Nanoscale design of polarization in ultrathin ferroelectric heterostructures

Polarization Boost and Ferroelectricity Down to One Unit Cell in Layered Carpy-Galy La2Ti2O7 Thin Films

Layered perovskite-based compounds offer a range of unconventional properties enabled by their naturally anisotropic structure. Among these, the Carpy-Galy phases (AnBnO3n+2), characterized by (110)-oriented perovskite planes interleaved with additional oxygen layers, stand out for robust in-plane polarization. However, the challenges associated with the synthesis of ultrathin Carpy-Galy films and understanding the impact of strain on their properties limit their integration into devices. Here, La2Ti2O7 (n = 4) films grown on substrates imposing tensile, compressive, or negligible epitaxial strains are investigated. Surprisingly, a 3% tensile strain from DyScO3 (100) substrates facilitates layer-by-layer growth mode, whereas compressive (LaAlO3-Sr2TaAlO6 (110)) or negligible (SrTiO3 (110)) epitaxial strains require post-deposition annealing to reach comparable crystallinity. Using density-functional theory calculations, scanning probe microscopy, X-ray diffraction, scanning transmission electron microscopy, and polarization switching experiments, it is confirmed that these films possess exceptional ferroelectric properties, including a polarization of 18 µCcm-2 - more than three times higher than previously reported - as well as persistence of ferroelectricity down to a single-unit-cell thickness. This study not only advances the understanding of Carpy-Galy phases as epitaxial thin films but also lays a foundation for their integration into advanced ferroelectric device architectures.

Inverse Size-Scaling Ferroelectricity in Centrosymmetric Insulating Perovskite Oxide DyScO3

The breaking of inversion symmetry dictates the emergence of electric polarization, whose topological states in superlattices and bulks have received tremendous attention for their intriguing physics brought for novel device design. However, as for substrate oxides such as LaAlO3, KTaO3, RScO3 (R = rare earth element), their centrosymmetric trivial attributes make their functionality poorly explored. Here, the discovery of nanoscale thickness gradient-induced nonpolar-to-polar phase transition in band insulator DyScO3 is reported by using atomic resolution transmission electron microscopy. As the free-standing specimen reduces to a critical thickness ≈5 nm, its inversion symmetry is spontaneously broken by surface charge transfer, which gives rise to asymmetric Dy atomic displacements and ferrodistortive octahedral order, as substantiated by the first-principles calculations. Apart from the observation of migratable polar vortex structures, the switchable electric polarization by applied electric field is demonstrated by the piezoresponse force microscopy experiments. Given the decisive role of critical size in generating ferroelectricity, a concept of "inverse size-scaling ferroelectric" is proposed to define a class of such materials. Distinct from the proper and improper ferroelectrics, the findings offer a new platform to explore novel low-dimensional ferroelectrics and device applications in the future.

Surface polarization profile of ferroelectric thin films probed by X-ray standing waves and photoelectron spectroscopy

Understanding the mechanisms underlying a stable polarization at the surface of ferroelectric thin films is of particular importance both from a fundamental point of view and to achieve control of the surface polarization itself. In this study, we demonstrate that the X-ray standing wave technique allows the surface polarization profile of a ferroelectric thin film, as opposed to the average film polarity, to be probed directly. The X-ray standing wave technique provides the average Ti and Ba atomic positions, along the out-of-plane direction, near the surface of three differently strained [Formula: see text] thin films. This technique gives direct access to the local ferroelectric polarization at and below the surface. By employing X-ray photoelectron spectroscopy, a detailed overview of the oxygen-containing species adsorbed on the surface is obtained. The different amplitude and orientation of the local ferroelectric polarizations are associated with surface charges attributed to different type, amount and spatial distribution of the oxygen-containing adsorbates.

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.

Artificial Domain Patterning in Ultrathin Ferroelectric Films via Modifying the Surface Electrostatic Boundary Conditions

Nanoscale spatially controlled modulation of the properties of ferroelectrics via artificial domain pattering is crucial to their emerging optoelectronics applications. New patterning strategies to achieve high precision and efficiency and to link the resultant domain structures with device functionalities are being sought. Here, we present an epitaxial heterostructure of SrRuO3/PbTiO3/SrRuO3, wherein the domain configuration is delicately determined by the charge screening conditions in the SrRuO3 layer and the substrate strains. Chemical etching of the top SrRuO3 layer leads to a transition from in-plane a domains to out-of-plane c domains, accompanied by a giant (>105) modification in the second harmonic generation response. The modulation effect, coupled with the plasmonic resonance effect from SrRuO3, enables a highly flexible design of nonlinear optical devices, as demonstrated by a simulated split-ring resonator metasurface. This domain patterning strategy may be extended to more thin-film ferroelectric systems with domain stabilities amenable to electrostatic boundary conditions.

Reversible Optical Control of Polarization in Epitaxial Ferroelectric Thin Films

Light is an effective tool to probe the polarization and domain distribution in ferroelectric materials passively, that is, non-invasively, for example, via optical second harmonic generation (SHG). With the emergence of oxide electronics, there is now a strong demand to expand the role of light toward active control of the polarization. In this work, optical control of the ferroelectric polarization is demonstrated in prototypical epitaxial PbZrx Ti1-x O3 (PZT)-based heterostructures. This is accomplished in three steps, using above-bandgap UV light, while tracking the response of the polarization with optical SHG. First, it is found that UV-light exposure induces a transient enhancement or suppression of the ferroelectric polarization in films with an upward- or downward-oriented polarization, respectively. This behavior is attributed to a modified charge screening driven by the separation of photoexcited charge carriers at the Schottky interface of the ferroelectric thin film. Second, by taking advantage of this optical handle on electrostatics, remanent optical poling from a pristine multi-domain into a single-domain configuration is accomplished. Third, via thermal annealing or engineered electrostatic boundary conditions, a complete reversibility of the optical poling is further achieved. Hence, this work paves the way for the all-optical control of the spontaneous polarization in ferroelectric thin films.

Surface Effect of Thickness-Dependent Polarization and Domain Evolution in BiFeO3 Epitaxial Ultrathin Films

With the trend of device miniaturization, ultrathin ferroelectric films are gaining more and more attention. However, understanding ferroelectricity in this nanoscale context remains a formidable challenge, primarily due to the heightened relevance of surface effects, which often leads to the loss of net polarization. Here, the influence of surface effects on the polarization as a function of thickness in ultrathin BiFeO3 films is investigated using phase-field simulations. The findings reveal a notable increase in ferroelectric polarization with increasing thickness, with a particularly discernible change occurring below the 10 nm threshold. Upon accounting for surface effects, the polarization is marginally lower than the case without such considerations, with the disparity becoming more pronounced at smaller thicknesses. Moreover, the hysteresis loop and butterfly loop of the ultrathin film were simulated, demonstrating that the ferroelectric properties of films remain robust even down to a thickness of 5 nm. Our investigations provide valuable insights into the significance of ferroelectric thin films in device miniaturization.

Tuning Rashba-Dresselhaus effect with ferroelectric polarization at asymmetric heterostructural interface

The spin-orbit interaction (SOI) plays an essential role in materials properties, and controlling its intensity has great potential in the design of materials. In this work, asymmetric [(La0.7Sr0.3MnO3)8/(BaTiO3)t/(SrTiO3)2]8 superlattices were fabricated on (001) SrTiO3 substrate with SrO or TiO2 termination, labelled as SrO-SL and TiO2-SL, respectively. The in-plane angular magnetoresistance of the superlattices shows a combination of two- and four-fold symmetry components. The coefficient of two-fold symmetry component has opposite sign with current I along [100] and [110] directions for TiO2-SL, while it has the same sign for SrO-SL. Detailed study shows that the asymmetric cation inter-mixing and ferroelectricity-modulated electronic charge transfer induce asymmetric electronic potential for SrO-SL with dominating Rashba SOI, and symmetric electronic potential for TiO2-SL with dominating Dresselhaus SOI induced by BaTiO3. This work shows that the Rashba and Dresselhaus SOIs are sensitive to the ferroelectric polarization in the asymmetric structure.

Defeating depolarizing fields with artificial flux closure in ultrathin ferroelectrics

Material surfaces encompass structural and chemical discontinuities that often lead to the loss of the property of interest in so-called dead layers. It is particularly problematic in nanoscale oxide electronics, where the integration of strongly correlated materials into devices is obstructed by the thickness threshold required for the emergence of their functionality. Here we report the stabilization of ultrathin out-of-plane ferroelectricity in oxide heterostructures through the design of an artificial flux-closure architecture. Inserting an in-plane-polarized ferroelectric epitaxial buffer provides the continuity of polarization at the interface; despite its insulating nature, we observe the emergence of polarization in our out-of-plane-polarized model of ferroelectric BaTiO3 from the very first unit cell. In BiFeO3, the flux-closure approach stabilizes a 251° domain wall. Its unusual chirality is probably associated with the ferroelectric analogue to the Dzyaloshinskii-Moriya interaction. We, thus, see that in an adaptively engineered geometry, the depolarizing-field-screening properties of an insulator can even surpass those of a metal and be a source of functionality. This could be a useful insight on the road towards the next generation of oxide electronics.

Onset of Multiferroicity in Prototypical Single-Spin Cycloid BiFeO3 Thin Films

In the room-temperature magnetoelectric multiferroic BiFeO3, the noncollinear antiferromagnetic state is coupled to the ferroelectric order, opening applications for low-power electric-field-controlled magnetic devices. While several strategies have been explored to simplify the ferroelectric landscape, here we directly stabilize a single-domain ferroelectric and spin cycloid state in epitaxial BiFeO3 (111) thin films grown on orthorhombic DyScO3 (011). Comparing them with films grown on SrTiO3 (111), we identify anisotropic in-plane strain as a powerful handle for tailoring the single antiferromagnetic state. In this single-domain multiferroic state, we establish the thickness limit of the coexisting electric and magnetic orders and directly visualize the suppression of the spin cycloid induced by the magnetoelectric interaction below the ultrathin limit of 1.4 nm. This as-grown single-domain multiferroic configuration in BiFeO3 thin films opens an avenue both for fundamental investigations and for electrically controlled noncollinear antiferromagnetic spintronics.

Origin of the Critical Thickness in Improper Ferroelectric Thin Films

Improper ferroelectrics are expected to be more robust than conventional ferroelectrics against depolarizing field effects and to exhibit a much-desired absence of critical thickness. Recent studies, however, revealed the loss of ferroelectric response in epitaxial improper ferroelectric thin films. Here, we investigate improper ferroelectric hexagonal YMnO₃ thin films and find that the polarization suppression, and hence functionality, in the thinner films is due to oxygen off-stoichiometry. We demonstrate that oxygen vacancies form on the film surfaces to provide the necessary charge to screen the large internal electric field resulting from the positively charged YMnO₃ surface layers. Additionally, we show that by modifying the oxygen concentration of the films, the phase transition temperatures can be substantially tuned. We anticipate that our findings are also valid for other ferroelectric oxide films and emphasize the importance of controlling the oxygen content and cation oxidation states in ferroelectrics for their successful integration in nanoscale applications.

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.

Signatures of enhanced out-of-plane polarization in asymmetric BaTiO3 superlattices integrated on silicon

In order to bring the diverse functionalities of transition metal oxides into modern electronics, it is imperative to integrate oxide films with controllable properties onto the silicon platform. Here, we present asymmetric LaMnO₃/BaTiO₃/SrTiO₃ superlattices fabricated on silicon with layer thickness control at the unit-cell level. By harnessing the coherent strain between the constituent layers, we overcome the biaxial thermal tension from silicon and stabilize c-axis oriented BaTiO₃ layers with substantially enhanced tetragonality, as revealed by atomically resolved scanning transmission electron microscopy. Optical second harmonic generation measurements signify a predominant out-of-plane polarized state with strongly enhanced net polarization in the tricolor superlattices, as compared to the BaTiO₃ single film and conventional BaTiO₃/SrTiO₃ superlattice grown on silicon. Meanwhile, this coherent strain in turn suppresses the magnetism of LaMnO₃ as the thickness of BaTiO₃ increases. Our study raises the prospect of designing artificial oxide superlattices on silicon with tailored functionalities.

Integrating multifunctional oxides on silicon is highly desirable. Here, the authors present asymmetric BaTiO3 superlattices on silicon exhibiting enhanced out-of-plane polarization by harnessing the interfacial strain and broken inversion symmetry.

Monitoring Electrical Biasing of Pb(Zr0.2Ti0.8)O3 Ferroelectric Thin Films In Situ by DPC-STEM Imaging

Increased data storage densities are required for the next generation of nonvolatile random access memories and data storage devices based on ferroelectric materials. Yet, with intensified miniaturization, these devices face a loss of their ferroelectric properties. Therefore, a full microscopic understanding of the impact of the nanoscale defects on the ferroelectric switching dynamics is crucial. However, collecting real-time data at the atomic and nanoscale remains very challenging. In this work, we explore the ferroelectric response of a Pb(Zr0.2Ti0.8)O3 thin film ferroelectric capacitor to electrical biasing in situ in the transmission electron microscope. Using a combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and differential phase contrast (DPC)-STEM imaging we unveil the structural and polarization state of the ferroelectric thin film, integrated into a capacitor architecture, before and during biasing. Thus, we can correlate real-time changes in the DPC signal with the presence of misfit dislocations and ferroelastic domains. A reduction in the domain wall velocity of 24% is measured in defective regions of the film when compared to predominantly defect-free regions.

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.

Layer and spontaneous polarizations in perovskite oxides and their interplay in multiferroic bismuth ferrite

We review the concept of surface charge, first, in the context of the polarization in ferroelectric materials and, second, in the context of layers of charged ions in ionic insulators. While the former is traditionally discussed in the ferroelectrics community and the latter in the surface science community, we remind the reader that the two descriptions are conveniently unified within the modern theory of polarization. In both cases, the surface charge leads to electrostatic instability-the so-called "polar catastrophe"-if it is not compensated, and we review the range of phenomena that arise as a result of different compensation mechanisms. We illustrate these concepts using the example of the prototypical multiferroic bismuth ferrite, BiFeO₃, which is unusual in that its spontaneous ferroelectric polarization and the polarization arising from its layer charges can be of the same magnitude. As a result, for certain combinations of polarization orientation and surface termination, its surface charge is self-compensating. We use density functional calculations of BiFeO₃ slabs and superlattices, analysis of high-resolution transmission electron micrographs, and examples from the literature to explore the consequences of this peculiarity.

Inversion-Symmetry Engineering in Layered Oxide Thin Films

Inversion-symmetry breaking is a ubiquitous concept in condensed-matter science: It is a prerequisite for technologically relevant effects such as piezoelectricity, nonlinear optical properties, and spin-transport phenomena. It also determines abstract properties, like the electronic topology in quantum materials. Therefore, the creation of materials where inversion symmetry can be turned on or off by design may be a versatile approach for controlling parity-related functionalities. Here, we engineer inversion symmetry on a sub-unit-cell level in ultrathin hexagonal manganite films. Although an odd number of half-unit-cell layers breaks inversion symmetry, an even number of such layers remains centrosymmetric. Optical second harmonic generation as an inversion-symmetry-sensitive functionality is thus activated and deactivated on demand and at the same time used for in situ tracking of the symmetry state of our films. Symmetry engineering on the sub-unit-cell level thus suggests a new platform for controlled activation and deactivation of symmetry-governed functionalities in oxide-electronic epitaxial thin films.

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.

Interface and surface stabilization of the polarization in ferroelectric thin films

With an ever-increasing societal demand for energy for electronic devices and in the face of the current climate issues, the need for low-energy-consuming electronics has never been greater. Ferroelectrics are promising energy-efficient device components for digital information storage, with the functionality relying on the manipulation of their polarization in ultrathin films. Polar discontinuities at the thin film interfaces and surfaces, however, can cause loss of polarization and thus functionality. Here we show how the interface and surface influence the overall polarization of the thin film. We show that the structure of the interface and surface can be tailored toward a specific polarization direction and strength, and that great control in the engineering of ferroelectrics thin films can be achieved.

Ferroelectric perovskites present a switchable spontaneous polarization and are promising energy-efficient device components for digital information storage. Full control of the ferroelectric polarization in ultrathin films of ferroelectric perovskites needs to be achieved in order to apply this class of materials in modern devices. However, ferroelectricity itself is not well understood in this nanoscale form, where interface and surface effects become particularly relevant and where loss of net polarization is often observed. In this work, we show that the precise control of the structure of the top surface and bottom interface of the thin film is crucial toward this aim. We explore the properties of thin films of the prototypical ferroelectric lead titanate (PbTiO₃) on a metallic strontium ruthenate (SrRuO₃) buffer using a combination of computational (density functional theory) and experimental (optical second harmonic generation) methods. We find that the polarization direction and strength are influenced by chemical and electronic processes occurring at the epitaxial interface and at the surface. The polarization is particularly sensitive to adsorbates and to surface and interface defects. These results point to the possibility of controlling the polarization direction and magnitude by engineering specific interface and surface chemistries.

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.

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.

The Impact of a Controlled-Release Fertilizer on Greenhouse Gas Emissions and the Efficiency of the Production of Chinese Cabbage

Optimization of plant fertilization is an important element of all quality systems in primary production, such as Integrated Production, GLOBAL G.A.P. (Good Agriculture Practice) or SAI (Sustainable Agriculture Initiative). Fertilization is the most important element of agricultural treatments, affecting the quantity and quality of crops. The aim of the study was to assess greenhouse gas (GHG) emissions in the cultivation of Chinese cabbage, depending on the technological variant. The factor modifying the production technology was the use of fertilizers with a slow release of nutrients. One tonne of marketable Chinese cabbage crop was selected as the functional unit. To achieve the research goal, a strict field experiment was carried out. Calculation of the total amount of GHG emitted from the crop was made in accordance with ISO 14040 and ISO 14044. The system boundaries included the production and use of fertilizers and pesticides, energy consumption for agricultural practices and the emission of gases from soil resources and harvesting residue. The use of slow-release fertilizers resulted in a greater marketable yield of cabbage compared to conventional fertilizers. The results of the research indicate a significant potential for the use of slow-release fertilizers in reducing agricultural emissions. From the environmental and production point of view, the most favourable variant is the one with 108 kg N·ha−1 slow-release fertilizers. At a higher dose of this element, no increase in crop yield was observed. At this nitrogen dose, a 30% reduction in total GHG emissions and a 50% reduction in fertilizer emissions from the use of per product functional unit were observed. The reference object was fertilization in accordance with production practice in the test area.

Probing local order in multiferroics by transmission electron microscopy

The ongoing trend toward miniaturization has led to an increased interest in the magnetoelectric effect, which could yield entirely new device concepts, such as electric field-controlled magnetic data storage. As a result, much work is being devoted to developing new robust room temperature (RT) multiferroic materials that combine ferromagnetism and ferroelectricity. However, the development of new multiferroic devices has proved unexpectedly challenging. Thus, a better understanding of the properties of multiferroic thin films and the relation with their microstructure is required to help drive multiferroic devices toward technological application. This review covers in a concise manner advanced analytical imaging methods based on (scanning) transmission electron microscopy which can potentially be used to characterize complex multiferroic materials. It consists of a first broad introduction to the topic followed by a section describing the so-called phase-contrast methods, which can be used to map the polar and magnetic order in magnetoelectric multiferroics at different spatial length scales down to atomic resolution. Section 3 is devoted to electron nanodiffraction methods. These methods allow measuring local strains, identifying crystal defects and determining crystal structures, and thus offer important possibilities for the detailed structural characterization of multiferroics in the ultrathin regime or inserted in multilayers or superlattice architectures. Thereafter, in Section 4, methods are discussed which allow for analyzing local strain, whereas in Section 5 methods are addressed which allow for measuring local polarization effects on a length scale of individual unit cells. Here, it is shown that the ferroelectric polarization can be indirectly determined from the atomic displacements measured in atomic resolution images. Finally, a brief outlook is given on newly established methods to probe the behavior of ferroelectric and magnetic domains and nanostructures during in situ heating/electrical biasing experiments. These in situ methods are just about at the launch of becoming increasingly popular, particularly in the field of magnetoelectric multiferroics, and shall contribute significantly to understanding the relationship between the domain dynamics of multiferroics and the specific microstructure of the films providing important guidance to design new devices and to predict and mitigate failures.

Linear Magnetoelectric Phase in Ultrathin MnPS_{3} Probed by Optical Second Harmonic Generation

The transition metal thiophosphates MPS_{3} (M=Mn, Fe, Ni) are a class of van der Waals stacked insulating antiferromagnets that can be exfoliated down to the ultrathin limit. MnPS_{3} is particularly interesting because its Néel ordered state breaks both spatial-inversion and time-reversal symmetries, allowing for a linear magnetoelectric phase that is rare among van der Waals materials. However, it is unknown whether this unique magnetic structure of bulk MnPS_{3} remains stable in the ultrathin limit. Using optical second harmonic generation rotational anisotropy, we show that long-range linear magnetoelectric type Néel order in MnPS_{3} persists down to at least 5.3 nm thickness. However an unusual mirror symmetry breaking develops in ultrathin samples on SiO_{2} substrates that is absent in bulk materials, which is likely related to substrate induced strain.

The ultrathin limit of improper ferroelectricity

The secondary nature of polarization in improper ferroelectrics promotes functional properties beyond those of conventional ferroelectrics. In technologically relevant ultrathin films, however, the improper ferroelectric behavior remains largely unexplored. Here, we probe the emergence of the coupled improper polarization and primary distortive order parameter in thin films of hexagonal YMnO₃. Combining state-of-the-art in situ characterization techniques separately addressing the improper ferroelectric state and its distortive driving force, we reveal a pronounced thickness dependence of the improper polarization, which we show to originate from the strong modification of the primary order at epitaxial interfaces. Nanoscale confinement effects on the primary order parameter reduce the temperature of the phase transition, which we exploit to visualize its order-disorder character with atomic resolution. Our results advance the understanding of the evolution of improper ferroelectricity within the confinement of ultrathin films, which is essential for their successful implementation in nanoscale applications.

Evolution of improper ferroelectricity within the confinement of ultrathin films is essential for their successful implementation in nanoscale applications. Here, the authors show thickness dependence of the improper polarization originating from the strong modification of the primary order at epitaxial interfaces.

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.

Observation of Strong Polarization Enhancement in Ferroelectric Tunnel Junctions

Ferroelectric heterostructures, with capability of storing data at ultrahigh densities, could act as the platform for next-generation memories. The development of new device paradigms has been hampered by the long-standing notion of inevitable ferroelectricity suppression under reduced dimensions. Despite recent experimental observation of stable polarized states in ferroelectric ultrathin films, the out-of-plane polarization components in these films are strongly attenuated compared to thicker films, implying a degradation of device performance in electronic miniaturization processes. Here, in a model system of BiFeO₃/La_0.7Sr_0.3MnO₃, we report observation of a dramatic out-of-plane polarization enhancement that occurs with decreasing film thickness. Our electron microscopy analysis coupled with phase-field simulations reveals a polarization-enhancement mechanism that is dominated by the accumulation of oxygen vacancies at interfacial layers. The results shed light on the interplay between polarization and defects in nanoscale ferroelectrics and suggest a route to enhance functionality in oxide devices.

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.

Advances in magnetoelectric multiferroics

The manipulation of magnetic properties by an electric field in magnetoelectric multiferroic materials has driven significant research activity, with the goal of realizing their transformative technological potential. Here, we review progress in the fundamental understanding and design of new multiferroic materials, advances in characterization and modelling tools to describe them, and the exploration of devices and applications. Focusing on the translation of the many scientific breakthroughs into technological innovations, we identify the key open questions in the field where targeted research activities could have maximum impact in transitioning scientific discoveries into real applications.

Water printing of ferroelectric polarization

Ferroelectrics, which generate a switchable electric field across the solid-liquid interface, may provide a platform to control chemical reactions (physical properties) using physical fields (chemical stimuli). However, it is challenging to in-situ control such polarization-induced interfacial chemical structure and electric field. Here, we report that construction of chemical bonds at the surface of ferroelectric BiFeO3 in aqueous solution leads to a reversible bulk polarization switching. Combining piezoresponse (electrostatic) force microscopy, X-ray photoelectron spectroscopy, scanning transmission electron microscopy, first-principles calculations and phase-field simulations, we discover that the reversible polarization switching is ascribed to the sufficient formation of polarization-selective chemical bonds at its surface, which decreases the interfacial chemical energy. Therefore, the bulk electrostatic energy can be effectively tuned by H+/OH- concentration. This water-induced ferroelectric switching allows us to construct large-scale type-printing of polarization using green energy and opens up new opportunities for sensing, high-efficient catalysis, and data storage.

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.

Bulk and Thin Film Synthesis of Compositionally Variant Entropy-stabilized Oxides

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (Cu variant) entropy-stabilized oxides. Phase pure and chemically homogeneous (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (x = 0.20, 0.27, 0.33) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (x = 0.11, 0.27) ceramic pellets are synthesized and used in the deposition of ultra-high quality, phase pure, single crystalline thin films of the target stoichiometry. A detailed methodology for the deposition of smooth, chemically homogeneous, entropy-stabilized oxide thin films by pulsed laser deposition on (001)-oriented MgO substrates is described. The phase and crystallinity of bulk and thin film materials are confirmed using X-ray diffraction. Composition and chemical homogeneity are confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The surface topography of thin films is measured with scanning probe microscopy. The synthesis of high quality, single crystalline, entropy-stabilized oxide thin films enables the study of interface, size, strain, and disorder effects on the properties in this new class of highly disordered oxide materials.

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.