Ferroelectric Domain Walls Research Articles

Magnetoelectric Phase Control at Domain‐Wall‐Like Epitaxial Oxide Multilayers

AbstractFerroelectric domain walls are nanoscale objects that can be created, positioned, and erased on demand. They often embody functional properties that are distinct from the surrounding bulk material. Enhanced conductivity, for instance, is observed at charged ferroelectric domain walls. Regrettably, domain walls of this type are scarce because of the energetically unfavorable electrostatics. This hinders the current technological development of domain‐wall nanoelectronics. Here this constraint is overcome by creating robust domain‐wall‐like objects in epitaxial oxide heterostructures. Charged head‐to‐head (HH) and tail‐to‐tail (TT) junctions are designed with two ferroelectric layers (BaTiO3 and BiFeO3) that have opposing out‐of‐plane polarization. To test domain‐wall‐like functionalities, an ultrathin ferromagnetic La0.7Sr0.3MnO3 layer is inserted into the junctions. The interfacial electron or hole accumulation at the interfaces, set by the HH and TT polarization configurations, respectively, controls the LSMO conductivity and magnetization. Thus it is proposed that trilayers reminiscent of artificial domain walls provide magnetoelectric functionality and may constitute an important building block in the design of oxide‐based electronic devices.

Graphene-enhanced ferroelectric domain wall high-output memristor

Recent studies on memristive materials and technologies have expanded beyond conventional memory elements, driven by their potential application in novel information processing concepts. Among these materials, conductive domain walls in ferroics are especially promising, offering conductive tunability suitable for reconfigurable multi-state devices. However, challenges such as domain stability, time-dependent conductivity, and low current output have impeded progress in the field. Here, we study the graphene/Pb(Zr,Ti)O3/SrRuO3 system, which demonstrates robust domain wall conduction up to 100 nA/μm2 for 2 V bias, while addressing the critical issue of stability of switched domains. The introduction of graphene electrodes enhances low-voltage stochastic domain formation with limited domain expansion that promotes the emergence of multi-domain states. The developed micrometer sized capacitor devices enable electrically programmable multiple distinct conduction states with robust retention combined with high current output and low operation voltage. These features are highly desirable for memristors and mark the significant potential of domain wall electronics for neuromorphic computing.

Electrical conduction of ferroelectric domains and domain walls in polycrystalline BiFeO3 and Bi5Ti3FeO15 thin films

The unique properties of domain wall conductivity have garnered significant interest for their potential application in non-volatile ferroelectric domain wall memory. In this study, we investigated the electrical conduction within ferroelectric domains and domain walls of polycrystalline BiFeO3 (BFO) and Bi5Ti3FeO15 (BTFO) thin films, which were deposited on Pt/Ta/glass substrates via pulsed laser deposition. BFO thin film consistently demonstrated a (111) orientation, while BTFO thin film exhibited mixed crystallinity, featuring both c-axis and a-axis orientations. This mixed crystallinity in BTFO thin film contributed to a higher remanent polarization of 38.2 μC/cm2 compared to 20.3 μC/cm2 in BFO thin film, which is attributed to the a-oriented crystallinity within the Bi-layered perovskite structure of BTFO thin film. Additionally, BTFO thin film displayed a greater prevalence of 90° domain walls, which enhanced electrical conduction due to charge accumulation, particularly when compared to 180° domain walls. A significant change in resistance was observed when the domain wall was present versus absent, with a more pronounced effect in the BTFO capacitor compared to the BFO capacitor. This is attributed to the higher domain wall conductivity in BTFO thin film, confirming their superiority for use in ferroelectric capacitor devices that leverage domain wall conductivity.

Intrinsically Stable Charged Domain Walls in Molecular Ferroelectric Thin Films

AbstractCharged domain walls in ferroelectrics hold great promise for applications in ferroelectric random‐access memory (FeRAM), with advantages such as low energy consumption, high density, and non‐destructive operation. Due to the mechanical compatibility condition, the neutral domain walls are dominant in traditional ferroelectric thin films. Herein, using phase‐field simulations, the formation of intrinsically stable charged domain walls (CDWs) in the molecular ferroelectric films is demonstrated, which can be mainly attributed to the small mechanical stiffness. The switching kinetics are further investigated for the CDWs, showing a lower switching barrier as compared to the neutral counterparts. Moreover, it is indicated that increasing the compressive misfit strain can lead to prolonged switching time, with a significantly increased switching energy barrier. These findings pave the way for the potential applications of metal‐free organic ferroelectric materials in FeRAM devices.

Mobile intrinsic point defects for conductive neutral domain walls in LiNbO3.

Conductive ferroelectric domain walls (DWs) hold great promise for neuromorphic nanoelectronics as they can contribute to realize multi-level diodes and nanoscale memristors. Point defects accumulating at DWs will change the local electrical transport properties. Hence, local, inter-switchable n- and p-type conductivity at DWs can be achieved through point defect population control. Here, we study the impact of point defects on the electronic structure at neutral domain walls in LiNbO3 by density functional theory (DFT). Segregation of Li and O vacancies was found to be energetically favourable at neutral DWs, implying that charge-compensating electrons or holes can give rise to n- or p-type conductivity. Changes in the electronic band gap and defect transition levels are discussed with respect to local property engineering, opening the pathway for reversible tuning between n- and p-type conduction at neutral ferroelectric DWs. Specifically, the high Curie temperature of LiNbO3 and the significant calculated mobility of O and Li vacancies suggest that thermal annealing and applied electric fields can be used experimentally to control point defect populations, and thus enable rewritable pn-junctions.

Toward the reproducible fabrication of conductive ferroelectric domain walls into lithium niobate bulk single crystals

Ferroelectric domain walls (DWs) are promising structures for assembling future nano-electronic circuit elements on a larger scale since reporting domain wall currents of up to 1 mA per single DW. One key requirement hereto is their reproducible manufacturing by gaining preparative control over domain size and domain wall conductivity (DWC). To date, most works on DWC have focused on exploring the fundamental electrical properties of individual DWs within single-shot experiments, with an emphasis on quantifying the origins of DWC. Very few reports exist when it comes to comparing the DWC properties between two separate DWs, and literally nothing exists where issues of reproducibility in DWC devices have been addressed. To fill this gap while facing the challenge of finding guidelines for achieving predictable DWC performance, we report on a procedure that allows us to reproducibly prepare single hexagonal domains of a predefined diameter into uniaxial ferroelectric lithium niobate single crystals of 200 and 300 μm thickness, respectively. We show that the domain diameter can be controlled with an uncertainty of a few percent. As-grown DWs are then subjected to a standard procedure of current-limited high-voltage DWC enhancement, and they repetitively reach a DWC increase of six orders of magnitude. While all resulting DWs show significantly enhanced DWC values, their individual current–voltage (I–V) characteristics exhibit different shapes, which can be explained by variations in their 3D real structure reflecting local heterogeneities by defects, DW pinning, and surface-near DW inclination.

Observation of Antiferroelectric Domain Walls in a Uniaxial Hyperferroelectric.

Ferroelectric domain walls are a rich source of emergent electronic properties and unusual polar order. Recent studies show that the configuration of ferroelectric walls can go well beyond the conventional Ising-type structure. Néel-, Bloch-, and vortex-like polar patterns have been observed, displaying strong similarities with the spin textures at magnetic domain walls. Here, the discovery of antiferroelectric domain walls in the uniaxial ferroelectric Pb5Ge3O11 is reported. Highly mobile domain walls with an alternating displacement of Pb atoms are resolved, resulting in a cyclic 180° flip of dipole direction within the wall. Density functional theory calculations show that Pb5Ge3O11 is hyperferroelectric, allowing the system to overcome the depolarization fields that usually suppress the antiparallel ordering of dipoles along the longitudinal direction. Interestingly, the antiferroelectric walls observed under the electron beam are energetically more costly than basic head-to-head or tail-to-tail walls. The results suggest a new type of excited domain-wall state, expanding previous studies on ferroelectric domain walls into the realm of antiferroic phenomena.

Intrinsic Conductance of Ferroelectric Charged Domain Walls

Intrinsic Conductance of Ferroelectric Charged Domain Walls

Antipolar 2D Metallicity with Tunable Valence Wx+ (5 ≤ x ≤ 5.6) in the Layered Monophosphate Tungsten Bronzes [Ba(PO4)2]WmO3m-3.

The newly discovered series of layered monophosphate tungsten bronzes (L-MPTB) [Ba(PO4)2]WmO3m-3 consist of m-layer-thick slabs of WO6 octahedra separated by barium-phosphate spacers. They display a 2D metallic behavior confined in the central part of the perovskite slabs. Here, we report the missing m = 2 member of this series, containing the rather uncommon W5+ oxidation state. We have analyzed its structure-property relationships in relation to the other members of the L-MPTB family. In particular, we have determined its crystal structure by means of single-crystal X-ray and electron diffraction and investigated its physical properties from resistivity, Seebeck-coefficient and heat-capacity measurements combined with first-principles calculations. All the L-MPTB compounds show metallic behavior down to 1.8 K without any clear charge-density-wave (CDW) order. The m = 2 member, however, displays an increased influence of the spacer that translates into anisotropic negative thermal expansion, reversed thermopower and reversed crystal-field splitting of the tungsten t2g orbitals. Our analysis of the full [Ba(PO4)2]WmO3m-3 series reveals a systematic and significant W off-centering in their octahedral coordination. We identify the resulting anti-polar character of these W displacements as the crucial aspect behind the 2D metallicity of these systems: It leads to the presence of bound charges whose screening determines the distribution of mobile charges, tending to accumulate at the center of the [WmO3-m] block. We argue that this mechanism is analogous to enhanced conductivity observed for charged domain walls in ferroelectrics, thus providing a general design rule to promote 2D metallicity in layered systems.

Growth of ferroelectric domain nuclei: Insight from a sharp-interface model

We present an analytical framework to study the impact of electromechanical properties on the growth of a ferroelectric nucleus. Ferroelectric domain evolution is typically simulated by phase-field models, which have shown that nuclei evolve from needle-like structures into complex domain patterns. However, there has been limited in-depth analysis of the interplay between electrostatics, mechanics and piezoelectricity and their effect on nucleus growth because of the complexity involved in the phase-field description. In this study, we describe the ferroelectric domain wall as a sharp interface and solve for the fields inside an elliptic ferroelectric nucleus via Eshelby’s inclusion problem. We analytically determine the driving traction profile around the nucleus to gain insight into the movement of the domain wall with and without applied electromechanical loading. We analyze how the growth is affected by the permittivity, elasticity, and piezoelectricity as well as the nucleus’ eccentricity. We further demonstrate that applied loads do not significantly affect nucleus growth, which is primarily determined by the self-equilibrated mechanical and electric field, and that the anisotropy in material properties is essential in determining the growth of a ferroelectric nucleus.

Ferroelectric polarization and magnetic structure at domain walls in a multiferroic film

Domain walls affect significantly ferroelectric and magnetic properties of magnetoelectric multiferroics. The stereotype is that the ferroelectric polarization will reduce at the domain walls due to the incomplete shielding of depolarization field or the effects of gradient energy. By combining transmission electron microscopy and first-principles calculations, we demonstrate that the ferroelectric polarization of tail-to-tail 180° domain walls in ε-Fe2O3 is regulated by the bound charge density. A huge enhancement (43%) of ferroelectric polarization is observed in the type I domain wall with a low bound charge density, while the ferroelectric polarization is reduced to almost zero at the type II domain wall with a high bound charge density. The magnetic coupling across the type I and type II ferroelectric domain walls are antiferromagnetic and ferromagnetic, respectively. Revealing mechanisms for enhancing ferroelectric polarization and magnetic behaviors at ferroelectric domain walls may promote the fundamental research and potential applications of magnetoelectric multiferroics.

Bulk Photovoltaic Effect in Single Ferroelectric Domain of SnS Crystal and Control of Local Polarization by Strain

AbstractThe bulk photovoltaic effect (BPVE) in ferroelectrics, wherein spontaneous polarization can be reversed within crystals lacking centrosymmetry, encompasses the significant contribution of ferroelectric domain walls (DWs), known as DW‐PVE. Nevertheless, the separation between intrinsic BPVE within the domain and DW‐PVE remains unexplored in 2D ferroelectrics, notwithstanding its significant importance. In this study, sizable crystals of 2D ferroelectric SnS are successfully grown, facilitating a comprehensive yet intricate examination of domain configurations utilizing polarized optical microscopy and piezoresponse force microscopy. By properly selecting the large ferroelectric single domain within SnS crystals, uniform intrinsic BPVE across the domain is unequivocally demonstrated. Furthermore, to further enhance intrinsic BPVE, manipulation of strain poling increased photocurrent, suggesting that locally distributed polarizations due to imperfection introduced in SnS crystals are aligned by strain. These results will offer a new avenue for rigorous comprehension of DW‐PVE in 2D ferroelectrics.

Multi‐Functional Ferroelectric Domain Wall Nanodevices for In‐Memory Computing and Light Sensing

AbstractIn‐memory computing and sensing can break through the limit of the traditional Von Neumann architecture where information storing and computing units are separated. Recently the topographic creation of a ferroelectric domain wall in separation of two antiparallel domains in LiNbO3 single‐crystal films integrated with the Si substrate has been the focus of such research. Here, the additional freedom of the light stimulus is reported to modulate the domain wall current significantly, enabling applications in nonvolatile domain wall memory, photodetector, and image recognition processor. These measurements shows the fantastic photosensitivity of 2D domain walls where the photocurrent varies nonlinearly against the accumulative pulse number. With this observation, an associated learning model is constructed to stimulate neuromorphic computing (with a recognition accuracy of 90%) using an artificial neuromorphic network of domain walls. The research promotes the multifold development of artificial intelligence using domain‐wall nanodevices sensitive to both optical and electrical stimulus.

Optical Control of In-Plane Domain Configuration and Domain Wall Motion in Ferroelectric and Ferroelastic Materials.

The sensitivity of ferroelectric domain walls to external stimuli makes them functional entities in nanoelectronic devices. Specifically, optically driven domain reconfiguration with in-plane polarization is advantageous and thus is highly sought. Here, we show the existence of in-plane polarized subdomains imitating a single domain state and reversible optical control of its domain wall movement in a single-crystal of ferroelectric BaTiO3. Similar optical control in the domain configuration of nonpolar ferroelastic material indicates that long-range ferroelectric polarization is not essential for the optical control of domain wall movement. Instead, flexoelectricity is found to be an essential ingredient for the optical control of the domain configuration, and hence, ferroelastic materials would be another possible candidate for nanoelectronic device applications.

Comparative study of photo-induced electronic transport along ferroelectric domain walls in lithium niobate single crystals

Ferroelectric domain wall conductivity (DWC) is an intriguing and promising functional property that can be elegantly controlled and steered through a variety of external stimuli such as electric and mechanical fields. Optical-field control, as a noninvasive and flexible tool, has rarely been applied so far, but it significantly expands the possibility for both tuning and probing DWC. On the one hand, as known from second-harmonic or Raman micro-spectroscopy, the optical approach provides information on DW distribution and inclination, while simultaneously probing the DW vibrational modes; on the other hand, photons might be applied to directly generate charge carriers, thereby acting as a functional and spectrally tunable probe to deduce the local absorption properties and bandgaps of conductive DWs. Here, we report on investigating the photo-induced DWC (PI-DWC) of three lithium niobate crystals, containing a very different number of DWs, namely: (A) none, (B) one, and (C) many conductive DWs. All three samples are inspected for their current–voltage behavior in darkness and for different illumination wavelengths swept from 500 nm down to 310 nm. All samples show their maximum PI-DWC at 310 nm; moreover, sample (C) reaches PI-DWCs of several microampere. Interestingly, a noticeable PI-DWC is also observed for sub-bandgap illumination, hinting toward the existence and decisive role of electronic in-gap states that contribute to the electronic charge transport along DWs. Finally, complementary conductive atomic force microscopy investigations under illumination proved that the PI-DWC indeed is confined to the DW area and does not originate from photo-induced bulk conductivity.

High-Performance LiNbO3 Domain Wall Memory Devices with Enhanced Selectivity via Optimized Metal-Semiconductor Contact.

Lithium niobate (LiNbO3) single-crystal nanodevices featuring elevated readout domain wall currents exhibit significant potential for integrated circuits in memory computing applications. Nevertheless, challenges stem from suboptimal electrode-LiNbO3 single crystal contact characteristics, which impact the stability of high currents within these devices. In this work, we concentrate on augmenting the domain wall current by refining the fabrication processes of domain wall random access memory (DWRAM). Each LiNbO3 domain wall nanodevice was fabricated using a self-aligned process. Device performance was significantly enhanced by introducing a 10 nm interlayer between the LiNbO3 and Cu electrodes. A comparative analysis of electrical properties was conducted on devices with interlayers made of chromium (Cr) and titanium (Ti), as well as devices without interlayers. After the introduction of the Ti interlayer, the device's coercive voltage demonstrated an 82% reduction, while the current density showed a remarkable 94-fold increase. A 100 nm sized device with the Ti interlayer underwent positive down-negative up pulse testing, demonstrating a writing time of 82 ns at 8 V and an erasing time of 12 μs at -9 V. These operating speeds are significantly faster than those of devices without interlayers. Moreover, the enhanced devices exhibited symmetrical domain switching hysteresis loops with retention times exceeding 106 s. Notably, the coercive voltage (Vc) dispersion remained narrow after more than 1000 switching cycles. At an elevated temperature of 400 K, the device's on/off ratio was maintained at 105. The device's embedded selector demonstrated an ultrahigh selectivity (>106) across various reading voltages. These results underscore the viability of high-density nanoscale integration of ferroelectric domain wall memory.

Universal Dimensionality of Ferroelectric Domain Walls in Ultrathin Films

AbstractThe dimensionality of dynamic interfaces—domain walls (DWs) —is greatly influenced by symmetry and physical dimensions, irrespective of the microscopic details of the system. To address this fundamental question for the ferroelectric model system, the DW scaling criticality and dimensionality is investigated in ultrathin films of varied ferroelectric materials, compositions, and electrode–ferroelectric interfaces, grown on nominally flat and vicinal substrates. In spite of significant variations among ferroelectric systems, the observed prevalence of 1D DWs is consistent with a random bond disorder, elucidated through a scenario of 2D nucleation and growth‐driven DW creep. These findings highlight the thickness size‐dominated universal behavior of ferroelectric DWs, uncovering fascinating prospects for dimensionally engineered DW‐based nanoelectronics.

Recent progresses in transmission electron microscopy studies of two-dimensional ferroelectrics

The rich potential of two-dimensional materials endows them with superior properties suitable for a wide range of applications, thereby attracting substantial interest across various fields. The ongoing trend towards device miniaturization aligns with the development of materials at progressively smaller scales, aiming to achieve higher integration density in electronics. In the realm of nano-scaling ferroelectric phenomena, numerous new two-dimensional ferroelectric materials have been predicted theoretically and subsequently validated through experimental confirmation. However, the capabilities of conventional tools, such as electrical measurements, are limited in providing a comprehensive investigation into the intrinsic origins of ferroelectricity and its interactions with structural factors. These factors include stacking, doping, functionalization, and defects. Consequently, the progress of potential applications, such as high-density memory devices, energy conversion systems, sensing technologies, catalysis, and more, is impeded. In this paper, we present a review of recent research that employs advanced transmission electron microscopy (TEM) techniques for the direct visualization and analysis of ferroelectric domains, domain walls, and other crucial features at the atomic level within two-dimensional materials. We discuss the essential interplay between structural characteristics and ferroelectric properties on the nanoscale, which facilitates understanding of the complex relationships governing their behavior. By doing so, we aim to pave the way for future innovative applications in this field.

The Operation of Lithium Niobate Ferroelectric Domain Wall Memory at 673 K

The Operation of Lithium Niobate Ferroelectric Domain Wall Memory at 673 K

Polar orientation and extension in a novel crystallographic model for PbTiO3-based perovskites explaining the experimental ferroelectric thermal anomalies

PbTiO3-based ferroelectric solid-solution ceramics have been widely used for electromechanical devices. However, it is still challenging to separate and control the contributions to the electromechanical functionalities, mainly as a function of temperature, where thermal anomalies and phase transitions can be observed. This study investigates the ultrasonic velocity and attenuation and the dielectric, ferroelectric and structural features of Pb0.55Ca0.45TiO3 ceramics from low temperatures (10 or 115 K) up to room temperature as an example of A-site isovalent substitution in PbTiO3. Such a combination of information makes possible the phenomenological deconvolution of the effects of ferroelectric domain wall pinning and structural features on spontaneous electric polarization. The room-temperature symmetry was determined as Pna21. The results show that this model refined by the Rietveld method for synchrotron X-ray diffraction patterns from 115 K to room temperature can explain the polarization extension features of these materials during heating. This study shows a correlation between structural thermal anomalies and low-temperature electric polarization in PbTiO3-based ferroelectric ceramics.