der Waals crystals CrI3 and Cr2Ge2Te6, however, their Curie temperature is significantly lowered when reducing down to monolayer/few layers. Herein, using renormalized spin-wave theory and first-principles electronic structure theory, we present a theoretical study of electrically tunable 2D ferromagnetism in van der Waals layered CrSBr and CrSeBr semiconductors with a high Curie temperature of ∼150 K and a sizable bandgap. The high transition temperature is attributed to the strong anion-mediated superexchange interaction and a sizable spin-wave excitation gap due to large exchange and single-ion anisotropy. Remarkably, hole and electron doping can switch the magnetization easy axis from the in-plane to the out-of-plane direction. These unique characteristics establish monolayer CrSBr and CrSeBr as a promising platform for realizing 2D spintronics and magnetoelectrics such as 2D spin valves and spin field effect transistors. [ABSTRACT FROM AUTHOR]

Two-dimensional multiferroics have attracted tremendous attention due to their intriguing physics and promising applications. However, it has been a major challenge to discover and design two-dimensional multiferroic materials with large electric polarization and strong magnetoelectric coupling. In this work, we propose a strategy to design a two-dimensional van der Waals heterostructure with strong magnetoelectric coupling by stacking a transition metal phthalocyanine (TMPc) molecule with ferroelectric monolayer In2Se3. By first-principles electronic structure calculations, we predict that the magnetic states of the TMPc molecule can be controlled by electrically switching the polarization direction of In2Se3 using an external electric field. This strong magnetoelectric coupling effect originates from the interfacial charge transfer and orbital splitting, resulting in the different magnetic states of TMPc/In2Se3 heterostructures in two opposite ferroelectric phases. Based on the TMPc/In2Se3 heterostructure, a high-density magnetic memory device is proposed for pure electric writing and magnetic reading. Our predictions may open avenues for finding and designing multiferroic heterostructures by using two-dimensional ferroelectric materials and zero-dimensional magnetic molecules with a strong proximity effect. [ABSTRACT FROM AUTHOR]

We comparatively studied the structural and magnetic properties of ZnS nanowires (NWs) doped with one or two transition-metal (TM) atoms (Cr, Mn, Fe, Co, and Ni) using first-principles calculations. The results indicate that the TM atoms prefer to be at the middle position of the NWs and have no tendency to form clusters. The formation energies of doped NWs were smaller than that of the pristine NW, indicating that doing process is an exothermic reaction. All of the NWs had atomlike magnetic moments mainly due to the 3d component of the TM atoms. Significant hybridization was observed between the d states of the TM atoms and the p states of the S atoms. Based on the half-metallic characteristic and long-range ferromagnetic interactions, we demonstrated that the ZnS NWs doped with Cr atoms have great potential for successful implementation into spintronic devices. [ABSTRACT FROM AUTHOR]

We propose a van der Waals heterostructure CuInP2S6/germanene by combining two dimensional ferroelectric semiconductor CuInP2S6 with germanene. By density functional theory calculations, we find that the metal-semiconductor transition can be realized in the CuInP2S6/germanene heterostructure by controlling the ferroelectric polarization direction. CuInP2S6 induces the sublattice imbalance of germanene by interface interaction and thus makes it become a normal semiconductor. Then, two opposite ferroelectric polarization states in CuInP2S6/germanene lead to a different band alignment and finally determine its metallic or semiconductor properties. Large transition barriers from ferroelectric to antiferroelectric phases ensure its stability at room temperature. This is a pure electric field controlled metal-semiconductor transition, which has great application potential for exploring nonvolatile ferroelectric switches and memory devices. [ABSTRACT FROM AUTHOR]

Low-dimensional multiferroicity, though highly scarce in nature, has attracted great attention due to both fundamental and technological interests. Using first-principles density functional theory, we show that ferromagnetism and ferroelectricity can coexist in monolayer transition metal phosphorus chalcogenides (TMPCs)-CuMP2X6 (M = Cr, V; X = S, Se). These van der Waals layered materials represent a class of 2D multiferroic semiconductors that simultaneously possess ferroelectric and ferromagnetic orders. In these monolayer materials, Cu atoms spontaneously move away from the center atomic plane, giving rise to nontrivial electric dipole moment along the plane normal. In addition, their ferromagnetism originates from indirect exchange interaction between Cr/V atoms, and their out-of-plane ferroelectricity suggests the possibility of controlling electric polarization by external vertical electric field. Monolayer semiconducting TMPCs thus provide a solid-state 2D materials platform for realizing 2D nanoscale switches and memory devices patterned with top and bottom electrodes. [ABSTRACT FROM AUTHOR]

Monolayer manganese chalcogenophosphates MnPSe3 possess a pair of non-degenerate energy valleys in the vicinities of the vertices of the hexagonal Brillouin zone, which exhibit fascinating optical valley polarization. However, the spin is still degenerate due to antiferromagnetic coupling between Mn ions. In this paper we propose a strategy to realize simultaneously the spin and valley degeneracy splitting by the doping-induced Zeeman effect in monolayer MnPSe3. Using first-principles calculations we demonstrate that in Zn-doped monolayer MnPSe3 a valley splitting of larger than 20 meV and a spin splitting of larger than 100 meV can be achieved simultaneously. The doping-induced spin and valley splitting is attractive for spintronics and valleytronics as it creates differences in various energy scales between valleys and between spins, which will facilitate the access and manipulation of the valley and spin degrees of freedom by electrical gating in addition to optical pumping. [ABSTRACT FROM AUTHOR]

In this letter we propose a universal strategy combining external electric field with the ripple of membrane to tune the bandgap of semiconducting atomic monolayer. By first-principles calculations we show that the bandgap of rippled MoS2 monolayer can be tuned in a large range by vertical external electric field, which is expected to have little effect on MoS2 monolayer. This phenomenon can be explained from charge redistribution under external electric field by a simple model. This may open an avenue of optimizing monolayer MoS2 for electronic and optoelectronic applications by surface patterning. [ABSTRACT FROM AUTHOR]