Switching Of Electric Polarization Research Articles

Polarization Saturation in Multilayered Interfacial Ferroelectrics.

Van der Waals polytypes of broken inversion and mirror symmetries have been recently shown to exhibit switchable electric polarization even at the ultimate two-layer thin limit. Their out-of-plane polarization has been found to accumulate in a ladder-like fashion with each successive layer, offering 2D building blocks for the bottom-up construction of 3D ferroelectrics. Here, it is demonstrated experimentally that beyond a critical stack thickness, the accumulated polarization in rhombohedral polytypes of molybdenum disulfide saturates. The underlying saturation mechanism, deciphered via density functional theory and self-consistent Poisson-Schrödinger calculations, point to a purely electronic redistribution involving: 1.Polarization-induced bandgap closure that allows for cross-stack charge transfer and the emergence of free surface charge; 2. Reduction of the polarization saturation value, as well as the critical thickness at which it is obtained, by the presence of free carriers. The resilience of polar layered structures to atomic surface reconstruction, which is essentially unavoidable in polar 3D crystals, potentially allows for the design of new devices with mobile surface charges. The findings, which are of general nature, should be accounted for when designing switching and/or conductive devices based on ferroelectric layered materials.

La2SrSc2O7: A-Site Cation Disorder Induces Ferroelectricity in Ruddlesden-Popper Layered Perovskite Oxide.

Rational design of ferroelectrics in layered perovskites, like n = 2 Ruddlesden-Popper (RP) phase A3B2O7, has been achieved by the hybrid-improper ferroelectric (HIF) mechanism, in which an electric polarization is induced via a trilinear coupling to nonpolar BO6 octahedral rotation and tilt distortions around crystallographic axes. In the present work, hybrid improper ferroelectricity in n = 2 RP-type La2SrSc2O7 induced by the disordering of Sr2+/La3+ cations on the A-sites in rocksalt ([Sr/La]Rs = 25/75) and perovskite ([Sr/La]Pv = 50/50) layers is demonstrated through experimental and theoretical investigations. The ferroelectric A21am structure (a-a-c+ in Glazer notation) at room temperature and the second-order phase transition to paraelectric Amam structure (a-a-c0) at TC ∼ 600 K are determined by a combination of X-ray and neutron diffraction and optical second harmonic generation. The ferroelectric hysteresis loop measurements prove the switchable electric polarization indicative of ferroelectricity. These results represent an unprecedented example of ferroelectricity in the n = 2 RP family of Ln2AB2O7 with inequivalent Ln3+ and A2+ cations. Combining the abovementioned experimental results with the first-principles calculations, we verify the role of Sr/La distributions in regulating the interlayer rumpling, which, in addition to the structural tolerance factor, is key to controlling the structural distortions of RP phases. The stabilization of the ferroelectric, a-a-c+ distorted structure is a consequence of the disordered Sr/La distribution on the A-sites, which suppresses the rumpling-induced octahedral deformations in competition with the octahedral rotations and thus enables the concurrence of a0a0c+ rotations and a-a-c0 tilts required for the HIF mechanism. This work demonstrates the possibility of altering the crystal symmetry of RP phases through the A-site cation disorder and provides a complementary approach to the rational design of new HIF materials.

Observation of Electric Hysteresis, Polarization Oscillation, and Pyroelectricity in Nonferroelectric p-n Heterojunctions.

The switchable electric polarization is usually achieved in ferroelectric materials with noncentrosymmetric structures, which opens exciting opportunities for information storage and neuromorphic computing. In another polar system of p-n junction, there exists the electric polarization at the interface due to the Fermi level misalignment. However, the resultant built-in electric field is unavailable to manipulate, thus attracting less attention for memory devices. Here, we report the interfacial polarization hysteresis (IPH) in the vertical sidewall van der Waals heterojunctions of black phosphorus and quasi-two-dimensional electron gas on SrTiO_{3}. A nonvolatile switching of electric polarization can be achieved by reconstructing the space charge region (SCR) with long-lifetime nonequilibrium carriers. The resulting electric-field controllable IPH is experimentally verified by electric hysteresis, polarization oscillation, and pyroelectric effect. Further studies confirm the transition temperature of 340K, beyond which the IPH vanishes. The second transition is revealed with the temperature dropping below 230K, corresponding to the sharp improvement of IPH and the freezing of SCR reconstruction. This work offers new possibilities for exploring the memory phenomena in nonferroelectric p-n heterojunctions.

Unveiling a hidden multiferroic state under magnetic fields in BaHoFeO4

In this paper, we report field-induced ferroelectricity in ${\mathrm{BaHoFeO}}_{4}$ in a narrow range of applied magnetic fields ($1.5\phantom{\rule{4.pt}{0ex}}\text{T}\ensuremath{\le}H\ensuremath{\le}5\phantom{\rule{4.pt}{0ex}}\text{T}$). Magnetization measurements reveal three magnetic anomalies, the first one at ${T}_{N}^{\text{Fe}}\ensuremath{\sim}52.8$ K, the second one at ${T}_{{N}_{2}}\ensuremath{\sim}36.4$ K, and the third one at ${T}_{N}^{\text{Ho}}\ensuremath{\sim}6.8$ K, corresponding to the antiferromagnetic ordering of ${\mathrm{Fe}}^{3+}$, a possible spin structure change of ${\mathrm{Fe}}^{3+}$, and the antiferromagnetic ordering of ${\mathrm{Ho}}^{3+}$ sublattices, respectively. Surprisingly, the heat capacity data did not show any anomaly at ${T}_{N}^{\text{Fe}}$ but exhibited a broad maximum at ${T}_{N}^{\text{Ho}}$. Below 40 K, isothermal magnetization measurements indicate a metamagnetic transition at 1.5 T. The temperature-dependent dielectric measurements performed below 1 T reveal two anomalies, around 6 and 36 K, corresponding to ${\mathrm{Ho}}^{3+}$ ordering and a possible change in the magnetic phase of the Fe sublattice. These anomalies disappear at higher magnetic fields, and a different dielectric anomaly appears above the metamagnetic transition with a broad asymmetric peak around 13 K (${T}_{P}$), which shifts to higher temperatures, reaches a maximum under 2 T, and then is suppressed further on increasing the magnetic field up to 9 T. A switchable electric polarization appears below 25 K and above the metamagnetic transition field ($H\ensuremath{\ge}1.5$ T). Intriguingly, the polarization disappears above 5 T. The presence of electric polarization in intermediate magnetic fields and its strong magnetic field dependence are interesting and make this material unique.

Magnetic Domain Change Induced by In‐Plane Electric Polarization Switching in Bi(Fe, Co)O3 Thin Film

AbstractPerovskite BiFe0.9Co0.1O3 with electric‐field‐induced magnetization reversal is a promising material for low‐power non‐volatile memory devices because spontaneous magnetization perpendicular to the electric polarization can be reversed by out‐of‐plane polarization reversal. In this study, magnetic and ferroelectric domain changes in BiFe0.9Co0.1O3 after applying an in‐plane electric field with counter electrodes fabricated by lithography is investigated. Electric poling can be performed up to 100 times via 71° polarization switching without disrupting the ferroelectric domains. It is determined that the correlation between the ferroelectric and magnetic domains is preserved, while in‐plane 71° polarization switching without reconstructing the ferroelectric domains does not change its out‐of‐plane magnetization. Lastly, possible differences in the antiferromagnetic spin vector and magnetization direction for in‐plane and out‐of‐plane 71° switching is discussed.

Modeling of ultrafast polarization switching in PbTiO[formula omitted

Ultrafast switching of electric polarization in ferroelectric materials is an open scientific problem. Switching has not been achieved experimentally. In this paper, using an improved model describing switching in a two-dimensional space of generalized coordinates, we show that for PTO, one pulse with linear polarization cannot lead to stable switching. Only depolarization of the sample can be observed if the threshold pulse intensity is reached. It is also shown that friction plays an insignificant role in dynamics of switching, and the optimal frequency of the perturbing pulse is calculated at which the intensity is the lowest.

Observation of room‐temperature out‐of‐plane switchable electric polarization in supported 3R‐MoS2 monolayers

AbstractTwo‐dimensional (2D) ferroelectrics have attracted considerable attention due to their potential in the development of devices of miniaturization and multifunction. Although several van der Waals (vdW)‐layered materials show ferroelectricity, the experimental demonstrations of ferroelectric behavior in monolayers are very limited. Here we report the observation of room‐temperature out‐of‐plane switchable electric polarization in supported MoS2 monolayers exfoliated from 3R‐stacked bulk crystals under ambient conditions. Using in situ piezoelectric force microscopy and Kelvin probe force microscopy in a glovebox, we reveal that trapped water/ice molecules are responsible for this switchable electric polarization and this conclusion is strongly supported by theoretical simulations. It is worth noting that the water/ice trapping in the monolayers exfoliated from 2H‐stacked MoS2 crystals is not as much as that in 3R monolayers and, consequently, the out‐of‐plane electric polarization is missing there. Our findings indicate that monolayers with a trapped single layer of polar molecules might be emerging alternatives to 2D ferroelectrics. Furthermore, the stacking sequences may bring new properties and applications to 2D vdW materials not only when we stack them up but also when we thin them down.

Tailoring Bulk Photovoltaic Effects in Magnetic Sliding Ferroelectric Materials.

The bulk photovoltaic effect that is intimately associated with crystalline symmetry has been extensively studied in various nonmagnetic materials, especially ferroelectrics with a switchable electric polarization. In order to further engineer the symmetry, one could resort to spin-polarized systems possessing an extra magnetic degree of freedom. Here, we investigate the bulk photovoltaic effect in two-dimensional magnetic sliding ferroelectric (MSFE) systems, illustrated in VSe2, FeCl2, and CrI3 bilayers. The transition metal elements in these systems exhibit intrinsic spin polarization, and the stacking mismatch between the two layers produces a finite out-of-plane electric dipole. Through symmetry analyses and first-principles calculations, we show that photoinduced in-plane bulk photovoltaic current can be effectively tuned by their magnetic order and the out-of-plane dipole moment. The underlying mechanism is elucidated from the quantum metric dipole distribution in the reciprocal space. The ease of the fabrication and manipulation of MSFEs guarantee practical optoelectronic applications.

A Room-Temperature Ferroelectric Resonant Tunneling Diode.

Resonant tunneling is a quantum-mechanical effect in which electron transport is controlled by the discrete energy levels within a quantum-well (QW) structure. A ferroelectric resonant tunneling diode (RTD) exploits the switchable electric polarization state of the QW barrier to tune the device resistance. Here, the discovery of robust room-temperature ferroelectric-modulated resonant tunneling and negative differential resistance (NDR) behaviors in all-perovskite-oxide BaTiO3 /SrRuO3 /BaTiO3 QW structures is reported. The resonant current amplitude and voltage are tunable by the switchable polarization of the BaTiO3 ferroelectric with the NDR ratio modulated by ≈3 orders of magnitude and an OFF/ON resistance ratio exceeding a factor of 2×104 . The observed NDR effect is explained an energy bandgap between Ru-t2g and Ru-eg orbitals driven by electron-electron correlations, as follows from density functional theory calculations. This study paves the way for ferroelectric-based quantum-tunneling devices in future oxide electronics.

Berry curvature dipole senses topological transition in a moiré superlattice

Topological aspects of electron wavefunction play a crucial role in determining the physical properties of materials. Berry curvature and Chern number are used to define the topological structure of electronic bands. While Berry curvature and its effects in materials have been studied, detecting changes in the topological invariant, Chern number, is challenging. In this regard, twisted double bilayer graphene (TDBG) has emerged as a promising platform to gain electrical control over the Berry curvature hotspots and the valley Chern numbers of its flat bands. In addition, strain induced breaking of the three-fold rotation (C3) symmetry in TDBG, leads to a non-zero first moment of Berry curvature called the Berry curvature dipole (BCD), which can be sensed using nonlinear Hall (NLH) effect. We reveal, using TDBG, that the BCD detects topological transitions in the bands and changes its sign. In TDBG, the perpendicular electric field tunes the valley Chern number and the BCD simultaneously allowing us a tunable system to probe the physics of topological transitions. Furthermore, we find hysteresis of longitudinal and NLH responses with electric field that can be attributed to switching of electric polarization in moir\'e systems. Such a hysteretic response holds promise for next-generation Berry curvature-based memory devices. Probing topological transitions, as we show, can be emulated in other 3D topological systems.

Phase transitions and potential ferroelectricity in noncentrosymmetric KNaNbOF5

Increasing the number of known ferroelectrics requires expanding the chemical design space and mechanisms producing ferroelectricity. To that end, we examine the displacive, noncentrosymmetric-to-centrosymmetric phase transition in the oxyfluoride ${\mathrm{KNaNbOF}}_{5}$ using ab initio calculations and Landau theory. We predict an intermediate $Pnma$ phase occurs in the transition from the known low- and high-temperature phases, $Pna{2}_{1}$ and $Cmcm$, respectively, which we subsequently confirm using high-resolution and in situ $^{19}\mathrm{F}$ magic-angle spinning nuclear magnetic resonance. Using the intermediate structure, we then calculate the monodomain polarization switching barrier in ${\mathrm{KNaNbOF}}_{5}$ to be $\ensuremath{\sim}93$ meV per formula unit, comparable to existing ferroelectrics. The reversal of the polarization is due to uncompensated antipolar displacements of oxygen and fluorine and does not require oxygen and fluorine site exchange as in other nonswitchable heteroanionic materials, which makes ${\mathrm{KNaNbOF}}_{5}$ a viable oxyfluoride for a switchable electric polarization.

Large magnetization jumps in Ca-doped bismuth ferromanganite

Magnetic properties of the phase-separated (antiferromagnetic + glassy) Bi0·85Ca0·15Fe0·6Mn0·4O3 multiferroic have been studied over a wide temperature range. It has been found that the material exhibits a metamagnetic transition (shifting toward smaller fields with decreasing temperature) which is accompanied by the magnetization jumps whose value (up to ∼4 emu/g at T = 2 K) far exceeds that characteristic of the field-driven transformation from the cycloidal to the canted antiferromagnetic order in the pure BiFeO3. The metamagnetic behavior can be explained as originating from the spin-reorientation transition in the long range-ordered antiferromagnetic phase and involving magnetic moments from the glassy-like phase. The obtained results shed new light on the conditions favoring the coexistence of large switchable magnetization and spontaneous electric polarization in BiFeO3-based multiferroics.

InSitu Electric-Field Control of THz Nonreciprocal Directional Dichroism in the Multiferroic Ba_{2}CoGe_{2}O_{7}.

Nonreciprocal directional dichroism, also called the optical-diode effect, is an appealing functional property inherent to the large class of noncentrosymmetric magnets. However, the insitu electric control of this phenomenon is challenging as it requires a set of conditions to be fulfilled: Special symmetries of the magnetic ground state, spin excitations with comparable magnetic- and electric-dipole activity, and switchable electric polarization. We demonstrate the isothermal electric switch between domains of Ba_{2}CoGe_{2}O_{7} possessing opposite magnetoelectric susceptibilities. Combining THz spectroscopy and multiboson spin-wave analysis, we show that unbalancing the population of antiferromagnetic domains generates the nonreciprocal light absorption of spin excitations.

One Nanometer HfO2‐Based Ferroelectric Tunnel Junctions on Silicon

AbstractIn ferroelectric materials, spontaneous symmetry breaking leads to a switchable electric polarization, which offers significant promise for nonvolatile memories. In particular, ferroelectric tunnel junctions (FTJs) have emerged as a new resistive switching memory which exploits polarization‐dependent tunnel current across a thin ferroelectric barrier. This work integrates FTJs with complementary metal‐oxide‐semiconductor‐compatible Zr‐doped HfO2 (Zr:HfO2) ferroelectric barriers of just 1 nm thickness, grown by atomic layer deposition on silicon. These 1 nm Zr:HfO2 tunnel junctions exhibit large polarization‐driven electroresistance (>20 000%), the largest value reported for HfO2‐based FTJs. In addition, due to just a 1 nm ferroelectric barrier, these junctions provide large tunneling current (>1 A cm−2) at low read voltage, orders of magnitude larger than reported thicker HfO2‐based FTJs. Therefore, this proof‐of‐principle demonstration provides an approach to simultaneously overcome three major drawbacks of prototypical FTJs: a Si‐compatible ultrathin ferroelectric, large electroresistance, and large read current for high‐speed operation.

Switchable electric polarization of phosphorene in mixed dimensional van der Waals heterostructure

Switchable electric polarization in semiconductors has promising applications in nanoelectronic and optoelectronic devices. Here, we propose a type of two-dimensional (2D) polarization switchable by angstrom-scale position shifts of phosphorene on PbTe. As a homo-atomic analogue to the 2D ferroelectric monolayer GeS, non-ferroelectric phosphorene acquires in-plane electric polarization by breaking the in-plane and out-of-plane symmetry via proximity of phosphorene to PbTe compound in the mixed dimensional vertical P/PbTe heterostructure. We demonstrate via first-principles calculations the asymmetric charge distribution on the two P sublattices in phosphorene, rendered by the alternative cation and anion chains in PbTe substrate that breaks the glide reflection symmetry and inversion symmetry of phosphorene, leading to the in-plane polarization. It is found that the in-plane polarization in phosphorene can be significantly modulated by interlayer strain and external electric field. More interestingly, the in-plane electric polarization switches sign when phosphorene shifts on PbTe substrate by an interval between the cation and anion chains with a low transition barrier of a few meV/P-atom. Our study demonstrates that the switching of 2D in-plane polarization via spatial shifting in phosphorene-based van der Waals heterostructures, with promising applications in 2D polarization functionalized nanoelectronic and optoelectronic devices.

Emerging van der Waals ferroelectrics: Unique properties and novel devices

The past few decades have witnessed extensive and intensive studies on ferroelectric materials with switchable electric polarization due to their broad device applications. Emerging van der Waals (vdW) layered ferroelectrics ingeniously assemble strong covalent-bonded polar or non-polar monolayers through weak vdW forces. These atom arrangements contrast with the stacking of conventional oxide ferroelectrics, enabling unprecedented ferroelectric physics in terms of polarization origin, polar stabilization, and switching kinetics. Combined with other inherent optical and electrical features, the vdW ferroelectrics can undoubtedly provide a new, versatile platform for advancing fundamental physics and revolutionizing device technology. In this review, we summarize the unique ferroelectric properties in experimentally confirmed vdW ferroelectrics, particularly those properties that expand our understanding of ferroelectric switching. We also elucidate how some of these properties can intrinsically reduce depolarized instability at the atomic limit. Finally, we discuss innovative devices enabled by distinct properties of vdW ferroelectrics for electronic, optoelectronic, and energy-harvesting applications, and highlight possible future research lines.

Colloquium : Physical properties of group-IV monochalcogenide monolayers

We survey the state-of-the-art knowledge of ferroelectric and ferroelastic group-IV monochalcogenide monolayers. These semiconductors feature remarkable structural and mechanical properties, such as a switchable in-plane spontaneous polarization, soft elastic constants, structural degeneracies, and thermally-driven two-dimensional structural transformations. Additionally, these 2D materials also display selective valley excitations, valley Hall effects, and persistent spin helix behavior. After a description of their Raman spectra, a discussion of optical properties arising from their lack of centrosymmetry---such as an unusually strong second-harmonic intensity, large bulk photovoltaic effects, photostriction, and tunable exciton binding energies---is provided as well. The physical properties observed in these materials originate from (correlate with) their intrinsic and switchable electric polarization, and the physical behavior hereby reviewed could be of use in non-volatile memory, valleytronic, spintronic, and optoelectronic devices: these 2D multiferroics enrich and diversify the 2D materials toolbox.

Mechanical Manipulation of Nano‐Twinned Ferroelectric Domain Structures for Multilevel Data Storage

AbstractFerroelectric materials feature a switchable spontaneous electric polarization and can enable low‐power logic and nonvolatile memories. These applications require reliable and precise control of ferroelectric domains and domain walls in ferroelectric thin films. Mechanical manipulation is a promising route to engineer ferroelectric domains, but it has proved ineffective when going beyond a critical thickness. Here, multi‐step 90° switching polarization reversal processes in (111)‐oriented PbZr0.2Ti0.8O3 thin films by applying mechanical forces along the direction parallel to the domain bands are reported. By probing the interrelationships between the relevant order parameters, coupled lattice distortion and piezoelectricity is revealed to facilitate domain switching from downward to upward in PbZr0.2Ti0.8O3, a mechanism that is supported by the evolution of domains and electrical performances at different temperatures and under varying pressures, respectively. The multi‐step domain reversal processes render PbZr0.2Ti0.8O3 thin films an excellent candidate for multilevel data storage. The study's results have implications for the manipulation of polarization switching in ferroelectrics and open an avenue to domain reversal driven by mechanical loads for the development of next‐generation ferroelectric devices.

Single frequency vertical piezoresponse force microscopy

Piezoresponse force microscopy (PFM) uses a cantilever arm to track the electromechanical motion of the electric dipole moment to visualize the ferroelectric domain structure, which provides an important insight into the physics of switchable electric polarization—especially for memory devices and integrated microelectronics. Here, I provide a tutorial on single frequency vertical PFM, the most basic mode of PFM. I will start with the basic components of atomic force microscopy (AFM), including tip, cantilever, X–Y stage, Z actuator, and lock-in amplifier. Contact mode AFM will be briefly explained and discussed, where you can find two modes: constant deflection and constant height modes. Single-frequency vertical PFM splits the frequency domain of tip vibration into low and high frequencies and uses a low-pass filter to nullify any motion caused by topography (constant deflection). In contrast, the lock-in amplifier will pinpoint the vibration induced by the vertical piezoelectric strain along the sample’s surface normal (constant height). This tutorial will provide an overall and detailed step by step instruction to conduct PFM imaging and piezoresponse hysteresis loop measurement using atomic force microscopy and a lock-in amplifier and teach how to interpret the PFM images and the piezoresponse hysteresis loops for various applications.

Electric Polarization Switching on an Atomically Thin Metallic Oxide.

Materials with reduced dimensions have been shown to host a wide variety of exotic properties and novel quantum states that often defy textbook wisdom. Polarization switching and metallic screening are well-known examples of mutually exclusive properties that cannot coexist in bulk solids. Here we report the fabrication of (SrRuO3)1/(BaTiO3)10 superlattices that exhibits reversible polarization switching in an atomically thin metallic layer. A multipronged investigation combining structural analyses, electrical measurements, and first-principles electronic structure calculations unravels the coexistence of two-dimensional (2D) metallicity in the SrRuO3 layer accompanied by the breaking of inversion symmetry, supporting electric polarization along the out-of-plane direction. Such a 2D ferroelectric-like metal paves a novel way to engineer a quantum multistate with unusual coexisting properties, such as ferroelectrics and metals, manipulated by external fields.