Tuning of the depolarization field and nanodomain structure in ferroelectric thin films

Tunable Ferroionic Properties in CeO2/BaTiO3 Heterostructures

Ferroionic materials combine ferroelectric properties and spontaneous polarization with ionic phenomena of fast charge recombination and electrodic functionalities. In this paper, we propose the concept of tunable polarization in CeO2-δ (ceria) thin (5 nm) films induced by built-in remnant polarization of a BaTiO3 (BTO) ferroelectric thin film interface, which is buried under the ceria layer. Upward and downward fixed polarizations at the BTO thin film (10 nm) are achieved by the lattice termination engineering of the SrO or TiO2 terminated Nb:SrTiO3 (NSTO or STN) substrate. We find that the ceria layer punctually replicates the polarization of the BTO interface via a dynamic reconfiguration of its intrinsic defects, i.e., oxygen vacancies and small polarons. Tunable oxidative or reducing properties (redox) also arise at the surface from the built-in polarization. Opposite polarities at the ceria termination tune the chemo-physical dynamics toward water molecule adsorbates. The inversion of the surface potential leads to a modulation of the water adsorption-desorption equilibrium and water ionization (splitting) redox overpotentials within ±400 mV at room temperature, depending on the ceria termination's charges. Such tunability opens up the perspectives of using ferroionics for wireless electrochemically enhanced catalysis.

Brownian Electric Bubble Quasiparticles

Recent works on electric bubbles (including the experimental demonstration of electric skyrmions) constitute a breakthrough akin to the discovery of magnetic skyrmions some 15 years ago. So far research has focused on obtaining and visualizing these objects, which often appear to be immobile (pinned) in experiments. Thus, critical aspects of magnetic skyrmions-e.g., their quasiparticle nature, Brownian motion-remain unexplored (unproven) for electric bubbles. Here we use predictive atomistic simulations to investigate the basic dynamical properties of these objects in pinning-free model systems. We show that it is possible to find regimes where the electric bubbles can present long lifetimes (∼ns) despite being relatively small (diameter <2  nm). Additionally, we find that they can display stochastic dynamics with large and highly tunable diffusion constants. We thus establish the quasiparticle nature of electric bubbles and put them forward for the physical effects and applications (e.g., in token-based probabilistic computing) considered for magnetic skyrmions.

Electrically and mechanically driven rotation of polar spirals in a relaxor ferroelectric polymer

Topology created by quasi-continuous spatial variations of a local polarization direction represents an exotic state of matter, but field-driven manipulation has been hitherto limited to creation and destruction. Here we report that relatively small electric or mechanical fields can drive the non-volatile rotation of polar spirals in discretized microregions of the relaxor ferroelectric polymer poly(vinylidene fluoride-ran-trifluoroethylene). These polar spirals arise from the asymmetric Coulomb interaction between vertically aligned helical polymer chains, and can be rotated in-plane through various angles with robust retention. Given also that our manipulation of topological order can be detected via infrared absorption, our work suggests a new direction for the application of complex materials.

Spherical ferroelectric solitons

Spherical ferroelectric domains, such as electrical bubbles, polar skyrmion bubbles and hopfions, share a single and unique feature-their homogeneously polarized cores are surrounded by a vortex ring of polarization whose outer shells form a spherical domain boundary. The resulting polar texture, typical of three-dimensional topological solitons, has an entirely new local symmetry characterized by a high polarization and strain gradients. Consequently, spherical domains represent a different material system of their own with emergent properties drastically different from that of their surrounding medium. Examples of new functionalities inherent to spherical domains include chirality, optical response, negative capacitance and giant electromechanical response. These characteristics, particularly given that the domains naturally have an ultrafine scale, offer new opportunities in high-density and low-energy nanoelectronic technologies. This Perspective gives an insight into the complex polar structure and physical origin of these spherical domains, which facilitates the understanding and development of spherical domains for device applications.

Epitaxial ferroelectric memristors integrated with silicon

Neuromorphic computing requires the development of solid-state units able to electrically mimic the behavior of biological neurons and synapses. This can be achieved by developing memristive systems based on ferroelectric oxides. In this work we fabricate and characterize high quality epitaxial BaTiO3-based memristors integrated with silicon. After proving the ferroelectric character of BaTiO3 we tested the memristive response of LaNiO3/BaTiO3/Pt microstructures and found a complex behavior which includes the co-existence of volatile and non-volatile effects, arising from the modulation of the BaTiO3/Pt Schottky interface by the direction of the polarization coupled to oxygen vacancy electromigration to/from the interface. This produces remanent resistance loops with tunable ON/OFF ratio and asymmetric resistance relaxations. These properties might be harnessed for the development of neuromorphic hardware compatible with existing silicon-based technology.

Multilevel polarization switching in ferroelectric thin films

Ferroic order is characterized by hystereses with two remanent states and therefore inherently binary. The increasing interest in materials showing non-discrete responses, however, calls for a paradigm shift towards continuously tunable remanent ferroic states. Device integration for oxide nanoelectronics furthermore requires this tunability at the nanoscale. Here we demonstrate that we can arbitrarily set the remanent ferroelectric polarization at nanometric dimensions. We accomplish this in ultrathin epitaxial PbZr_0.52Ti_0.48O₃ films featuring a dense pattern of decoupled nanometric 180° domains with a broad coercive-field distribution. This multilevel switching is achieved by driving the system towards the instability at the morphotropic phase boundary. The phase competition near this boundary in combination with epitaxial strain increases the responsiveness to external stimuli and unlocks new degrees of freedom to nano-control the polarization. We highlight the technological benefits of non-binary switching by demonstrating a quasi-continuous tunability of the non-linear optical response and of tunnel electroresistance.

Setting any polarization value in ferroelectric thin films is a key step for their implementation in neuromorphic devices. Here, the authors demonstrate continuous modulation of the remanent polarization at the nanoscale in PbZr_0.52Ti_0.48O₃ films.

Electrostatically Driven Polarization Flop and Strain-Induced Curvature in Free-Standing Ferroelectric Superlattices

The combination of strain and electrostatic engineering in epitaxial heterostructures of ferroelectric oxides offers many possibilities for inducing new phases, complex polar topologies, and enhanced electrical properties. However, the dominant effect of substrate clamping can also limit the electromechanical response and often leaves electrostatics to play a secondary role. Releasing the mechanical constraint imposed by the substrate can not only dramatically alter the balance between elastic and electrostatic forces, enabling them to compete on par with each other, but also activates new mechanical degrees of freedom, such as the macroscopic curvature of the heterostructure. In this work, an electrostatically driven transition from a predominantly out-of-plane polarized to an in-plane polarized state is observed when a PbTiO₃ /SrTiO₃ superlattice with a SrRuO₃ bottom electrode is released from its substrate. In turn, this polarization rotation modifies the lattice parameter mismatch between the superlattice and the thin SrRuO₃ layer, causing the heterostructure to curl up into microtubes. Through a combination of synchrotron-based scanning X-ray diffraction imaging, Raman scattering, piezoresponse force microscopy, and scanning transmission electron microscopy, the crystalline structure and domain patterns of the curved superlattices are investigated, revealing a strong anisotropy in the domain structure and a complex mechanism for strain accommodation.

High-precision and linear weight updates by subnanosecond pulses in ferroelectric tunnel junction for neuro-inspired computing

The rapid development of neuro-inspired computing demands synaptic devices with ultrafast speed, low power consumption, and multiple non-volatile states, among other features. Here, a high-performance synaptic device is designed and established based on a Ag/PbZr_0.52Ti_0.48O₃ (PZT, (111)-oriented)/Nb:SrTiO₃ ferroelectric tunnel junction (FTJ). The advantages of (111)-oriented PZT (~1.2 nm) include its multiple ferroelectric switching dynamics, ultrafine ferroelectric domains, and small coercive voltage. The FTJ shows high-precision (256 states, 8 bits), reproducible (cycle-to-cycle variation, ~2.06%), linear (nonlinearity <1) and symmetric weight updates, with a good endurance of >10⁹ cycles and an ultralow write energy consumption. In particular, manipulations among 150 states are realized under subnanosecond (~630 ps) pulse voltages ≤5 V, and the fastest resistance switching at 300 ps for the FTJs is achieved by voltages <13 V. Based on the experimental performance, the convolutional neural network simulation achieves a high online learning accuracy of ~94.7% for recognizing fashion product images, close to the calculated result of ~95.6% by floating-point-based convolutional neural network software. Interestingly, the FTJ-based neural network is very robust to input image noise, showing potential for practical applications. This work represents an important improvement in FTJs towards building neuro-inspired computing systems.

Brain-inspired computing demands high-performance synapses. Here, the authors report a subnanosecond ferroelectric tunnel junction with 256 conductance states, 10⁹ endurance, and 5.3 fJ/bit energy consumption, satisfactory to build synaptic devices.

In situmonitoring of epitaxial ferroelectric thin-film growth

In ferroelectric thin films, the polarization state and the domain configuration define the macroscopic ferroelectric properties such as the switching dynamics. Engineering of the ferroelectric domain configuration during synthesis is in permanent evolution and can be achieved by a range of approaches, extending from epitaxial strain tuning over electrostatic environment control to the influence of interface atomic termination. Exotic polar states are now designed in the technologically relevant ultrathin regime. The promise of energy-efficient devices based on ultrathin ferroelectric films depends on the ability to create, probe, and manipulate polar states in ever more complex epitaxial architectures. Because most ferroelectric oxides exhibit ferroelectricity during the epitaxial deposition process, the direct access to the polarization emergence and its evolution during the growth process, beyond the realm of existing structuralin situdiagnostic tools, is becoming of paramount importance. We review the recent progress in the field of monitoring polar states with an emphasis on the non-invasive probes allowing investigations of polarization during the thin film growth of ferroelectric oxides. A particular importance is given to optical second harmonic generationin situ. The ability to determine the net polarization and domain configuration of ultrathin films and multilayers during the growth of multilayers brings new insights towards a better understanding of the physics of ultrathin ferroelectrics and further control of ferroelectric-based heterostructures for devices.

Multiferroic heterostructures for spintronics

For next-generation technology, magnetic systems are of interest due to the natural ability to store information and, through spin transport, propagate this information for logic functions. Controlling the magnetization state through currents has proven energy inefficient. Multiferroic thin-film heterostructures, combining ferroelectric and ferromagnetic orders, hold promise for energy efficient electronics. The electric field control of magnetic order is expected to reduce energy dissipation by 2–3 orders of magnitude relative to the current state-of-the-art. The coupling between electrical and magnetic orders in multiferroic and magnetoelectric thin-film heterostructures relies on interfacial coupling though magnetic exchange or mechanical strain and the correlation between domains in adjacent functional ferroic layers. We review the recent developments in electrical control of magnetism through artificial magnetoelectric heterostructures, domain imprint, emergent physics and device paradigms for magnetoelectric logic, neuromorphic devices, and hybrid magnetoelectric/spin-current-based applications. Finally, we conclude with a discussion of experiments that probe the crucial dynamics of the magnetoelectric switching and optical tuning of ferroelectric states towards all-optical control of magnetoelectric switching events.

In-situ monitoring of interface proximity effects in ultrathin ferroelectrics

The development of energy-efficient nanoelectronics based on ferroelectrics is hampered by a notorious polarization loss in the ultrathin regime caused by the unscreened polar discontinuity at the interfaces. So far, engineering charge screening at either the bottom or the top interface has been used to optimize the polarization state. Yet, it is expected that the combined effect of both interfaces determines the final polarization state; in fact the more so the thinner a film is. The competition and cooperation between interfaces have, however, remained unexplored so far. Taking PbTiO₃ as a model system, we observe drastic differences between the influence of a single interface and the competition and cooperation of two interfaces. We investigate the impact of these configurations on the PbTiO₃ polarization when the interfaces are in close proximity, during thin-film synthesis in the ultrathin limit. By tailoring the interface chemistry towards a cooperative configuration, we stabilize a robust polarization state with giant polarization enhancement. Interface cooperation hence constitutes a powerful route for engineering the polarization in thin-film ferroelectrics towards improved integrability for oxide electronics in reduced dimension.

How to maintain a robust polarization in ferroelectrics despite its inherent suppression when going to the thin-film limit is a long-standing issue. Here, the authors propose the concept of competitive and cooperative interfaces and establish robust polarization states in the ultrathin regime.

Tailoring nanoscale polarization patterns and transport properties in ferroelectric tunnel junctions by octahedral tilts in electrodes

Oxygen octahedral tilts are known for the ability to tailor polarization patterns in perovskites. We propose a way to manipulate nanoscale polarization patterns in ferroelectric tunnel junctions (FTJs) by oxygen octahedral tilts in electrodes combined with interface engineering. Here the electrode is epitaxial SrRuO3 on SrTiO3, and the ferroelectric barrier is a BaTiO3/CaTiO3 superlattice. The octahedral tilt mismatch between electrodes and the barrier is found to be eliminated at one of the two most stable interfaces by the imprinting of in-phase oxygen octahedral tilt from electrodes into the barrier, which further results in anti-polar order in the barrier, while uniform polar order is retained for another stable interface. Further analysis of electronic transport properties shows an increased transmission in FTJ with anti-polar order, which is mainly caused by octahedral tilt induced out-of-plane ferroelectric domain walls. Our results indicate that the oxygen octahedral tilts in electrodes can be used to manipulate polarization patterns and create charged domain walls in FTJs, which further expand the application prospects of FTJ-based devices.

Dynamic Manipulation in Piezoresponse Force Microscopy: Creating Nonequilibrium Phases with Large Electromechanical Response

Domain walls and topological defects in ferroelectric materials have emerged as a powerful tool for functional electronic devices including memory and logic. Similarly, wall interactions and dynamics underpin a broad range of mesoscale phenomena ranging from giant electromechanical responses to memory effects. Exploring the functionalities of individual domain walls, their interactions, and controlled modifications of the domain structures is crucial for applications and fundamental physical studies. However, the dynamic nature of these features severely limits studies of their local physics since application of local biases or pressures in piezoresponse force microscopy induce wall displacement as a primary response. Here, we introduce an approach for the control and modification of domain structures based on automated experimentation, whereby real-space image-based feedback is used to control the tip bias during ferroelectric switching, allowing for modification routes conditioned on domain states under the tip. This automated experiment approach is demonstrated for the exploration of domain wall dynamics and creation of metastable phases with large electromechanical response.

Skewed electronic band structure induced by electric polarization in ferroelectric BaTiO3

Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (FTJs). Nonvolatile ferroelectric random access memory (FeRAM) and the artificial neural network device based on the FTJ system are rapidly developing. However, because the actual ferroelectric band structure has not been elucidated, precise designing of devices has to be advanced through appropriate heuristics. Here, we perform angle-resolved hard X-ray photoemission spectroscopy of ferroelectric BaTiO₃ thin films for the direct observation of ferroelectric band skewing structure as the depth profiles of atomic orbitals. The depth-resolved electronic band structure consists of three depth regions: a potential slope along the electric polarization in the core, the surface and interface exhibiting slight changes. We also demonstrate that the direction of the energy shift is controlled by the polarization reversal. In the ferroelectric skewed band structure, we found that the difference in energy shifts of the atomic orbitals is correlated with the atomic configuration of the soft phonon mode reflecting the Born effective charges. These findings lead to a better understanding of the origin of electric polarization.

Role of ferroelectric polarization during growth of highly strained ferroelectric materials

In ferroelectric thin films and superlattices, the polarization is intricately linked to crystal structure. Here we show that it can also play an important role in the growth process, influencing growth rates, relaxation mechanisms, electrical properties and domain structures. This is studied by focusing on the properties of BaTiO₃ thin films grown on very thin layers of PbTiO₃ using x-ray diffraction, piezoforce microscopy, electrical characterization and rapid in-situ x-ray diffraction reciprocal space maps during the growth using synchrotron radiation. Using a simple model we show that the changes in growth are driven by the energy cost for the top material to sustain the polarization imposed upon it by the underlying layer, and these effects may be expected to occur in other multilayer systems where polarization is present during growth. This motivates the concept of polarization engineering as a complementary approach to strain engineering.

Ferroelectric (FE) materials are used in a wide range of applications, which often requires sizable FE polarization. Here, the authors report a growth procedure to enhance the FE polarization by exploiting the polarization of a FE substrate during growth to obtain higher strains and polarizations in the final material.

Nanowire Tunnel FET with Simultaneously Reduced Subthermionic Subthreshold Swing and Off-Current due to Negative Capacitance and Voltage Pinning Effects

Nanowire tunnel field-effect transistors (TFETs) have been proposed as the most advanced one-dimensional (1D) devices that break the thermionic 60 mV/decade of the subthreshold swing (SS) of metal oxide semiconductor field-effect transistors (MOSFETs) by using quantum mechanical band-to-band tunneling and excellent electrostatic control. Meanwhile, negative capacitance (NC) of ferroelectrics has been proposed as a promising performance booster of MOSFETs to bypass the aforementioned fundamental limit by exploiting the differential amplification of the gate voltage under certain conditions. We combine these two principles into a single structure, a negative capacitance heterostructure TFET, and experimentally demonstrate a double beneficial effect: (i) a super-steep SS value down to 10 mV/decade and an extended low slope region that is due to the NC effect and, (ii) a remarkable off-current reduction that is experimentally observed and explained for the first time by the effect of the ferroelectric dipoles, which set the surface potential in a slightly negative value and further blocks the source tunneling current in the off-state. State-of-the-art InAs/InGaAsSb/GaSb nanowire TFETs are employed as the baseline transistor and PZT and silicon-doped HfO₂ as ferroelectric materials.

Real-time studies of ferroelectric domain switching: a review

Ferroelectric materials have been utilized in a broad range of electronic, optical, and electromechanical applications and hold the promise for the design of future high-density nonvolatile memories and multifunctional nano-devices. The applications of ferroelectric materials stem from the ability to switch polarized domains by applying an electric field, and therefore a fundamental understanding of the switching dynamics is critical for design of practical devices. In this review, we summarize the progress in the study of the microscopic process of ferroelectric domain switching using recently developed in situ transmission electron microscopy (TEM). We first briefly introduce the instrumentation, experimental procedures, imaging mechanisms, and analytical methods of the state-of-the-art in situ TEM techniques. The application of these techniques to studying a wide range of complex switching phenomena, including domain nucleation, domain wall motion, domain relaxation, domain-defect interaction, and the interplay between different types of domains, is demonstrated. The underlying physics of these dynamic processes are discussed.

Interfacial phenomena in nanocapacitors with multifunctional oxides

The analysis of the structure, chemical stability, electronic and ferroelectric properties of the interfaces between Pt(001) and PbZrTiO₃(001) have been performed with ab initio methods. We show that the chemical environment plays a critical role in determining the interfacial reconstruction and charge redistribution at the metal/oxide interfaces. We demonstrate that the difference in interfacial bonds formed at the Pt/PZT interfaces with (TiZr)O₂- and PbO-termination of PZT essentially defines the effectiveness of the screening, and ease of polarisation switching in PZT-based capacitors.

Rotational polarization nanotopologies in BaTiO3/SrTiO3 superlattices

Ferroelectrics are characterized by domain structures as are other ferroics. At the nanoscale though, ferroelectrics may exhibit non-trivial or exotic polarization configurations under proper electrostatic and elastic conditions. These polar states may possess emerging properties not present in the bulk compounds and are promising for technological applications. Here, the observation of rotational polarization topologies at the nanoscale by means of aberration-corrected scanning transmission electron microscopy is reported in BaTiO₃/SrTiO₃ superlattices grown on cubic SrTiO₃(001). The transition from a highly homogeneous polarization state to the formation of rotational nanodomains has been achieved by controlling the superlattice period while maintaining compressive clamping of the superlattice to the cubic SrTiO₃ substrate. The nanodomains revealed in BaTiO₃ prove that its nominal tetragonal structure also allows rotational polar textures.

Depolarizing-Field Effects in Epitaxial Capacitor Heterostructures

We identify a transient enhancement of the depolarizing field, leading to an unexpected quench of net polarization, during the growth of a prototypical metal-ferroelectric-metal epitaxial system made of BaTiO_{3} and SrRuO_{3}. Reduced conductivity and, hence, charge screening efficiency in the early growth stage of the SrRuO_{3} top electrode promotes a breakdown of ferroelectric BaTiO_{3} into domains. We demonstrate how a thermal annealing procedure can recover the single-domain state. By tracking the polarization state in situ, using optical second harmonic generation, we bring new understanding to interface-related electrostatic effects in ferroelectric capacitors.

Design and Manipulation of Ferroic Domains in Complex Oxide Heterostructures

The current burst of device concepts based on nanoscale domain-control in magnetically and electrically ordered systems motivates us to review the recent development in the design of domain engineered oxide heterostructures. The improved ability to design and control advanced ferroic domain architectures came hand in hand with major advances in investigation capacity of nanoscale ferroic states. The new avenues offered by prototypical multiferroic materials, in which electric and magnetic orders coexist, are expanding beyond the canonical low-energy-consuming electrical control of a net magnetization. Domain pattern inversion, for instance, holds promises of increased functionalities. In this review, we first describe the recent development in the creation of controlled ferroelectric and multiferroic domain architectures in thin films and multilayers. We then present techniques for probing the domain state with a particular focus on non-invasive tools allowing the determination of buried ferroic states. Finally, we discuss the switching events and their domain analysis, providing critical insight into the evolution of device concepts involving multiferroic thin films and heterostructures.

Structures and electronic properties of domain walls in BiFeO3 thin films

Domain walls (DWs) in ferroelectrics are atomically sharp and can be created, erased, and reconfigured within the same physical volume of ferroelectric matrix by external electric fields. They possess a myriad of novel properties and functionalities that are absent in the bulk of the domains, and thus could become an essential element in next-generation nanodevices based on ferroelectrics. The knowledge about the structure and properties of ferroelectric DWs not only advances the fundamental understanding of ferroelectrics, but also provides guidance for the design of ferroelectric-based devices. In this article, we provide a review of structures and properties of DWs in one of the most widely studied ferroelectric systems, BiFeO₃ thin films. We correlate their conductivity and photovoltaic properties to the atomic-scale structure and dynamic behaviors of DWs.

Self-Organized Ferroelectric Domains Controlled by a Constant Bias from the Atomic Force Microscopy Tip

The ferroelectric polarization switching along an external electric field is most important for the applications of ferroelectric memories and piezoelectric sensors and actuators; however, the depolarization commonly occurs randomly and cannot be controlled exactly until now. Here, a tip bias introduces the polarization switching and a ∼10 μm-scale domain in a triglycine sulfate crystal, and then the polarization backswitching as a special depolarization introduces a series of ordered granular domains along a line being parallel to the c axis and through the tip which divides the original domain to two similar parts. Such backswitching is controlled by the surface charge change as a result of the interplay among polarization charges, mobile H⁺ ions at the surface, and the strong crystal anisotropy. The self-organized ferroelectric domains offer us a new freedom to design novel ferroelectric or piezoelectric devices in future.

Control of Domain Structures in Multiferroic Thin Films through Defect Engineering

Domain walls (DWs) have become an essential component in nanodevices based on ferroic thin films. The domain configuration and DW stability, however, are strongly dependent on the boundary conditions of thin films, which make it difficult to create complex ordered patterns of DWs. Here, it is shown that novel domain structures, that are otherwise unfavorable under the natural boundary conditions, can be realized by utilizing engineered nanosized structural defects as building blocks for reconfiguring DW patterns. It is directly observed that an array of charged defects, which are located within a monolayer thickness, can be intentionally introduced by slightly changing substrate temperature during the growth of multiferroic BiFeO₃ thin films. These defects are strongly coupled to the domain structures in the pretemperature-change portion of the BiFeO₃ film and can effectively change the configuration of newly grown domains due to the interaction between the polarization and the defects. Thus, two types of domain patterns are integrated into a single film without breaking the DW periodicity. The potential use of these defects for building complex patterns of conductive DWs is also demonstrated.

Temperature dependence of piezo- and ferroelectricity in ultrathin P(VDF-TrFE) films

The polymer poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) is highly desirable for piezoelectric and ferroelectric functional applications owing to its considerable electromechanical activity and reliable electrical polarization. However, a clear understanding of the effect of the thermal annealing on the electromechanical behavior and polarization nature of ultrathin crystalline P(VDF-TrFE) films is severely lacking. Here we report the thermally induced structural reorganization, and piezo- and ferroelectric features in the ultrathin P(VDF-TrFE) films. On applying a 40 °C annealing treatment, the polarization-patterned electrostrictive strain reaches the highest value of ∼53.7 pm. Besides, the ultrathin film exhibits a highly ordered antiparallel dipole alignment, the highest local piezoelectric activity, and an improved polarization relaxation time. The optimum film properties are achieved owing to a high degree of polymer chains oriented parallel to the substrate plane. Our results can reveal a promising avenue for nano-electro-mechanical and nano-ferroelectric electronic applications using ultrathin P(VDF-TrFE) films.

Self-Polarization in Epitaxial Fully Matched Lead-Free Bismuth Sodium Titanate Based Ferroelectric Thin Films

The Bi_0.5Na_0.5TiO₃-based ferroelectric is one of the most promising candidates for environment-friendly lead-free ferroelectric/piezoelectric materials for various applications such as actuators and micro-electromechanical systems. The understanding and tailoring of the ferro-(piezo-)electric properties of thin films, however, are strongly hindered by the formation of the defects such as dislocations, ion vacancies in the film, as well as by the complexity of the interface between the film and the substrate. An ideal system for the study of the polarization behavior in the ferro-(piezo-)electric film would be a fully matched system. In this work, monocrystalline 0.89Bi_0.5Na_0.5TiO₃-0.11BaTiO₃ thin films were epitaxially grown on (001)-oriented Nb-doped SrTiO₃ substrates using a sol-gel technique. The films were almost fully lattice- and thermally matched with the substrate, thus avoiding the impact of dislocations and thermal stress. The films were self-poled by a built-in electric field, originating from the sedimentation of heavier atoms during the film preparation. As a consequence, an upward self-polarization was introduced into the films, giving rise to asymmetric phase hysteresis loops and domain switching current responses. These results highlight the importance of the interface complexity for the self-polarization of piezoelectric thin films, even for fully matched films, which will therefore facilitate the control of properties of piezoelectric films and their applications for various functional devices.

Probing Ferroic States in Oxide Thin Films Using Optical Second Harmonic Generation

Forthcoming low-energy consumption oxide electronics rely on the deterministic control of ferroelectric and multiferroic domain states at the nanoscale. In this review, we address the recent progress in the field of investigation of ferroic order in thin films and heterostructures, with a focus on non-invasive optical second harmonic generation (SHG). For more than 50 years, SHG has served as an established technique for probing ferroic order in bulk materials. Here, we will survey the specific new aspects introduced to SHG investigation of ferroelectrics and multiferroics by working with thin film structures. We show how SHG can probe complex ferroic domain patterns non-invasively and even if the lateral domain size is below the optical resolution limit or buried beneath an otherwise impenetrable cap layer. We emphasize the potential of SHG to distinguish contributions from individual (multi-) ferroic films or interfaces buried in a device or multilayer architecture. Special attention is given to monitoring switching events in buried ferroic domain- and domain-wall distributions by SHG, thus opening new avenues towards the determination of the domain dynamics. Another aspect studied by SHG is the role of strain. We will finally show that by integrating SHG into the ongoing thin film deposition process, we can monitor the emergence of ferroic order and properties in situ, while they emerge during growth. Our review closes with an outlook, emphasizing the present underrepresentation of ferroic switching dynamics in the study of ferroic oxide heterostructures.

Domain Wall Orientations in Ferroelectric Superlattices Probed with Synchrotron X-Ray Diffraction

Ferroelectric domains in PbTiO_{3}/SrTiO_{3} superlattices are studied using synchrotron x-ray diffraction. Macroscopic measurements reveal a change in the preferential domain wall orientation from {100} to {110} crystallographic planes with increasing temperature. The temperature range of this reorientation depends on the ferroelectric layer thickness and domain period. Using a nanofocused beam, local changes in the domain wall orientation within the buried ferroelectric layers are imaged, both in structurally uniform regions of the sample and near defect sites and argon ion-etched patterns. Domain walls are found to exhibit a preferential alignment with the straight edges of the etched patterns as well as with structural features associated with defect sites. The distribution of out-of-plane lattice parameters is mapped around one such feature, showing that it is accompanied by inhomogeneous strain and large strain gradients.

Unveiling the piezoelectric nature of polar α-phase P(VDF-TrFE) at quasi-two-dimensional limit

Piezoelectric response of P(VDF-TrFE), which is modulated by the dipole density due to the polarization switching on applying an electric field, allows it act as the fundamental components for electromechanical systems. As proposed since the 1970s, its polar α-phase is supposed to yield an enhanced piezoelectric activity. However, its experimental verification has never been reported, hampered by a substantial challenge for the achievement of a smooth, neat α-phase film. Here, we prepare ultrathin crystalline α-phase P(VDF-TrFE) films on the AlO_x/Al-coated SiO₂/Si substrates via a solution-based approach at room temperature. Thus, we unveil the piezoelectric nature of the polar α-phase P(VDF-TrFE) at a quasi-two-dimensional limit. The obtained values of the relative morphological deformation, the local effective piezoelectric coefficient, and the electric field-induced strain reach up to 37 pm, −46.4 pm V⁻¹, and 4.1%, respectively. Such a robust piezoelectric response is even higher than that of the β-phase. Besides, the evolution of piezoelectricity, which is related to the piezoelectric properties of two polarization states, is also studied. Our work can enable the exploration of the prospective applications of polar α-phase P(VDF-TrFE) films.

Nanoscale Bubble Domains and Topological Transitions in Ultrathin Ferroelectric Films

Observation of a new type of nanoscale ferroelectric domains, termed as "bubble domains"-laterally confined spheroids of sub-10 nm size with local dipoles self-aligned in a direction opposite to the macroscopic polarization of a surrounding ferroelectric matrix-is reported. The bubble domains appear in ultrathin epitaxial PbZr_0.2 Ti_0.8 O₃ /SrTiO₃ /PbZr_0.2 Ti_0.8 O₃ ferroelectric sandwich structures due to the interplay between charge and lattice degrees of freedom. The existence of the bubble domains is revealed by high-resolution piezoresponse force microscopy (PFM), and is corroborated by aberration-corrected atomic-resolution scanning transmission electron microscopy mapping of the polarization displacements. An incommensurate phase and symmetry breaking is found within these domains resulting in local polarization rotation and hence impart a mixed Néel-Bloch-like character to the bubble domain walls. PFM hysteresis loops for the bubble domains reveal that they undergo an irreversible phase transition to cylindrical domains under the electric field, accompanied by a transient rise in the electromechanical response. The observations are in agreement with ab-initio-based calculations, which reveal a very narrow window of electrical and elastic parameters that allow the existence of bubble domains. The findings highlight the richness of polar topologies possible in ultrathin ferroelectric structures and bring forward the prospect of emergent functionalities due to topological transitions.

Manipulating Ferroelectrics through Changes in Surface and Interface Properties

Ferroelectric materials are used in many applications of modern technologies including information storage, transducers, sensors, tunable capacitors, and other novel device concepts. In many of these applications, the ferroelectric properties, such as switching voltages, piezoelectric constants, or stability of nanodomains, are crucial. For any application, even for material characterization, the material itself needs to be interfaced with electrodes. On the basis of the structural, chemical, and electronic properties of the interfaces, the measured material properties can be determined by the interface. This is also true for surfaces. However, the importance of interfaces and surfaces and their effect on experiments are often neglected, which results in many dramatically different experimental results for nominally identical samples. Therefore, it is crucial to understand the role of the interface and surface properties on internal bias fields and the domain switching process. Here, the nanoscale ferroelectric switching process and the stability of nanodomains for Pb(Zr,Ti)O₃ thin films are investigated by using scanning probe microscopy. Interface and surface properties are modulated through the selection/redesign of electrode materials as well as tuning the surface-near oxygen vacancies, which both can result in changes of the electric fields acting across the sample, and consequently this controls the measured ferroelectric and domain retention properties. By understanding the role of surfaces and interfaces, ferroelectric properties can be tuned to eliminate the problem of asymmetric domain stability by combining the effects of different electrode materials. This study forms an important step toward integrating ferroelectric materials in electronic devices.

Room temperature ferroelectricity in fluoroperovskite thin films

The NaMnF₃ fluoride-perovskite has been found, theoretically, to be ferroelectric under epitaxial strain becoming a promising alternative to conventional oxides for multiferroic applications. Nevertheless, this fluoroperovskite has not been experimentally verified to be ferroelectric so far. Here we report signatures of room temperature ferroelectricity observed in perovskite NaMnF₃ thin films grown on SrTiO₃. Using piezoresponse force microscopy, we studied the evolution of ferroelectric polarization in response to external and built-in electric fields. Density functional theory calculations were also performed to help understand the strong competition between ferroelectric and paraelectric phases as well as the profound influences of strain. These results, together with the magnetic order previously reported in the same material, pave the way to future multiferroic and magnetoelectric investigations in fluoroperovskites.

Controlled Charging of Ferroelastic Domain Walls in Oxide Ferroelectrics

Conductive domain walls (DWs) in ferroic oxides as device elements are a highly attractive research topic because of their robust and agile response to electric field. Charged DWs possessing metallic-type conductivity hold the highest promises in this aspect. However, their intricate creation, low stability, and interference with nonconductive DWs hinder their investigation and the progress toward future applications. Here, we find that conversion of the nominally neutral ferroelastic 90° DWs into partially charged DWs in Pb(Zr_0.1Ti_0.9)O₃ thin films enables easy and robust control over the DW conductivity. By employing transmission electron microscopy, conductive atomic force microscopy and phase-field simulation, our study reveals that charging of the ferroelastic DWs is controlled by mutually coupled DW bending, type of doping, polarization orientation and work-function of the adjacent electrodes. Particularly, the doping outweighs other parameters in controlling the DW conductivity. Understanding the interplay of these key parameters not only allows us to control and optimize conductivity of such ferroelastic DWs in the oxide ferroelectrics but also paves the way for utilization of DW-based nanoelectronic devices in the future.

Interface Engineering of Domain Structures in BiFeO3 Thin Films

A wealth of fascinating phenomena have been discovered at the BiFeO₃ domain walls, examples such as domain wall conductivity, photovoltaic effects, and magnetoelectric coupling. Thus, the ability to precisely control the domain structures and accurately study their switching behaviors is critical to realize the next generation of novel devices based on domain wall functionalities. In this work, the introduction of a dielectric layer leads to the tunability of the depolarization field both in the multilayers and superlattices, which provides a novel approach to control the domain patterns of BiFeO₃ films. Moreover, we are able to study the switching behavior of the first time obtained periodic 109° stripe domains with a thick bottom electrode. Besides, the precise controlling of pure 71° and 109° periodic stripe domain walls enable us to make a clear demonstration that the exchange bias in the ferromagnet/BiFeO₃ system originates from 109° domain walls. Our findings provide future directions to study the room temperature electric field control of exchange bias and open a new pathway to explore the room temperature multiferroic vortices in the BiFeO₃ system.

Single-domain multiferroic BiFeO3 films

The strong coupling between antiferromagnetism and ferroelectricity at room temperature found in BiFeO₃ generates high expectations for the design and development of technological devices with novel functionalities. However, the multi-domain nature of the material tends to nullify the properties of interest and complicates the thorough understanding of the mechanisms that are responsible for those properties. Here we report the realization of a BiFeO₃ material in thin film form with single-domain behaviour in both its magnetism and ferroelectricity: the entire film shows its antiferromagnetic axis aligned along the crystallographic b axis and its ferroelectric polarization along the c axis. With this we are able to reveal that the canted ferromagnetic moment due to the Dzyaloshinskii–Moriya interaction is parallel to the a axis. Furthermore, by fabricating a Co/BiFeO₃ heterostructure, we demonstrate that the ferromagnetic moment of the Co film does couple directly to the canted moment of BiFeO₃.

The coupling of ferroelectric and antiferromagnetic order in BiFeO₃ makes it appealing for applications however the presence of domain structure acts to undermine this potential. Here, the authors demonstrate BiFeO₃ thin films with a single domain of electrical polarization and canted antiferromagnetic order.

New modalities of strain-control of ferroelectric thin films

Ferroelectrics, with their spontaneous switchable electric polarization and strong coupling between their electrical, mechanical, thermal, and optical responses, provide functionalities crucial for a diverse range of applications. Over the past decade, there has been significant progress in epitaxial strain engineering of oxide ferroelectric thin films to control and enhance the nature of ferroelectric order, alter ferroelectric susceptibilities, and to create new modes of response which can be harnessed for various applications. This review aims to cover some of the most important discoveries in strain engineering over the past decade and highlight some of the new and emerging approaches for strain control of ferroelectrics. We discuss how these new approaches to strain engineering provide promising routes to control and decouple ferroelectric susceptibilities and create new modes of response not possible in the confines of conventional strain engineering. To conclude, we will provide an overview and prospectus of these new and interesting modalities of strain engineering helping to accelerate their widespread development and implementation in future functional devices.

Overcoming the Fundamental Barrier Thickness Limits of Ferroelectric Tunnel Junctions through BaTiO3/SrTiO3 Composite Barriers

Ferroelectric tunnel junctions (FTJs) have attracted increasing research interest as a promising candidate for nonvolatile memories. Recently, significant enhancements of tunneling electroresistance (TER) have been realized through modifications of electrode materials. However, direct control of the FTJ performance through modifying the tunneling barrier has not been adequately explored. Here, adding a new direction to FTJ research, we fabricated FTJs with BaTiO3 single barriers (SB-FTJs) and BaTiO3/SrTiO3 composite barriers (CB-FTJs) and reported a systematic study of FTJ performances by varying the barrier thicknesses and compositions. For the SB-FTJs, the TER is limited by pronounced leakage current for ultrathin barriers and extremely small tunneling current for thick barriers. For the CB-FTJs, the extra SrTiO3 barrier provides an additional degree of freedom to modulate the barrier potential and tunneling behavior. The resultant high tunability can be utilized to overcome the barrier thickness limits and enhance the overall CB-FTJ performances beyond those of SB-FTJ. Our results reveal a new paradigm to manipulate the FTJs through designing multilayer tunneling barriers with hybrid functionalities.

Polarization Rotation in Ferroelectric Tricolor PbTiO3/SrTiO3/PbZr0.2Ti0.8O3 Superlattices

In ferroelectric thin films, controlling the orientation of the polarization is a key element to controlling their physical properties. We use laboratory and synchrotron X-ray diffraction to investigate ferroelectric bicolor PbTiO3/PbZr0.2Ti0.8O3 and tricolor PbTiO3/SrTiO3/PbZr0.2Ti0.8O3 superlattices and to study the role of the SrTiO3 layers on the domain structure. In the tricolor superlattices, we demonstrate the existence of 180° ferroelectric stripe nanodomains, induced by the depolarization field produced by the SrTiO3 layers. Each ultrathin SrTiO3 layer modifies the electrostatic boundary conditions between the ferroelectric layers compared to the corresponding bicolor structures, leading to the suppression of the a/c polydomain states. Combined with the electrostatic effect, the tensile strain induced by PbZr0.2Ti0.8O3 in the PbTiO3 layers leads to polarization rotation in the system as evidenced by grazing incidence X-ray measurements. This polarization rotation is associated with the monoclinic Mc phase as revealed by the splitting of the (HHL) and (H0L) reciprocal lattice points. This work demonstrates that the tricolor paraelectric/ferroelectric superlattices constitute a tunable system to investigate the concomitant effects of strains and depolarizing fields. Our studies provide a pathway to stabilize a monoclinic symmetry in ferroelectric layers, which is of particular interest for the enhancement of the piezoelectric properties.

A review of molecular beam epitaxy of ferroelectric BaTiO3 films on Si, Ge and GaAs substrates and their applications

SrTiO3 epitaxial growth by molecular beam epitaxy (MBE) on silicon has opened up the route to the monolithic integration of various complex oxides on the complementary metal-oxide-semiconductor silicon platform. Among functional oxides, ferroelectric perovskite oxides offer promising perspectives to improve or add functionalities on-chip. We review the growth by MBE of the ferroelectric compound BaTiO3 on silicon (Si), germanium (Ge) and gallium arsenide (GaAs) and we discuss the film properties in terms of crystalline structure, microstructure and ferroelectricity. Finally, we review the last developments in two areas of interest for the applications of BaTiO3 films on silicon, namely integrated photonics, which benefits from the large Pockels effect of BaTiO3, and low power logic devices, which may benefit from the negative capacitance of the ferroelectric.

Some current problems in perovskite nano-ferroelectrics and multiferroics: kinetically-limited systems of finite lateral size

We describe some unsolved problems of current interest; these involve quantum critical points in ferroelectrics and problems which are not amenable to the usual density functional theory, nor to classical Landau free energy approaches (they are kinetically limited), nor even to the Landau-Kittel relationship for domain size (they do not satisfy the assumption of infinite lateral diameter) because they are dominated by finite aperiodic boundary conditions.

Understanding ferroelectricity in layered perovskites: new ideas and insights from theory and experiments

Recent theoretical and experimental studies showing how polar structures or ferroelectricity arise in layered perovskites are highlighted.