An important requirement for the integration of ferroelectric thin films into devices is deterministic control of the polarization state in films of only a few unit cells in thickness. Here, we utilize the charged atomic planes of (001)-oriented SmNiO_3 (SNO) buffer layers as a polarizing template to stabilize the polarization in ferroelectric BaTiO_3 (BTO) model system thin films. We show that an upwards (downwards) oriented polarization is achieved by selection of the [SmO]^+ ([NiO_2]^-) buffer termination. Most importantly, the charged atomic planes of SNO suppress the depolarizing-field-induced critical thickness in BTO, and we record the emergence of a net polarization in our BTO films from the first unit cell deposited. Our experiments, guided by density-functional-theory (DFT) calculations, further highlight the impact of charged defects on the polarizing effectiveness of the SNO buffer. Specifically, oxygen vacancies counteract the polarizing field of the negatively charged, [NiO_2]^--terminated surface of the SNO buffer. Our findings provide important insights into the interplay of defect chemistry and polarizing interfaces to stabilize ferroelectric polarization down to the single-unit-cell limit.

Electrically tunable metasurfaces leveraging the strong Pockel's effect in barium titanate (BaTiO$_3$ or BTO) are a promising platform for reconfigurable free-space optical devices. However, the high cost, limited scalability, and restricted substrate compatibility of epitaxial BTO films hinder its exploitation. Here, we demonstrate free-space optical modulators based on imprinted BTO metasurfaces with targeted designs for optical and electric field confinement within the active material. With resonances exhibiting high quality factors of up to 200, we demonstrate improved transmission modulation at sub-volt driving amplitudes and frequencies up to 5 MHz. Additional enhancement is achieved via ferroelectric domain alignment, resulting in up to 25 % higher modulation strength compared to the unbiased case and up to 75 % compared to previous demonstrations. This enhanced EO response, arising from the effective permittivity engineering and domain orientation in these polycrystalline metasurfaces, holds significant potential for scalable and efficient EO modulators and active metasurfaces.

Antiferroelectrics attract attention due to their unusual physical characteristics, chief among which is the double hysteresis loop that separates their antipolar ground state from the voltage-induced polar phase. This behaviour is useful for energy storage and promising for electrocaloric cooling and other applications. However, the defining features of antiferroelectrics (antipolar ground state and double hysteresis loops) are increasingly challenged: materials with non-collinear and/or hybrid polar-antipolar order have been discovered, and double hysteresis has been realized in materials without a conventional antipolar ground state. These developments add to the already intense interest in the fundamental and practical aspects of antiferroelectrics and call for a fresh look at antiferroelectricity. In this Perspective, we revise the definition of antiferroelectricity, discuss material systems with new antipolar orders and/or engineered double hysteresis, and reflect on emergent properties and theoretical approaches.

Ferroelectric materials are established candidates for beyond complementary metal-oxide-semiconductor technology, owing to their non-volatile spontaneous electrical polarization. The recent boom in electric dipole texture engineering and manipulation in such materials has revealed exciting routes for controlling ferroelectric polarization, offering alternatives to the classical, sometimes challenging, application of electrical fields. In this short perspective, we shed light on electrode-free external stimuli enabling control over polar states in thin films. We bring awareness to the polarizing role of chemically-engineered surface contributions and provide insights into the combination of chemical substitution and mechanical pressure, complementing the polar state tuning capabilities readily enabled by flexoelectricity. Finally, we describe recent developments in the optical modulation of polarization. Thus, our perspective aims to stimulate the advancement of alternative means to act on polarization states and facilitate the development of ferroelectric-based applications.

ABSTRACT Epitaxial strain in antiferromagnetic orthoferrite thin films is predicted to significantly enhance magnetic and polar properties with a shift of the polar response from below 4 K to room temperature and above by strengthening the rare earth‐Fe interaction. In DyFeO 3 , the Fe‐spins undergo a spin reorientation transition between 40–50 K, and a magnetic field‐induced ferroelectric phase transition is triggered by Dy ordering and displacement below 4 K. Here we report an increase in the spin reorientation temperature by more than 20 K in compressively strained DyFeO 3 films. The expected sharp spin transition is several 10 K wide, incomplete and largely suppressed, with the magnetic point group remaining at Γ 4 at all temperatures. The strain‐induced Dy ordering above 4 K reduces the magnetic symmetry and introduces room‐temperature polar order with the electrical polarization vector oriented along [100] and polarization values of more than 3 µC/cm 2 .

The balance between interfacial chemistry, electrostatics, and epitaxial strain plays a crucial role in stabilizing polarization in ferroelectric thin films. Here, these contributions are brought into competition in BiFeO 3 (BFO) thin films grown on the charged‐surface‐terminated La 0.7 Sr 0.3 MnO 3 (LSMO)‐buffered NdGaO 3 (001) substrates. The large anisotropic compressive strain from the substrate promotes the formation of ferroelectric domains despite the expected stabilization of a uniform out‐of‐plane polarization by the (La,Sr)O 0.7+ termination of the metallic buffer. Piezoresponse force microscopy and scanning transmission electron microscopy reveal that the resulting nanoscale domain architecture is stabilized by the deterministic formation of a fluorite‐like Bi 2 O 2 surface layer on regions polarized oppositely to the LSMO‐imposed polarization orientation. Leveraging this polarization compensation mechanism, a uniform out‐of‐plane polarization is stabilized in the highly strained BFO films by inserting a Bi 2 O 2 ‐terminated Aurivillius film as a buffer layer. Additionally, signatures of homohelicity in polarization textures of the BFO films are revealed on the level of domain configurations using local polarization switching experiments. The work thus brings new strategies for controlling polarization direction and helicity in oxide ferroelectrics, opening pathways for functional domain‐wall and domain‐based electronics.

Employing light as a means of actively tuning material properties unlocks the potential for non-invasive, remote, and macroscopic control over technology-relevant functionalities. Here, we demonstrate optical control over multiferroic phases in prototypical magnetoelectric thin films, utilizing above-bandgap UV light illumination. Taking advantage of the enhanced response at the strain-driven morphotropic phase boundary, we show that by modifying the electrostatic boundary conditions with photoinduced charge carriers, the rhombohedral-like (R-like) phase of can be selectively suppressed within the tetragonal-like (T-like) phase matrix. Furthermore, the electronic origin of such an optical response permits a pronounced polarization-dependent R-to-T-phase conversion. Finally, using scanning nitrogen vacancy magnetometry, we correlate optically induced ferroelectric phase conversion with a change from uncompensated magnetic ordering to G-type antiferromagnetic ordering. Our work thus presents a novel approach to writing multiferroic states, which is key to magnetoelectric oxide electronics.

Ferroelectric materials are used in a broad range of functional applications because of their switchable, spontaneous polarization. The bulk ferroelectric response is linked to the underlying domain microstructure, motivating methods that can quantitatively resolve local polarization directions. However, in application-relevant polycrystalline materials exhibiting a distribution of grain orientations, a direct mapping of the polarization direction in three dimensions has remained inaccessible using conventional experimental approaches. Here, taking barium titanate single crystals and lead zirconium titanate polycrystals as our bulk model systems, we map the local polarization directions using a new electron backscatter diffraction indexing technique based on simulated pattern-matching. Through improved pre-processing techniques (including optimized pattern processing, a new pseudosymmetry-sensitive neighbor pattern averaging method, and DIC-based global sample-detector geometry calibration) and a new pseudosymmetry confidence index (which considers not only pattern similarity but pattern dissimilarity trends with other domain variant patterns), we successfully distinguish between the six polarization directions, despite the challengingly small unit cell aspect ratio of the selected materials. The methods developed in this work are not only applicable to ferroelectrics but any material which exhibits close crystallographic pseudosymmetries-extending the current capabilities of EBSD.

The interplay between ionic and electronic subsystems dictates the behavior of structural phase transitions in polar dielectrics, a coupling mediated by soft optical phonon modes. In incipient ferroelectrics such as SrTiO_3 (STO), strong local-field effects can drive the lattice into a non-perturbative regime near the phase boundary. However, disentangling the distinct contributions of local fields from those of spontaneous macroscopic polarization remains an experimental challenge. Here, we isolate these mechanisms by probing paraelectric STO deep within its symmetric phase, where macroscopic spontaneous polarization is suppressed. Linear terahertz (THz) spectroscopy reveals that the soft mode exhibits a hybrid character, predominantly driven by electronic polarizability. Utilizing two-dimensional THz spectroscopy, we map the underlying nonlinear signals, demonstrating that the system persists in a perturbative regime characterized by robust local-field coherence. By implementing a microscopic model of coupled electronic and lattice degrees of freedom mediated by local fields, we qualitatively reproduce these multidimensional coherent signatures. Our findings highlight that while local fields are necessary to initiate non-perturbative lattice dynamics, they are insufficient on their own. This reveals that spontaneous polarization plays a deterministic role in dictating soft-mode nonlinearities in strongly correlated polar dielectrics.

Recent advances in the fabrication of single-crystalline oxide membranes have opened a promising avenue for the realization of freestanding ferroelectric films with potential applications in the field of flexible electronics, such as sensors, actuators, and energy harvesting devices. A widely used fabrication method is the integration of water-soluble Sr3Al2O6 (SAO) as a sacrificial layer between the perovskite-type substrate and the ferroelectric film. Here we report the design of few-unit-cell-thick ferroelectric membranes using pulsed laser deposition. This work reveals the tendency for strontium diffusion from the SAO layer into the BiFeO3 (BFO) films during the pulsed deposition, leading to the spontaneous formation of an Aurivillius-like layered phase. Mitigation of such undesired effects is achieved here by introducing a SrRuO3 (SRO) buffer layer between the SAO and BFO layers. This prevents strontium diffusion and enables the fabrication of BFO membranes down to 5-unit cells with high structural integrity and a consistent polar tetragonal distortion. Our results demonstrate that freestanding ferroelectric films can be fabricated with a degree of precision comparable to that of molecular beam epitaxy. Our strategy can be further extended to other membrane compositions.

Natural interfaces in ferroic oxides have developed into versatile playgrounds for studying electronic correlation effects in 2D systems. The microscopic origin of the emergent local electronic properties is often debated, however, as quantitative atomic-scale characterization remains challenging. A prime example is enhanced conductivity at ferroelectric domain walls, attributed to mechanisms ranging from local band gap reduction to point defect accumulations. Here, we resolve the microscopic mechanisms for domain wall conduction in the ferroelectric model system BiFeO3, by combining transport measurements with atom probe tomography to quantify the local chemical composition and correlate it with the electrical properties. Significant chemical variations along the walls are observed, demonstrating an outstanding chemical flexibility at domain walls, which manifest in spatially varying physical properties. The results give a unifying explanation for the diverse electronic behavior observed and establish the fundamental notion that multiple conduction mechanisms can coexist within individual domain walls.

Ferroelectric materials exhibit a switchable, spontaneous polarization at the unit cell level–an attractive property utilized in many emerging technologies including, among others, high-density memory storage, low-power transistors, and high-speed fiber optic communication. Understanding the local polarization switching behavior, through domain nucleation and evolution, is critical to advancing these technologies and requires characterization of the local domain microstructure. However, in application-relevant polycrystalline materials exhibiting a distribution of grain orientations, a direct mapping of the polarization direction in three dimensions has remained inaccessible using conventional experimental approaches. Here, taking barium titanate single crystals and lead zirconium titanate polycrystals as our bulk model systems, we map the local polarization directions using a new electron backscatter diffraction indexing technique based on simulated pattern-matching. Through improved pre-processing techniques (including optimized pattern processing, a new pseudosymmetry-sensitive neighbor pattern averaging method, and DIC-based global sample-detector geometry calibration) and a new pseudosymmetry confidence index (which considers not only pattern similarity but pattern dissimilarity trends with other domain variant patterns), we successfully distinguish between the six polarization directions, despite the challengingly small unit cell aspect ratio of the selected materials. The methods developed in this work are not only applicable to ferroelectrics but any material which exhibits close crystallographic pseudosymmetries–extending the current capabilities of EBSD.

The balance between interfacial chemistry, electrostatics, and epitaxial strain plays a crucial role in stabilizing polarization in ferroelectric thin films. Here, these contributions are brought into competition in BiFeO3 (BFO) thin films grown on the charged-surface-terminated La0.7Sr0.3MnO3 (LSMO)-buffered NdGaO3 (001) substrates. The large anisotropic compressive strain from the substrate promotes the formation of ferroelectric domains despite the expected stabilization of a uniform out-of-plane polarization by the (La,Sr)O0.7+ termination of the metallic buffer. Piezoresponse force microscopy and scanning transmission electron microscopy reveal that the resulting nanoscale domain architecture is stabilized by the deterministic formation of a fluorite-like Bi2O2 surface layer on regions polarized oppositely to the LSMO-imposed polarization orientation. Leveraging this polarization compensation mechanism, a uniform out-of-plane polarization is stabilized in the highly strained BFO films by inserting a Bi2O2-terminated Aurivillius film as a buffer layer. Additionally, signatures of homohelicity in polarization textures of the BFO films are revealed on the level of domain configurations using local polarization switching experiments. The work thus brings new strategies for controlling polarization direction and helicity in oxide ferroelectrics, opening pathways for functional domain-wall and domain-based electronics.

The demand for efficient data processing motivates a shift toward in-memory computing architectures. Ferroelectric materials, particularly AlScN, show great promise for next-generation memory devices. However, their widespread application is limited due to challenges such as high coercive fields, leakage currents, and limited stability. Our work introduces a novel synthesis approach for ferroelectric AlScN thin films using high-power impulse magnetron sputtering (HiPIMS). Through a combinatorial study, we investigate the effect of scandium content and substrate bias on the ferroelectric properties of AlScN films deposited using metal-ion synchronized (MIS) HiPIMS. Leveraging the high ionization rates of HiPIMS and optimally timed substrate bias potentials, we enhance the adatom mobility at low temperatures. Our films exhibit a high degree of texture and crystallinity as well as low roughness at temperatures as low as 250 °C. Most importantly, the films exhibit coercive fields comparable to state-of-the-art values (5 MV/cm) with significantly enhanced remanent polarization (158–172 μC/cm2). Notably, the remanent polarization remains stable across varying scandium concentrations. We further evaluate cycling stability and leakage current to assess suitability for memory applications. This study demonstrates HiPIMS as a scalable and CMOS compatible technique for synthesizing high-quality ferroelectric AlScN films, paving the way for their application in non-volatile memory applications.

Ferroelectric thin films present a powerful platform for next generation computing and memory applications. However, domain morphology and dynamics in buried ferroelectric stacks have remained underexplored, despite the importance for real device performance. Here, nanoprobe X-ray diffraction (nano-XRD) is used to image ferroelectric domains inside BiFeO3-based capacitors, revealing striking differences from bare films such as local disorder in domain architecture and partial polarization reorientation. We demonstrate sensitivity to ferroelectric reversal in poled capacitors, revealing expansive/compressive (001) strain for up-/down-polarization using nano-XRD. We observe quantitative and qualitative differences between poling by piezoresponse force microscopy (PFM) and in devices. Further, biasing induces lattice tilt at electrode edges which may modify performance in down-scaled devices. Direct comparison with PFM polarized structures even demonstrates potential nano-XRD sensitivity to domain walls. Our results establish nano-XRD as a noninvasive probe of buried ferroelectric domain morphologies and dynamics, opening avenues for operando characterization of energy-efficient nanoscale devices.

We present a novel experimental approach employing high-energy X-ray scattering in ultra-small-angle grazing-incidence geometry to investigate local atomic structures in single-crystalline thin films. This non-destructive and non-invasive method overcomes the limitations of conventional moderate-energy grazing-incidence diffraction, achieving both high reciprocal-space resolution and coverage and high surface sensitivity. By leveraging high-energy X-ray diffraction, we enable quantitative analysis of local structures in the model system of ferroelectric PbTiO3 and dielectric SrTiO3 superlattices through three-dimensional difference pair distribution function analysis. The approach provides detailed insights into atomic structures in single-crystalline thin films with local order, capturing information on spatial correlations within and across unit cells.

The nanowatt-level power density of current biobased piezoelectric energy harvesters restricts their applicative potential for the efficient conversion of biomechanical energy. A high-performing, fully renewable piezoelectric device incorporating green piezo-active Rochelle salt in a laser-drilled wood template is demonstrated to form ordered crystal pillar arrays by melt crystallization. Investigating the effect of different crystal pillar configurations on the piezoelectric response, a shearing design (45 degrees-oriented pillars) shows potential of up to 30 V and a current of 4 mu A - corresponding to a 10-fold power increase compared to single-crystalline Rochelle salt. A concept of direct laser graphitization on the crystal surfaces are demonstrated using a fully renewable ink to create electrodes of low resistance (36 Omega sq-1). The entire device can be disassembled, fully recycled, and reused. This nanogenerator outperforms state-of-the-art biobased ones and competes with conventional lead-based devices in power generation while showing a significantly lower environmental footprint, as indicated by life-cycle assessment.

Ferroelectric thin films present a powerful platform for next-generation computing and memory applications. However, domain morphology and dynamics in buried ferroelectric stacks have remained underexplored, despite their importance for real device performance. Here, nanoprobe X-ray diffraction (nano-XRD) is used to image ferroelectric domains inside BiFeO3-based capacitors, revealing local disorder in domain architecture and partial polarization reorientation caused by the capacitor electrostatic boundary conditions and internal stress. We demonstrate sensitivity to ferroelectric reversal in poled capacitors, highlighting expansive/compressive (001) strain for up-/down-polarization using nano-XRD. We observe significant quantitative and qualitative differences between poling by piezoresponse force microscopy and in devices. Further, electrical poling induces lattice tilt at electrode edges, which may modify performance in downscaled devices. Our results establish nano-XRD as a noninvasive probe of buried ferroelectric domain morphologies and dynamics, opening avenues for operando characterization of energy-efficient nanoscale devices.

The balance between interfacial chemistry, electrostatics, and epitaxial strain plays a crucial role in stabilizing polarization in ferroelectric thin films. Here, we bring these contributions into competition in BiFeO_3 (BFO) thin films grown on the charged-surface-terminated La_0.7Sr_0.3MnO_3 (LSMO)-buffered NdGaO_3 (001) substrates. The large anisotropic compressive strain from the substrate promotes the formation of ferroelectric domains despite the expected stabilization of a uniform out-of-plane polarization by the (La,Sr)O^0.7+ termination of the metallic buffer. Piezoresponse force microscopy and scanning transmission electron microscopy reveal that the resulting nanoscale domain architecture is stabilized by the deterministic formation of a fluorite-like Bi_2O_2 surface layer on regions polarized oppositely to the LSMO-imposed polarization orientation. Leveraging this polarization compensation mechanism, we stabilize a uniform out-of-plane polarization in our highly strained BFO films by inserting a Bi_2O_2-terminated Aurivillius film as a buffer layer. Additionally, we reveal signatures of homochiral polarization textures in our BFO films on the level of domain configurations using local polarization switching experiments. Our work thus brings new strategies for controlling polarization direction and chiral textures in oxide ferroelectrics, opening pathways for functional domain-wall and domain-based electronics.