Abstract The functional properties of ferroelectric materials are strongly influenced by their polarization orientation; as such, precise characterization of polarization vectors is important to ferroelectrics research. Here, we develop a fully automated three-dimensional piezoresponse force microscopy (Auto-3DPFM) technique, which integrates all essential steps in interferometric PFM for 3D polarization vector characterization, including laser alignment, tip calibration and approach, image acquisition, polarization vector reconstruction, and visualization. The automation reduces the experimental burden of ferroelectric polarization vector characterization, while continual calibration ensures consistency and reproducibility of 3D polarization reconstruction. An algorithmic workflow is also developed to identify domain walls and calculate their characteristic angles via a spatial vector-angle-difference method, presenting one capability enabled by Auto-3DPFM and inaccessible through traditional PFM techniques. When integrated with machine learning and adaptive sampling strategies in self-driving labs, Auto-3DPFM serves as a valuable tool for advancing ferroelectric physics and microelectronics development.

A polar oxide on the brink displays enhanced electromechanical response, classically from a combination of phase competition and domain wall motion. In epitaxial thin films, however, such enhancement is widely suppressed by substrate clamping, which immobilizes structural interfaces. Here, we show that by using a substrate that imposes directionally asymmetric strain, one can fundamentally engineer this constraint to stabilize a special class of glissile interfaces that amplify electromechanical response through their mobility. In BiFeO3 films grown on LaAlO3 (103), strain asymmetry stabilizes elastically compatible low-symmetry variants separated by dislocation-free, glissile interphase boundaries. These interfaces translate reversibly under weak electrical or mechanical perturbations, mediating collective nanoscale phase transformations that are forbidden under conventional biaxial strain. The resulting dynamics exhibit scale-free avalanche behavior characteristic of systems poised near an elastic instability. Our findings establish glissile interphase boundaries as an emergent interfacial state in anisotropically-strained oxides and provide a general framework for engineering phase competition and collective switching phenomena in epitaxial materials.

The recent observation of topologically protected polar soliton textures in multiferroic BiFeO3/SrTiO3 (BFO/STO) superlattices has given rise to new prospects for next-generation electronic devices, with the inherent room-temperature polar and spin coupling. Herein, we employed phase-field simulations to explore the phase diagrams of the BFO/STO superlattice under various thermodynamic conditions and experimentally validated them using piezoresponse force microscopy. It is revealed that by modulating external factors such as epitaxial strain, film thickness, and external temperature, different phases including polar solitons, polar skyrmions, and trivial domains can be designed within the BFO/STO superlattice system. For instance, reducing the magnitude of compressive strain can induce transitions from polar skyrmions to polar solitons and finally to a multidomain state. On the other hand, increasing the superlattice periodicity can shift the system from a single domain like state to polar solitons and a multidomain state. Furthermore, it is predicted that heating can transform the polar soliton state into a polar skyrmion state. This research offers deep insights into the stability of polar topologies in BFO-based systems and provides practical guidelines for the manipulation and design of polar solitons in multiferroic materials.

The nature of lead zirconate, the historical antiferroelectric material, has recently been challenged. In PbZrO3 epitaxial films, thickness reduction engenders competition among antiferroelectric, ferrielectric and ferroelectric phases. All studies so far on PbZrO3 films have utilized commercially-available oxide single crystals with large compressive lattice mismatch, causing the films to undergo strain relaxation. First-principles calculations have predicted that tensile strain can stabilize antiferroelectricity down to the nanometre scale. Here we use tensile strain imposed by artificial substrates of LaLuO3 to stabilize a pure antiferroelectric phase in PbZrO3. Sharp double hysteresis loops of polarization vs electric field show zero remanent polarization, and polar displacement maps reveal the characteristic up-up-down-down antipolar pattern down to 9 nanometre film thicknesses. Moreover, the electron beam can move this antipolar pattern through the nucleation and annihilation of translational boundaries. These results highlight the critical role of coherent epitaxial strain in the phase stability of PbZrO3.

Ferroelectric tunnel junctions (FTJs) harness the combination of ferroelectricity and quantum tunneling and thus herald opportunities in next-generation nonvolatile memory technologies. Recent advancements in the fabrication of ultrathin heterostructures have enabled the integration of ferroelectrics with various functional materials, forming hybrid tunneling-diode junctions. These junctions benefit from the modulation of the functional layer/ferroelectric interface through ferroelectric polarization, thus enabling further modalities and functional capabilities in addition to tunneling electroresistance. This Perspective aims to provide in-depth insight into the physical phenomena of several typical ferroelectric hybrid junctions, ranging from ferroelectric/dielectric, ferroelectric/multiferroic, and ferroelectric/superconducting to ferroelectric/2D materials, and finally their expansion into the realm of ferroelectric resonant tunneling diodes (FeRTDs). This latter aspect, i.e., resonant tunneling, offers an approach to exploiting tunneling behavior in ferroelectric heterostructures. We discuss examples that have successfully shown room-temperature ferroelectric control of parameters such as the resonant peak, tunnel current ratio at peak, and negative differential resistance. We conclude the Perspective by summarizing the challenges and highlighting the opportunities for the future development of hybrid FTJs, with a special emphasis on a possible type of FeRTD device. The prospects for enhanced performance and expanded functionality ignite tremendous excitement in hybrid FTJs and FeRTDs for future nanoelectronics.

Topological defects in ferroic materials are attracting significant attention due to their role as a platform for distinctive physical phenomena and their potential applications in electronic devices. Bismuth ferrite stands out as a well-known multiferroic material, exhibiting both ferroelectric and antiferromagnetic properties at room temperature. While magnetic topological defects in bismuth ferrite begin to be well documented, the influence of cycloids with different propagation directions on the magnetic structure at ferroelectric domain walls remains relatively unexplored. In this study, we employ a first-principles-based effective Hamiltonian method in conjunction with Monte Carlo simulations to predict the formation of distinct magnetic textures at ferroelectric domain walls. An analysis of the Pontryagin charge density reveals them to be localized Bloch points, merons, and homogeneous Pontryagin charge-density tubes, which hold promise for future usage in electronic devices.

The functional properties of ferroelectric materials are strongly influenced by ferroelectric polarization orientation; as such, access to consistent and precise characterization of polarization vectors is of substantial importance to ferroelectrics research. Here, we develop a fully automated three-dimensional piezoresponse force microscopy (Auto-3DPFM) technique automating all essential steps in interferometric PFM for 3D polarization vector characterization, including laser alignment, tip calibration and approach, image acquisition, polarization vector reconstruction, and visualization. The automation reduces the experimental burden of ferroelectric polarization vector characterization, while the back-and-forth calibration ensures consistency and reproducibility of 3D polarization reconstruction. An algorithmic workflow is also developed to identify domain walls and calculate their characteristic angles via a spatial vector-angle-difference method, presenting one unique capability enabled by Auto-3DPFM that is not accessible with traditional PFM techniques. Beyond representing a significant step forward in 3D polarization mapping, Auto-3DPFM promises to accelerate discovery via high-throughput and autonomous characterization in ferroelectric materials research. When integrated with machine learning and adaptive sampling strategies in self-driving labs, Auto-3DPFM will serve as a valuable tool for advancing ferroelectric physics and microelectronics development.

Single crystals of (001)-oriented 0.7Pb(Mg1/3Nb2/3)-0.3PbTiO(3) (PMN-30PT) with a composition near the morphotropic phase boundary have attracted considerable attention due to their superior dielectric and electromechanical performance. Recently, a new alternating current (electric field) poling approach used for the enhancement of dielectric and piezoelectric properties. However, the microscopic domain variants that govern the performance, especially under high-frequency alternating current (AC) voltages, remain largely unexplored. In this work, the domain microstructure under AC poling reveals the presence of four monoclinic (M-A) domain variants using a suite of scanning probe microscopy methods, and X-ray diffraction (XRD) reciprocal space mapping is tuned. It is reported on the emergence of hierarchical fine domains - needle-shaped, and 109 degrees domain walls under applied high-frequency AC poling. Time-resolved Kelvin probe force microscopy (KPFM) reveals the charge dynamics and relaxation behavior of these needle domains and walls. The findings provide new insight and guidance to the domain engineering by high-frequency AC poling for the development of advanced transducer technology.

Domain walls are quasi-one-dimensional topological defects in ferroic materials, which can harbor emergent functionalities. In the case of ferroelectric domain wall (FEDW) devices, an exciting frontier has emerged: memristor-based information storage and processing approaches. Memristor solid-state FEDW devices presented thus far, however predominantly utilize a complex network of domain walls to achieve the desired regulation of density and charge state. Here, using epitaxial bismuth ferrite thin films as a prototype ferroelectric and advanced scanning probe microscopy and stroboscopic methods, we demonstrate controlled single-wall memristive behavior through deterministic electric field-driven conformal changes. Our design exploits memristive functionality through surface pinning of topological domain walls. That is, although the FEDW is constricted by the surface, it is free to twist in the thickness direction. The resulting vertical variation of FEDW morphology creates a complex interplay between surface-induced pinning and field-induced wall bending. This gives rise to metastable electronic transitions, and hence memristive attributes. Moreover, being a single FEDW memristor, once injected there is no need for repeated injection or erasure. Microscopic insight into device operation from phase field modeling indicates controllable warping of the wall, causing memristive conductivity changes. Our results reaffirm the promise of FEDWS for brain-inspired neuromorphic and in-memory computing applications based on integrated ferroelectric devices.

Abstract Epitaxially strain‐engineered tetragonal (T)‐like BiFeO3 (BFO) is a multiferroic material with unique crystallographic and physical properties compared to its bulk rhombohedral parent. While the effect of this structural change on ferroelectric properties is understood, the influence on correlated antiferromagnetic (AFM) properties, especially with reduced film thickness, is less clear. Here, the AFM behavior of T‐like BFO films 9–58 nm thick on LaAlO3 (001) substrates fabricated by pulsed laser deposition was studied using conversion electron Mössbauer spectroscopy and X‐ray diffraction. The key findings include: i) Ultrathin T‐like BFO films (<10 nm) show a decoupling of magnetic and structural transitions, with the polar vector tilted 32 degrees from [001] in 9–13 nm films. ii) Films thinner than 13 nm exhibit no structural transition down to 150 K, with a Néel (TN) transition at ≈290 K, ≈35 K lower than thicker films. Interestingly, the TN scaling with thickness suggests realistic scaling exponents considering a critical correlation length for C‐type AFM order, rather than G‐type. The results show that finite size effects can tailor transition temperatures and modulate AFM wave modes in antiferromagnetic oxides, with implications for AFM spintronics for future information technologies.

In ferroic materials, giant susceptibilities can be realized at artificially constructed phase boundaries through deterministic manipulation of the order parameter. Here, emergent ferroelectric structural phase evolution behavior is demonstrated through a synergistic combination of A-site doping and strain engineering. Using chemical solution deposition derived (001)-oriented Sm-substituted bismuth ferrite (Bi1-xSmxFeO3) films as a prototypical system, a morphotropic phase boundary comprising a coexistence of four distinct crystallographic phases is uncovered. These ferroelectric, polar, and nonpolar phases form a nanoscale mixture without the presence of crystallographically hard boundaries. The system thus possesses the ability to show both polarization rotation and extension, effectively releasing the polarization from its crystallographic constraint. Consequently, both robust ferroelectric properties and giant electromechanical responses are obtained. For instance, the optimized composition with x = 0.14 has a remnant polarization of 2P(r) = 103 mu C cm(-2) and electromechanical response 175% that of undoped BFO. These findings showcase the tremendous potential of synthetic phase boundaries, particularly in the context of lead-free functional multiferroics.

We report the observation of a magnetocapacitance effect at the interface between Ni and epitaxial nonpolar BiInO3 thin films at room temperature. A detailed surface study using X-ray photoelectron spectroscopy (XPS) reveals the formation of an intermetallic Ni-Bi alloy at the Ni/BiInO3 interface and a shift in the Bi 4f and In 3d core levels to higher binding energies with increasing Ni thickness. The latter infers band bending in BiInO3, corresponding to the formation of a p-type Schottky barrier. The current-voltage characteristics of the Ni/BiInO3/(Ba,Sr)RuO3/NdScO3(110) heterostructure show a significant dependence on the applied magnetic field and voltage cycling, which can be attributed to voltage-controlled band bending and spin-polarized charge accumulation in the vicinity of the Ni/BiInO3 interface. The magnetocapacitance effect can be realized at room temperature without involving multiferroic materials.

In ferroelectric oxides, it is well-known that defect engineering can be exploited to stabilize novel polar phases. In bismuth ferrite (BiFeO3; BFO) epitaxial films, coherent bismuth oxide inclusions stabilize a pure tetragonal-like (T') phase for film thicknesses greater than 70 nm. Here, we investigate the atomic and chemical structure of such bismuth oxide inclusions within T' BFO thin films grown on (001) LaAlO3 substrates using atomic-resolution aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy. The coherent bismuth oxide inclusions are dispersed throughout the BFO thin film and have a unique structure. In contrast to bulk bismuth oxide, they possess a tetragonally distorted cubic delta-Bi2O3 structure with a = 5.23 & Aring; and c = 6.04 & Aring;, which helps maintain the three-dimensional epitaxial constraint with the surrounding BFO matrix. The T' phase BFO is found to possess the following unit cell parameters; a = 3.77 & Aring;, c = 4.65 & Aring;, c/a = 1.24. Moreover, the appearance of superlattice reflections in the selected area electron diffraction pattern of the BFO thin film implies that the T' phase BFO matrix has a doubled unit cell in the a and c directions compared to the primitive simple perovskite cell, consistent with a Cm monoclinic structure. Our structural and chemical insights into these 'designer defects' and how they interface with the thick T' BFO films will further assist in the development of this material into nanoelectronics.

Gelation chemistry and the related phase development play important roles in chemical solution-derived ferroelectric oxide thin films. Here, we report a detailed study on the gelation chemistry behavior of sol-gel Sm-doped bismuth ferrite (Bi1-xSmxFeO(3)-BSFO) thin films. The precursor solutions were first investigated to elucidate the role of Sm dopant on film molecular networks. The hydrolysis and esterification reactions were found to be dependent on the metal cation species; compared to Bi3+ and Fe3+, the introduction of Sm3+ accelerates the esterification reaction. The subsequent crystallization process of deposited BSFO films was further analyzed to understand the phase structure development as a function of Sm dopant concentration and heating temperature. (00l)-oriented BSFO phases were observed in all films. With the increase of the Sm dopant, the paraelectric orthorhombic (O) phase and antipolar phase start to form at x = 0.10. The domain structure and ferroelectric properties show a concomitant dependence on the Sm doping component. Upon increasing the Sm doping component, the virgin domain amplitude decreases, while the poled domains show an enhanced amplitude. The amplitude variation between the unpoled and poled domains is as high as 760% at x = 0.10. The ferroelectric properties of BSFO films are greatly improved due to the Sm doping. At x = 0.14, a fully developed polarization hysteresis loop is acquired. With the further increase of Sm composition to 0.15, a slightly distorted loop, which indicates an electric-field-induced paraelectric O phase to ferroelectric R phase transition, is first observed in chemical solution deposition (CSD)-derived BSFO films. These results provide a deep insight into the development of rare-earth-doped BFO films through a commercially viable chemical solution method.

Ultrashort light pulses induce rapid deformations of crystalline lattices. In ferroelectrics, lattice deformations couple directly to the polarization, which opens the perspective to modulate the electric polarization on an ultrafast time scale. Here, we report on the temporal and spatial tracking of strain and polar modulation in a single-domain BiFeO 3 thin film by ultrashort light pulses. To map the light-induced deformation of the BiFeO 3 unit cell, we perform time-resolved optical reflectivity and time-resolved x-ray diffraction. We show that an optical femtosecond laser pulse generates not only longitudinal but also shear strains. The longitudinal strain peaks at a large amplitude of 0.6%. The access of both the longitudinal and shear strains enables to quantitatively reconstruct the ultrafast deformation of the unit cell and to infer the corresponding reorientation of the ferroelectric polarization direction in space and time. Our findings open new perspectives for ultrafast manipulation of strain-coupled ferroic orders.

In ferroelectrics, complex interactions among various degrees of freedom enable the condensation of topologically protected polarization textures. Known as ferroelectric solitons, these particle-like structures represent a new class of materials with promise for beyond-CMOS technologies due to their ultrafine size and sensitivity to external stimuli. Such polarization textures have scarcely been demonstrated in multiferroics. Here, we present evidence for ferroelectric solitons in (BiFeO 3 )/(SrTiO 3 ) superlattices. High-resolution piezoresponse force microscopy and Cs-corrected high-angle annular dark-field scanning transmission electron microscopy reveal a zoo of topologies, and polarization displacement mapping of planar specimens reveals center-convergent/divergent topological defects as small as 3 nm. Phase-field simulations verify that some of these structures can be classed as bimerons with a topological charge of ±1, and first-principles-based effective Hamiltonian computations show that the coexistence of such structures can lead to non-integer topological charges, a first observation in a BiFeO 3 -based system. Our results open new opportunities in multiferroic topotronics.

Multiferroic BiFe0.5Cr0.5O3 (BFCO) thin films are promising candidates for emerging optoelectronics and all-oxide solar absorbers. Yet, a thorough understanding of the structural evolution and associated changes in the functional properties of BFCO is lacking. Here, we explore thickness-dependent structural phase transitions in the epitaxial BFCO films and ascertain the impact of the accompanying crystallographic distortions on their photoresponse. The results show that the strain imposed by the substrate changes the crystal symmetry, inducing a transition from a tetragonal-like phase to a rhombohedral-like phase through a rather complex strain relaxation mechanism upon increasing film thickness. This change in crystallographic distortion also induces a shift of similar to 150 meV in the bandgap. Moreover, wavelength- resolved photocurrent measurements reveal that the absorption onset is red-shifted for the tetragonal-like structure, implying light absorption up to wavelengths of 800 nm. First-principles calculations shed further light on the symmetry-induced changes in the electronic structure of the BFCO films. The crystallographic symmetry is shown to be a decisive factor in modifying the valence band maximum and conduction band minimum characteristics in the perovskite oxides, revealing an emerging type of Mott multiferroic in the BFCO system. This work provides a practical strategy to further engineer the optoelectronic properties of the multiferroic oxide films through thickness-induced phase transitions.

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

The understanding of the lattice dynamics in ferroic compounds driven by an ultrashort light pulse is an exciting research direction due to the exceptional non-linear properties (optical, elastic, electric and magnetic) of ferroic and multiferroic materials. Photo-induced strain in ferroic materials is driven by a complex interplay between charge, phonon and spin dynamics with microscopic mechanisms that still need to be elucidated. We present recent experiments where ultrafast photoinduced strain is evaluated in BiFeO3-based multiferroic materials, with a focus on the description of the ultrafast symmetry change of the unit-cell that appears after an ultrashort laser pulse. A combination of optical and X-ray time-resolved techniques will be presented. We show that it is possible to modulate at the picosecond time scale the ferroic order by playing with the out-of-plane and in-plane light-induced strains. These new results provide new insights for the understanding of the physics of photo-induced strain, in relation with the light-induced ferroelectric modulation in nanostructured ferroic compounds and could be the first step towards their use as on-purpose ferroic architectures in devices like actuators or modulators with ultra-short light pulses.

Ferroelectric materials exhibit spontaneous polarization that can be switched by electric field. Beyond traditional applications as nonvolatile capacitive elements, the interplay between polarization and electronic transport in ferroelectric thin films has enabled a path to neuromorphic device applications involving resistive switching. A fundamental challenge, however, is that finite electronic conductivity may introduce considerable power dissipation and perhaps destabilize ferroelectricity itself. Here, tunable microwave frequency electronic response of domain walls injected into ferroelectric lead zirconate titanate (PbZr0.2Ti0.8O3) on the level of a single nanodomain is revealed. Tunable microwave response is detected through first‐order reversal curve spectroscopy combined with scanning microwave impedance microscopy measurements taken near 3 GHz. Contributions of film interfaces to the measured AC conduction through subtractive milling, where the film exhibited improved conduction properties after removal of surface layers, are investigated. Using statistical analysis and finite element modeling, we inferred that the mechanism of tunable microwave conductance is the variable area of the domain wall in the switching volume. These observations open the possibilities for ferroelectric memristors or volatile resistive switches, localized to several tens of nanometers and operating according to well‐defined dynamics under an applied field.