Reversible Manipulation of Polar Topologies in Oxide Ferroelectrics via Electric Fields.

Polar topologies show great potentials in memories and other nano-micro devices. To integrate with silicon conducting circuits, it is vital to understand the dynamic evolution and the transformation of different domain configurations under external stimulus. Here in situ transmission electron microscopy is performed and the electrically controlled creation and annihilation of large-scale polar flux-closure array from typical c/a domains in PbTiO/SrTiO bilayers is directly observed.

A Roadmap for Ferroelectric-Antiferroelectric Phase Transition.

Antiferroelectric materials have shown great potential in electronic devices benefiting from the reversible phase transition between ferroelectric and antiferroelectric phases. Understanding the dipole arrangements and clear phase transition pathways is crucial for design of antiferroelectric materials-based energy storage and conversion devices. However, the specific phase transition details remain largely unclear and even controversial to date.

Moiré fringe imaging of heterostructures by scanning transmission electron microscopy.

A heterostructured crystalline bilayer specimen is known to produce moiré fringes (MFs) in the conventional transmission electron microscopy (TEM). However, the understanding of how these patterns form in scanning transmission electron microscopy (STEM) remains limited. Here, we extended the double-scattering model to establish the imaging theory of MFs in STEM for a bilayer sample and applied this theory to successfully explain both experimental and simulated STEM images of a perovskite PbZrO/SrTiO system.

Polar Bloch points in strained ferroelectric films.

Topological domain structures have drawn great attention as they have potential applications in future electronic devices. As an important concept linking the quantum and classical magnetism, a magnetic Bloch point, predicted in 1960s but not observed directly so far, is a singular point around which magnetization vectors orient to nearly all directions. Here we show polar Bloch points in tensile-strained ultrathin ferroelectric PbTiO films, which are alternatively visualized by phase-field simulations and aberration-corrected scanning transmission electron microscopic imaging.

Large Polarization Near 50 μC/cm in a Single Unit Cell Layer SrTiO.

The generally nonpolar SrTiO has attracted more attention recently because of its possibly induced novel polar states and related paraelectric-ferroelectric phase transitions. By using controlled pulsed laser deposition, high-quality, ultrathin, and strained SrTiO layers were obtained. Here, transmission electron microscopy and theoretical simulations have unveiled highly polar states in SrTiO films even down to one unit cell at room temperature, which were stabilized in the PbTiO/SrTiO/PbTiO sandwich structures by in-plane tensile strain and interfacial coupling, as evidenced by large tetragonality (∼1.

Absence of critical thickness for polar skyrmions with breaking the Kittel's law.

The period of polar domain (d) in ferroics was commonly believed to scale with corresponding film thicknesses (h), following the classical Kittel's law of d ∝ [Formula: see text]. Here, we have not only observed that this relationship fails in the case of polar skyrmions, where the period shrinks nearly to a constant value, or even experiences a slight increase, but also discovered that skyrmions have further persisted in [(PbTiO)/(SrTiO)] ultrathin superlattices. Both experimental and theoretical results indicate that the skyrmion periods (d) and PbTiO layer thicknesses in superlattice (h) obey the hyperbolic function of d = Ah + [Formula: see text] other than previous believed, simple square root law.

Atomic Insight into the Successive Antiferroelectric-Ferroelectric Phase Transition in Antiferroelectric Oxides.

Antiferroelectrics characterized by voltage-driven reversible transitions between antiparallel and parallel polarity are promising for cutting-edge electronic and electrical power applications. Wide-ranging explorations revealing the macroscopic performances and microstructural characteristics of typical antiferroelectric systems have been conducted. However, the underlying mechanism has not yet been fully unraveled, which depends largely on the atomistic processes.

Room-Temperature Ferroelectricity of Paraelectric Oxides Tailored by Nano-Engineering.

Inducing clear ferroelectricity in the quantum paraelectric SrTiO is important for triggering methods to discover hidden phases in condensed matter physics. Several methods such as isotope substitution and freestanding membranes could introduce ferroelectricity in SrTiO toward nonvolatile memory applications. However, the stable transformation from quantum paraelectric SrTiO to ferroelectricity SrTiO at room temperature still remains challenging.

Achieving High-Temperature Multiferroism by Atomic Architecture.

Materials with multiple order parameters, typically, in which ferroelectricity and magnetism are coupled, are illuminative for next-generation multifunctional electronics. However, searching for such single-phase multiferroics is challenging owing to antagonistic orbital occupancy and chemical bonding requirements for polarity and magnetism. Appropriate multiferroic candidates have been proposed, but their practical implementation is impeded by the low working temperature, weak coupling between ferroic orders, or antiparallel spin alignment in magnetic sublattices.

Real-Time Transformation of Flux-Closure Domains with Superhigh Thermal Stability.

Polar topologies have received extensive attention due to their exotic configurations and functionalities. Understanding their responsive behaviors to external stimuli, especially thermal excitation, is highly desirable to extend their applications to high temperature, which is still unclear. Here, combining transmission electron microscopy and phase-field simulations, the thermal dynamics of the flux-closure domains were illuminated in PbTiO/SrTiO multilayers.

Polar Magnetism Above 600 K with High Adaptability in Perovskite Oxides.

High magnetic order temperature, sustainable polar insulating state, and tolerance to device integrations are substantial advantages for applications in next-generation spintronics. However, engineering such functionality in a single-phase system remains a challenge owing to the contradicted chemical and electronic requirements for polar nature and magnetism, especially with an ordering state highly above room temperature. Perovskite-related oxides with unique flexibility allow electron-unpaired subsystems to merge into the polar lattice to induce magnetic interactions, combined with their inherent asymmetry, thereby promising polar magnet design.

Magneto-Electric-Optical Coupling in Multiferroic BiFeO -Based Films.

Manipulating ferroic orders and realizing their coupling in multiferroics at room temperature are promising for designing future multifunctional devices. Single external stimulation has been extensively proved to demonstrate the ability of ferroelastic switching in multiferroic oxides, which is crucial to bridge the ferroelectricity and magnetism. However, it is still challenging to directly realize multi-field-driven magnetoelectric coupling in multiferroic oxides as potential multifunctional electrical devices.

Direct Observation of Large-Scale Screw Dislocation Grids in Oxide Heteroepitaxies.

Screw dislocation is important not only for understanding plastic deformation of crystals but also for optical and electrical properties of materials. However, characterizations of screw dislocations are still challenging since there is almost no atom distortion when viewed along the dislocation line. In particular, although it is theoretically known that shear strains in heteroepitaxy systems may be relaxed via screw dislocation grids, the specific structures and thickness-dependent evolutions of these grids are still largely unknown.

Thickness-Dependent Polar Domain Evolution in Strained, Ultrathin PbTiO Films.

Ferroelectric ultrathin films have great potential in electronic devices and device miniaturization with the innovation of technology. In the process of product commercialization, understanding the domain evolution and topological properties of ferroelectrics is a prerequisite for high-density storage devices. In this work, a series of ultrathin PbTiO (PTO) films with varying thicknesses were deposited on cubic KTaO substrates by pulsed laser deposition and were researched by Cs-corrected scanning transmission electron microscopy (STEM), reciprocal space mapping (RSM), and piezoresponse force microscopy (PFM).

Atomic-Scale Tunable Flexoelectric Couplings in Oxide Multiferroics.

Flexoelectricity is an effective tool in modulating the crystallographic structures and properties of oxides for multifunctional applications. However, engineering the nonuniform strain to obtain tunable flexoelectric behaviors at the atomic scale remains an ongoing challenge in conventional substrate-imposed ferroelectric films. Here, the regulatable flexoelectric behaviors are demonstrated at atomic scale in [110]-oriented BiFeO thin films, which are triggered by the strain-field coupling of high-density interfacial dislocations.

Periodic Polarization Waves in a Strained, Highly Polar Ultrathin SrTiO.

SrTiO is generally paraelectric with centrosymmetric structure exhibiting unique quantum fluctuation related ferroelectricity. Here we reveal highly polar and periodic polarization waves in SrTiO at room temperature, which is stabilized by periodic tensile strains in a sandwiched PbTiO/SrTiO/PbTiO structure. Scanning transmission electron microscopy reveals that periodic / domain structures in PbTiO layers exert unique periodic tensile strains in the ultrathin SrTiO layer and consequently make the highly polar and periodic states of SrTiO.

Atomic mapping of periodic dipole waves in ferroelectric oxide.

A dipole wave is composed of head-to-tail connected electric dipoles in the form of sine function. Potential applications in information carrying, transporting, and processing are expected, and logic circuits based on nonlinear wave interaction are promising for dipole waves. Although similar spin waves are well known in ferromagnetic materials for their roles in some physical essence, electric dipole wave behavior and even its existence in ferroelectric materials are still elusive.

Spinodal Decomposition-Driven Endurable Resistive Switching in Perovskite Oxides.

Common pursuits of developing nanometric logic and neuromorphic applications have motivated intensive research studies into low-dimensional resistive random-access memory (RRAM) materials. However, fabricating resistive switching medium with inherent stability and homogeneity still remains a bottleneck. Herein, we report a self-assembled uniform biphasic system, comprising low-resistance 3 nm-wide (Bi,La)FeO nanosheets coherently embedded in a high-resistance (Bi,La)FeO matrix, which were spinodally decomposed from an overall stoichiometry of the (Bi,La)FeO parent phase, as a promising nanocomposite to be a stable and endurable RRAM medium.

Tuning ferroelectricity and ferromagnetism in BiFeO/BiMnO superlattices.

Multiferroic materials with multifunctional characteristics play a critical role in the field of microelectronics. In a perovskite oxide, ferroelectric polarization and ferromagnetism usually cannot coexist in a single-phase material at the same time. In this work, we design a superlattice structure composed of alternating BiFeO and BiMnO layers and illustrate how tuning the supercell size of epitaxial BiFeO/BiMnO superlattices facilitates ferroelectric polarization while maintaining relatively strong ferromagnetism.

Modulation of charged a/a domains and piezoresponses of tensile strained PbTiO films by the cooling rate.

Controlling domain width, orientation, and patterns in oxide ferroelectrics are not only important for fundamental research but also for potential electronic application. Here, a series of PbTiO thin films under various cooling rates were deposited on (110)-oriented NdScO substrates by pulsed laser deposition and investigated by using conventional transmission electron microscopy, Cs-corrected scanning TEM and piezoresponse force microscopy. Contrast analysis and electron diffraction revealed that PbTiO films are a/a domain patterns under large tensile strains with different cooling rates.

Effect of transition metal (TM) doping on structural and magnetic properties in hexagonal YMnTMO systems.

Suitable TM doping at the Mn site is an important access to manipulate magnetic properties of hexagonal YMnO, however, it has not yet been systematically explored how the strength of antiferromagnetic interactions and the magnetic transition temperatures ( ) are modified in the doping YMnTMO systems. In the work, we have performed first-principles calculations to study the effect of TM doping on the structural and magnetic properties of hexagonal YMnTMO bulks; the results are combined with the available experimental results. The calculated results reveal that the planar TM-O bonds and O-TM-O angles of TMO bipyramid are both prominent structural features for the transformations of magnetic properties.

Rhombohedral-Orthorhombic Ferroelectric Morphotropic Phase Boundary Associated with a Polar Vortex in BiFeO Films.

Strongly correlated oxides exhibit multiple degrees of freedoms, which can potentially mediate exotic phases with exciting physical properties, such as the polar vortex recently found in ferroelectric oxide films. A polar vortex is stabilized by competition between charge, lattice, and/or orbital degrees of freedom, which displays vortex-ferroelectric phase transitions and emergent chirality, making it a potential candidate for designing information storage and processing devices. Here, by a combination of controlled film growth and aberration-corrected scanning transmission electron microscopy, we obtain nanoscale vortex arrays in [110]-oriented BiFeO films.

Oxygen Vacancy Ordering Modulation of Magnetic Anisotropy in Strained LaCoO Thin Films.

Oxygen vacancy configurations and concentration are coupled with the magnetic, electronic, and transport properties of perovskite oxides, and manipulating the physical properties by tuning the vacancy structures of thin films is crucial for applications in many functional devices. In this study, we report a direct atomic resolution observation of the preferred orientation of vacancy ordering structure in the epitaxial LaCoO (LCO) thin films under various strains from large compressive to large tensile strain utilizing scanning transmission electron microscopy (STEM). Under compressive strains, the oxygen vacancy ordering prefers to be along the planes parallel to the heterointerface.

Thickness-Dependent Evolution of Piezoresponses and Stripe 90° Domains in (101)-Oriented Ferroelectric PbTiO Thin Films.

High-index ferroelectric films as (101)-orientated ones exhibit enhanced dielectric responses, piezoelectric responses, and exotic ferroelectric switching behaviors, which are potential candidates for applications in memories and capacitors. However, possible domain patterns and domain wall structures in (101)-oriented ferroelectric thin films are still elusive, which results in difficulties in understanding the origin and further modulating their special properties. In this work, a series of PbTiO (PTO) thin films with 35, 50, 60, and 70 nm in thickness were grown on (101)-oriented (LaAlO)(SrTaAlO) (LSAT(101)) substrates by pulsed laser deposition and investigated by both piezoresponse force microscopy (PFM) and (scanning) transmission electron microscopy ((S)TEM).

Polarization Rotation in Ultrathin Ferroelectrics Tailored by Interfacial Oxygen Octahedral Coupling.

Multiple polar states and giant piezoelectric responses could be driven by polarization rotation in ferroelectric films, which have potential functionalities in modern material applications. Although theoretical calculations have predicted polarization rotation in pure PbTiO films without domain walls and strains, direct experiment has rarely confirmed such polar states under this condition. Here, we observed that interfacial oxygen octahedral coupling (OOC) can introduce an oxygen octahedral rotation, which induces polarization rotation in single domain PbTiO films with negligible strains.