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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. Topological structures could spark promising functionalities in next generation nanoelectronics. Here, the authors report the realization of complex topological polar textures in epitaxial multiferroic BiFeO 3 –SrTiO 3 superlattices induced by competing electrical and mechanical boundary conditions.

Ferroelectric materials possess a spontaneous polarization that is switchable by an electric field. Robust retention of switched polarization is critical for non-volatile nanoelectronic devices based on ferroelectrics, however, these materials often suffer from polarization relaxation, typically within days to a few weeks. Here we exploit designer-defect-engineered epitaxial BiFeO 3 films to demonstrate polarization retention with virtually no degradation in switched nanoscale domains for periods longer than 1 year. This represents a more than 2000% improvement over the best values hitherto reported. Scanning probe microscopy-based dynamic switching measurements reveal a significantly increased activation field for domain wall movement. Atomic resolution scanning transmission electron microscopy indicates that nanoscale defect pockets pervade the entire film thickness. These defects act as highly efficient domain wall pinning centres, resulting in anomalous retention. Our findings demonstrate that defects can be exploited in a positive manner to solve reliability issues in ferroelectric films used in functional devices. The use of ferroelectric materials in memory device applications is held back by low retention times. Here, the authors demonstrate that by intentionally introducing defective nanoregions which increase the activation field for domain wall motion, retention times larger than a year can be achieved.

The nature of lead zirconate, the historical antiferroelectric material, has recently been challenged. In PbZrO 3 epitaxial films, thickness reduction engenders competition among antiferroelectric, ferrielectric and ferroelectric phases. All studies so far on PbZrO 3 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 LaLuO 3 to stabilize a pure antiferroelectric phase in PbZrO 3 . 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 PbZrO 3 . Here the authors use tensile strain imposed by artificial substrates of LaLuO3 to stabilize a pure antiferroelectric phase in epitaxial thin films of PbZrO3.

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. The antiferromagnetic (AFM) behavior of tetragonal‐like BiFeO3 films fabricated on LaAlO3 substrates was studied. Scaling down the film thickness from 58 to 9 nm induces a decrease of ≈40 K in the Néel temperature, and ultrathin films are shown to have decoupled magnetic and structural transitions. These results are relevant for nanoscale AFM spintronics.

Understanding and manipulating complex spin texture in multiferroics can offer new perspectives for electric field-controlled spin manipulation. In BiFeO3, a well-known room temperature multiferroic, the competition between various exchange interactions manifests itself as non-collinear spin order, i.e., an incommensurate spin cycloid with period 64 nm. We report on the stability and systematic expansion of the length of the spin cycloid in (110)-oriented epitaxial Co-doped BiFeO3 thin films. Neutron diffraction shows (i) this cycloid, despite its partly out-of-plane canted propagation vector, can be stabilized in thinnest films; (ii) the cycloid length expands significantly with decreasing film thickness; (iii) theory confirms a unique [112¯] cycloid propagation direction; and (iv) in the temperature dependence the cycloid length expands significantly close to TN. These observations are supported by Monte Carlo simulations based on a first-principles effective Hamiltonian method. Our results therefore offer new opportunities for nanoscale magnonic devices based on complex spin textures.

Piezoelectrics interconvert mechanical energy and electric charge and are widely used in actuators and sensors. The best performing materials are ferroelectrics at a morphotropic phase boundary, where several phases coexist. Switching between these phases by electric field produces a large electromechanical response. In ferroelectric BiFeO 3 , strain can create a morphotropic-phase-boundary-like phase mixture and thus generate large electric-field-dependent strains. However, this enhanced response occurs at localized, randomly positioned regions of the film. Here, we use epitaxial strain and orientation engineering in tandem—anisotropic epitaxy—to craft a low-symmetry phase of BiFeO 3 that acts as a structural bridge between the rhombohedral-like and tetragonal-like polymorphs. Interferometric displacement sensor measurements reveal that this phase has an enhanced piezoelectric coefficient of ×2.4 compared with typical rhombohedral-like BiFeO 3 . Band-excitation frequency response measurements and first-principles calculations provide evidence that this phase undergoes a transition to the tetragonal-like polymorph under electric field, generating an enhanced piezoelectric response throughout the film and associated field-induced reversible strains. These results offer a route to engineer thin-film piezoelectrics with improved functionalities, with broader perspectives for other functional oxides. Strain in thin films can increase piezoelectric properties, but crystallographic constraints may restrict the enhanced response to localized regions. Here, by combining strain and orientation engineering, a low-symmetry bridging phase of BiFeO 3 with enhanced piezoresponse is stabilized uniformly throughout the film.

Epitaxially strain‐engineered tetragonal (T)‐like BiFeO 3 (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 LaAlO 3 (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 (T N ) transition at ≈290 K, ≈35 K lower than thicker films. Interestingly, the T N 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.

The nature of lead zirconate, the historical antiferroelectric material, has recently been challenged. In PbZrO epitaxial films, thickness reduction engenders competition among antiferroelectric, ferrielectric and ferroelectric phases. All studies so far on PbZrO 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 LaLuO to stabilize a pure antiferroelectric phase in PbZrO . 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 PbZrO .

Ferroelectric tunnel junctions (FTJs) harness the unique combination of ferroelectricity and quantum tunneling, and thus herald new 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 than in addition to tunneling electroresistance. This perspective aims to provide in-depth insight into novel physical phenomena of several typical ferroelectric hybrid junctions, ranging from ferroelectric/dielectric, ferroelectric/multiferroic, 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 a radically new 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 new possible type of FeRTD device. The prospects for enhanced performance and expanded functionality ignite tremendous excitement in hybrid FTJs and FeRTDs for future nanoelectronics.

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.