In a recent report, room-temperature vertical ferroelectricity was experimentally shown in WTe2 bilayer, while its mechanism of ferroelectric switching without vertical ion displacements remains unclarified. In this work, we reveal its origin by first-principles calculations that the polarization stems from uncompensated interlayer vertical charge transfer depending on in-plane translation, which can be switched upon interlayer sliding. The calculated results are consistent with experimental data, and a similar switching mechanism can be applied to a multilayer counterpart. Despite its small ferroelectric switching barrier and polarization, the in-plane rigidity of WTe2 layer gives rise to a high Curie temperature. A moire pattern of ferroelectric domain superlattice can be formed and tuned upon a small-angle twist of bilayer, which is unique compared with traditional ferroelectrics. Similar interlayer translational ferroelectricity may exist in a series of van der Waals bilayers or even bulk phases.

Origin of Two-Dimensional Vertical Ferroelectricity in WTe2 Bilayer and Multilayer.

Multiferroics, where multiple ferroic orders coexist and are intimately coupled, promise novel applications in conceptually new devices on one hand, and on the other hand provide fascinating physics that is distinctly different from the physics of high-TC superconductors and colossal magnetoresistance manganites. In this mini-review, we highlight the recent progress of single-phase multiferroics in the exploration of new materials, efficient roadmaps for functionality enhancement, new phenomena beyond magnetoelectric coupling, and underlying novel physics. In the meantime, a slightly more detailed description is given of several multiferroics with ferrimagnetic orders and double-layered perovskite structure and also of recently emerging 2D multiferroics. Some emergent phenomena such as topological vortex domain structure, non-reciprocal response, and hybrid mechanisms for multiferroicity engineering and magnetoelectric coupling in various types of multiferroics will be briefly reviewed.

/pdf/single-phase-multiferroics-new-materials-phenomena-and-3gooyqgli5.pdf

Single-phase multiferroics: new materials, phenomena, and physics.

The key physical property of multiferroic materials is the existence of coupling between magnetism and polarization, i.e. magnetoelectricity. The origin and manifestations of magnetoelectricity can be very different in the available plethora of multiferroic systems, with multiple possible mechanisms hidden behind the phenomena. In this review, we describe the fundamental physics that causes magnetoelectricity from a theoretical viewpoint. The present review will focus on mainstream physical mechanisms in both single-phase multiferroics and magnetoelectric heterostructures. The most recent tendencies addressing possible new magnetoelectric mechanisms will also be briefly outlined.

Magnetoelectricity in multiferroics: a theoretical perspective.

Multiferroics are rare in nature due to the mutual exclusive origins of magnetism and ferroelectricity. The simultaneous coexistence of robust magnetism/ferroelectricity and strong magnetoelectric coupling in single multiferroics is hitherto unreported, which may also be attributed to their potential conflictions. In this paper, we show the first-principles evidence of such desired coexistence in ultrathin-layer CuCrS2 and CuCrSe2. The vertical ferroelectricity is neither induced by an empty d shell nor spin-driven, giving rise to an alternative possibility of resolving those intrinsic exclusions and contradictions. Compared with their bulk phases, the ferromagnetism in the thin-layer structures (two-six layers) can be greatly stabilized due to the enhanced carrier density and orbital shifting by vertical polarization, and the Curie temperatures of both ferromagnetism and ferroelectricity can be above room temperature. Moreover, a considerable net magnetization can be reversed upon ferroelectric switching, where the change in spin-resolved band structure also renders efficient 'magnetic reading + electrical writing'. The thickness-different layers may even exhibit diversified types of magnetoelectric coupling, which both enriches the physics of multiferroics and facilitates their practical applications.

/pdf/room-temperature-multiferroicity-and-diversified-43fe5kjacl.pdf

Room-temperature multiferroicity and diversified magnetoelectric couplings in 2D materials

It is known that an isolated single-molecule magnet tends to become super-paramagnetic even at an ultralow temperature of a few Kelvin due to the low spin switching barrier. Herein, single-molecule ferroelectrics/multiferroics is proposed, as the ultimate size limit of memory, such that every molecule can store 1 bit data. The primary strategy is to identify polar molecules that possess bistable states, moderate switching barriers, and polarizations fixed along the vertical direction for high-density perpendicular recording. First-principles computation shows that several selected magnetic metal porphyrin molecules possess buckled structures with switchable vertical polarizations that are robust at ambient conditions. When intercalated within a bilayer of 2D materials such as bilayer MoS2 or CrI3, the magnetization can alter the spin distribution or can be even switched by 180° upon ferroelectric switching, rendering efficient electric writing and magnetic reading. It is found that the upper limit of areal storage density can be enhanced by four orders of magnitude, from the previous super-paramagnetic limit of ≈40 to ≈106 GB in.-2, on the basis of the design of cross-point multiferroic tunneling junction array and multiferroic hard drive.

/pdf/design-of-single-molecule-multiferroics-for-efficient-cui7lcdz8e.pdf

Design of Single‐Molecule Multiferroics for Efficient Ultrahigh‐Density Nonvolatile Memories

Oxyhydroxide minerals such as FeOOH have been a research focus in geology for studying the Earth's interior, and also in chemistry for studying their oxygen electrocatalysis activity. In this paper the first-principle evidence of a new class of ferroelectrics/multiferroics is given. In this class are: β-CrOOH (guyanaite), e-FeOOH, β-GaOOH, and InOOH, which are earth-abundant minerals which have been experimentally verified to possess distorted rutile structures, are ferroelectric with considerable polarizations (up to 24 μC cm−2) and piezoelectric coefficients. Their atomic-thick layer may possess vertical polarization will not be diminished by depolarizing field because of the formation of O–H⋯O bonds that can be hardly symmetrized. Furthermore, β-CrOOH is revealed to be a combination of a high Curie temperature (TC) in-plane type-I multiferroics and vertical type-II multiferroics, which is strain tunable and may give a desirable coupling between magnetism and ferroelectricity. Supported by experimental evidence on reversible conversion between metal oxyhydroxides and dioxides and their good lattice match that gives convenient epitaxial growth, a heterostructure composed of oxyhydroxides and common metal dioxides (e.g., TiO2, SnO2 and CrO2) may be constructed for various applications such as ferroelectric field-effect transistors and multiferroic tunneling junctions.

Proton transfer ferroelectricity/multiferroicity in rutile oxyhydroxides

Oxyhydroxide minerals like FeOOH have been a research focus in geology for studying the Earth interior, and also in chemistry for studying oxygen electrocatalysis activity. In this paper we provide first-principles evidence of a new class of ferroelectrics or multiferroics among them:GaOOH,InOOH,CrOOH,FeOOH, which are earth-abundant minerals and have been experimentally verified to possess distorted rutile structures, are ferroelectric with considerable polarizations(up to 24 muC/cm2) and piezoelectric coefficients. Their atomic-thick layer may possess vertical polarization robust against depolarizing field due to the formation of O-H O bonds that can hardly be symmetrized. Moreover,CrOOH (guyanaite) is revealed to be a combination of high-Tc in-plane type-I multiferroics and vertical type-II multiferroics, which is strain-tunable and may render a desirable coupling between magnetism and ferroelectricity. Supported by experimental evidence on reversible conversion between metal oxyhydroxides and dioxides and their nice lattice match that renders convenient epitaxial growth, heterostructure composed of oxyhydroxides and prevalent metal dioxides (e.g. TiO2, SnO2 and CrO2) may be constructed for various applications like ferroelectric field-effect transistors and multiferroic tunneling junctions.

Proton-transfer Ferroelectricity / Multiferroicity in Rutile Oxyhydroxides.

Antiferromagnetic (AFM) spintronics, where magneto-transport is governed by an antiferromagnet instead of a ferromagnet, opens fascinating new perspectives for both fundamental research and device technology, owing to their intrinsic appealing properties like rigidness to magnetic field, absence of stray field, and ultrafast spin dynamics. One of the urgent challenges, hindering the realization of the full potential of AFM spintronics, has been the performance gap between AFM metals and insulators. Here, we demonstrate the insulator-metal transition and persistently large anisotropic magnetoresistance (AMR) in single crystals Sr2(Ir1-xGax)O4 (0<x<0.09) which host the same basal-plane AFM lattice with strong spin-orbit coupling. The non-doped Sr2IrO4 shows the insulating transport with the AMR as big as ~16.8% at 50 K. The Ga substitution of Ir allows a gradual reduction of electrical resistivity, and a clear insulator-to-metal transition is identified in doped samples with x above 0.05, while the AMR can still have ~1%, sizable in comparison with those in AFM metals reported so far. Our experiments reveal that all the samples have the similar fourfold AMR symmetry, which can be well understood in the scenario of magnetocrystalline anisotropy. It is suggested that the spin-orbit coupled antiferromagnets Sr2(Ir1-xGax)O4 are promising candidate materials for AFM spintronics, providing a rare opportunity to integrate the superior spintronic functionalities of AFM metals and insulators.

Persistent large anisotropic magnetoresistance and insulator to metal transition in spin-orbit coupled antiferromagnets Sr2(Ir1-xGax)O4

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Proton transfer ferroelectricity/multiferroicity in rutile oxyhydroxides

Proton-transfer Ferroelectricity / Multiferroicity in Rutile Oxyhydroxides.

Persistent large anisotropic magnetoresistance and insulator to metal transition in spin-orbit coupled antiferromagnets Sr2(Ir1-xGax)O4