Ferroelectric Vortices Research Articles

Electric Field-Manipulated Optical Chirality in Ferroelectric Vortex Domains.

Manipulating optical chirality via electric fields has garnered considerable attention in the realm of both fundamental physics and practical applications. Chiral ferroelectrics, characterized by their inherent optical chirality and switchable spontaneous polarization, are emerging as a promising platform for electronic-photonic integrated circuits applications. Unlike organics with chiral carbon centers, integrating chirality into technologically mature inorganic ferroelectrics has posed a long-standing challenge. Here, the successful introduction of chirality is reported into self-assembly La-doped BiFeO3 nanoislands, which exhibit ferroelectric vortex domains. By employing synergistic experimental techniques with piezoresponse force microscopy and nonlinear optical second-harmonic generation probes, a clear correlation between chirality and polarization configuration within these ferroelectric nanoislands is established. Furthermore, the deterministic control of ferroelectric vortex domains and chirality is demonstrated by applying electric fields, enabling reversible and nonvolatile generation and elimination of optically chiral signals. These findings significantly expand the repertoire of field-controllable chiral systems and lay the groundwork for the development of innovative ferroelectric optoelectronic devices.

Probing ferroelectric domain structures and their switching dynamics in SrBi2Ta2O9 by in-situ electric biasing in transmission electron microscopy

Lead-free SrBi2Ta2O9 (SBT) has been a promising ferroelectric material for various applications such as electronics and data storage due to its outstanding ferroelectric properties including high fatigue endurance and low leakage current. However, the atomic-scale domain structure and switching dynamics of ferroelectric SBT remain elusive. This study reveals that spontaneous polarization arises from canted bismuth-cation displacements, forming 90° and Ising-type 180° domain walls. Interestingly, topological pairs of ferroelectric vortex and antivortex connect ferroelectric boundaries where three domain walls converge. In situ electrical biasing transmission electron microscopy (TEM) reveals the dominance of 180° switching over 90°, where oxygen octahedral connectivity is protected by ferroelastic energy in the 90° domain wall. Consequently, all 180° domain walls and (anti)vortices are removed, leaving only 90° domain walls in the electrically poled states. Chemical deterioration along domain walls highlights vulnerability of SBT to ferroelectric fatigue. This study provides insight into crucial aspects for practical applications of SBT.

Magnetoelectric coupling at the domain level in polycrystalline hexagonal ErMnO3

We explore the impact of a magnetic field on the ferroelectric domain pattern in polycrystalline hexagonal ErMnO3 at cryogenic temperatures. Utilizing piezoelectric force microscopy measurements at 1.65 K, we observe modifications of the topologically protected ferroelectric domain structure induced by the magnetic field. These alterations likely result from strain induced by the magnetic field, facilitated by intergranular coupling in polycrystalline multiferroic ErMnO3. Our findings give insights into the interplay between electric and magnetic properties at the domain scale and represent a so far unexplored pathway for manipulating topologically protected ferroelectric vortex patterns in hexagonal manganites.

Imaging and structure analysis of ferroelectric domains, domain walls, and vortices by scanning electron diffraction

Imaging and structure analysis of ferroelectric domains, domain walls, and vortices by scanning electron diffraction

Collective control of a vortex array in a ferroelectric ultrathin film

Recently, the observation of ferroelectric vortex arrays has triggered the investigation of topological domain structures and their characteristics. Vortices are typical topological domain structures with chirality in nanoscale ferroelectric materials. The chirality of a single vortex in a nanodot can be easily manipulated, but the collective control of a vortex array is exceptionally difficult and has not yet been realized. This Letter proposes an effective scheme for the collective control of a vortex array and investigates it via phase-field simulations. The results indicate that the collective control of a vortex array with bidirectional switching can be realized by introducing a bending film with periodic large curvatures under alternative electric fields. Furthermore, a general rule for determining the electrically controllable chirality of ferroelectric vortices is proposed. This Letter demonstrates the feasibility of the collective control of vortex arrays and provides insights for developing ferroelectric nanoelectronic devices based on vortex arrays.

Phase-field simulations of topological conservation in multi-vortex induced by surface charge in BiFeO3 thin films

The creations and manipulations of vortexes in ferroelectric materials with external stimuli are expected to be used in the design and fabrication of sensing materials and multifunctional electronic devices. In this work, we investigated the surface charge-induced multi-vortex evolution using the phase-field simulations in BiFeO3. A combination of domain morphology, polarization distribution and winding number calculation was considered. The results show that vortex and anti-vortex exist simultaneously in pairs, and the total value of winding numbers is always 0. In addition, the minimum distance [Formula: see text] between the surface charge regions is 9[Formula: see text]nm when the vortex domains are independent of each other. This work provides a reference for the manipulation of ferroelectric vortex induced by surface charges, which lays a theoretical foundation for the design and fabrication of high-density vortex memories.

Strain effects on stability of topological ferroelectric polar configurations in (PbTiO3)n/(SrTiO3)n superlattices

The (PbTiO3)n/(SrTiO3)n (PTO/STO) superlattice system has been shown to exhibit interesting topological phases (e.g., vortices and skyrmions) in addition to normal ferroelectric domain states. Existing studies are mostly focused on the dependence of topological polar distributions and properties of PTO/STO superlattice on its periodicity. Here, we study the strain effect on the topological phase transitions and ferroelectric domain structures employing phase-field simulations. We summarized in an isotropic strain (in-plane misfit strain along the x direction is equal to that along the y direction) periodicity phase diagram displaying the stability regions of different polar topological states, including normal ferroelectric twins, vortices, skyrmions, and mixtures of vortices and twins. We also analyzed the polarization configurations under anisotropic in-plane strains (in-plane misfit strain along the x direction is not equal to that along the y direction) and demonstrated that the strain anisotropy can be used to tune the directions of vortex arrays along either the [100] pc or [010] pc directions or labyrinth vortex arrays. This work offers guidance to manipulating polar structures in the PTO/STO superlattices via strain engineering.

Pure Chiral Polar Vortex Phase in PbTiO3/SrTiO3 Superlattices with Tunable Circular Dichroism.

Nontrivial polarization textures have been demonstrated in ferroelectric/dielectric superlattices, where the electrostatic, elastic, and different gradient energies compete in a delicate balance. When PbTiO3/SrTiO3 superlattices are grown on DyScO3, the coexistence of ferroelectric domains and vortex structure is observed for n = 12-20 unit cells. Here, we report an approach to achieve single-phase vortex structures in superlattices by controlling the epitaxial strain using Sr1.04Al0.12Ga0.35Ta0.50O3 substrates. The domain width follows Kittel's law with the thickness of the ferroelectric PbTiO3 layers. A phase transition from vortex to a disordered phase with temperature is characterized by the correlation length. Resonant soft X-ray diffraction circular dichroism at the titanium L-edge reveals enhanced chirality with the thickness of the ferroelectric layer. These results are supported by second-principles simulations, which demonstrate that the integrated helicity increases with n. The stabilization of chiral single-phase polar vortices in ferroelectric/dielectric superlattices can enable novel optoelectronic devices with enhanced ferroelectric-light interaction.

Vortex structure in relaxed BaTiO3/SrTiO3 superlattice

In this work, using the phase-field model, we demonstrated vortex structure can be generated in a lead-free BaTiO3/SrTiO3 superlattice structure. The labyrinth pattern (maze-like) domain structure was observed in a fully relaxed BaTiO3/SrTiO3 superlattice structure. Topological bubble-like rosette structures were found at an out-of-plane field, and polar skyrmions were obtained after the electric field was removed. In a designed fully relaxed nanowire of width ∼8 nm, near-perfect periodical clockwise-counterclockwise ferroelectric vortex pairs were generated due to the interfacial constraints. The simulated topological structures are potentially relevant to the application of high-density ferroelectrics memory devices.

Emergent Ferroelectric Switching Behavior from Polar Vortex Lattice.

Topologically protected polar textures have provided a rich playground for the exploration of novel, emergent phenomena. Recent discoveries indicate that ferroelectric vortices and skyrmions not only host properties markedly different from traditional ferroelectrics, but also that these properties can be harnessed for unique memory devices. Using a combination of capacitor-based capacitance measurements and computational models, it is demonstrated that polar vortices in dielectric-ferroelectric-dielectric trilayers exhibit classical ferroelectric bi-stability together with the existence of low-field metastable polarization states. This behavior is directly tied to the in-plane vortex ordering, and it is shown that it can be used as a new method of non-destructive readout-out of the poled state.

Novel Geometric Ferroelectric EuInO3 Single Crystals with Topological Vortex Domains

RInO3 (R: rare-earth element) has drawn unprecedented research attention due to its geometric ferroelectricity and spin liquid state. However, the structure–property relationship needs further investigation based on well-developed single crystals. A EuInO3 crystal was obtained for the first time by the laser floating zone method. The presence of ferroelectricity was revealed by polarization–electric field hysteresis loops of the bulk EuInO3 single crystal. Moreover, interesting topological vortex domains were revealed by vertical piezoresponse force microscopy. The lattice dynamics of EuInO3 was probed by correlating various Raman modes with the structural distortion of the unit cell. The improper ferroelectricity and topological ferroelectric vortices of the EuInO3 crystal provide great potential for vortex memory devices.

Phase-field method explored ferroelectric vortex topology structure and morphotropic phase boundaries

The perovskite crystal structure determines the appearance of ferroelectricity and the polarization direction of ferroelectric ceramics. When the polarization direction has a certain order, different domain structures will combine to form a multiparticle system with a specific morphology, i.e. the topological structure existing in ferroelectrics. In this study, the domain structures of potassium sodium niobate (<inline-formula><tex-math id="M2">\begin{document}$ {\rm{K}}_{0.5}{\rm{N}\rm{a}}_{0.5}\rm{N}\rm{b}{\rm{O}}_{3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20221898_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20221898_M2.png"/></alternatives></inline-formula>) thin films under different hysteresis electric fields and thickness are simulated and observed by the phase field method. According to the different switching paths of the domain structure under the electric field, the domain is divided into fast and slow switching process. Based on this, a method is proposed to first determine the domain switching state of the desired experiment and then conduct directional observation. Through the analysis of the domain structures combined with the polarization vector, a clear multi-domain combined with vortex-antivortex pair topological structure is observed for the first time in <inline-formula><tex-math id="M3">\begin{document}$ {\rm{K}}_{0.5}{\rm{N}\rm{a}}_{0.5}\rm{N}\rm{b}{\rm{O}}_{3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20221898_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20221898_M3.png"/></alternatives></inline-formula> film. The vortex structure is further analyzed for its switching process, and it is observed that this vortex topological microstructure can make the domain more likely to switch, so that more small-scale polarization vectors can be ordered, forming the desired multiparticle system topology. The mechanism of improving the dielectric properties of ferroelectric material by this polarization vector ordering is similar to that of the microscopic phase boundary formed by the specific polarization directions on both sides of the quasi morphotropic phase boundary.

Computing diffraction patterns of microstructures from phase-field simulations

The diffraction pattern of a material contains information not only on the crystal structures of its constituting phases, but also on its mesoscale spatial distributions of phases, grains, and ferroelastic, ferroelectric, and ferromagnetic domains. While diffraction patterns from experiments such as X-ray diffraction are presented in the reciprocal or Fourier space, mesoscale microstructure models such as the phase-field method naturally produce real-space images of spatial distribution of chemical composition, structural, and ferroic domains. Although one could rather readily compute the Fourier amplitudes of chemical and structural domain distributions generated by mesoscale simulations, they only contain information about the length scale and alignment of the real-space chemical and structure domains. Therefore, a direct comparison between diffraction experiments and mesoscale microstructure simulations is not possible. Here, we develop a theoretical approach to directly compute the crystal diffraction patterns of microstructures predicted by phase-field simulations. In particular, we consider five representative examples of microstructure patterns involving purely compositional domains, a single pair of tetragonal twin structures, multiple twin variants in a hexagonal system, ferroelectric polar vortices, and polycrystalline grains. The results are compared with previous experimental observations as well as X-ray diffraction experiments performed in the present study. The theoretical framework allows one to directly connect material microstructures and diffraction patterns predicted from phase-field simulations and the corresponding diffraction patterns from experiments, and thus providing guidance to experimental diffraction characterization and interpretation of microstructures.

From the flow to the polarization field: A cognitive way for ferroelectric vortex structures

The generation of topological domain structures can significantly influence the behaviors of ferroelectric materials, and, however, strategies and perspectives for treating and analyzing them are still lacking. Inspired by the fluid lines and vorticity in hydromechanics, a characterization method of polarization lines and polarization vorticity is proposed herein for characterizing the vortex structure evolution. The polarization vorticity has the same unit as charge density, and the polarization vortex size is identified as a structure for vorticity concentration. Observing the radius and vorticity of vortices, we find that the domain evolution can be divided into three stages and the vortices react to mechanical loading in two ways: increasing the vorticity and enlarging the radius. Moreover, vortices exhibit three-dimensional shape, just like the vortex tube, and their conservation is proved. This paper provides a basis for further research on topological domain structures and the mechanism of vortex evolution, which might help improve high energy density devices.

Inherent Spin-Polarization Coupling in a Magnetoelectric Vortex.

Solid-state materials are currently being explored as a platform for the manipulation of spins for spintronics and quantum information science. More broadly, a wide spectrum of ferroelectric materials, spanning from inorganic oxides to polymeric systems such as PVDF, present a different approach to explore quantum phenomena in which the spins are set and manipulated with electric fields. Using dilute Fe3+-doped ferroelectric PbTiO3-SrTiO3 superlattices as a model system, we demonstrate intrinsic spin-polarization control of spin directionality in complex ferroelectric vortices and skyrmions. Electron paramagnetic resonance (EPR) spectra show that the spins in the Fe3+ ion are strongly coupled to the local polarization and preferentially aligned perpendicular to the ferroelectric polar c axis in this complex vortex structure. The effect of polarization-spin directionality is corroborated by first-principles calculations, demonstrating the variation of the spin directionality with the polar texture and offering the potential for future quantum analogues of macroscopic magnetoelectric devices.

Phase-field simulations of vortex chirality manipulation in ferroelectric thin films

The ferroelectric chiral vortex domains are highly desirable for the application of data storage devices with low-energy consumption and high-density integration. However, the controllable switching of vortex chirality remains a challenge in the current ferroelectric community. Utilizing phase-field simulations, we investigate the vortex domain evolution and chirality formation in BiFeO3 thin films. By applying local surface charge or electric field, we demonstrate that the vorticity and the polarity can be manipulated by the initial bi-domain arrangement and the external field with different directions, respectively. By exchanging the domain arrangements, the opposite chirality can be obtained. Importantly, the topological vortex domain is retained after removing the external field. The vortex chirality can be switched reversibly with high reproducibility, which is beneficial to fatigue tolerance of the material in the operation. These results provide theoretical guidance for manipulating the vortex chirality in ferroelectric films.

Ferroelectric incommensurate spin crystals.

Ferroics, especially ferromagnets, can form complex topological spin structures such as vortices1 and skyrmions2,3 when subjected to particular electrical and mechanical boundary conditions. Simple vortex-like, electric-dipole-based topological structures have been observed in dedicated ferroelectric systems, especially ferroelectric-insulator superlattices such as PbTiO3/SrTiO3, which was later shown to be a model system owing to its high depolarizing field4-8. To date, the electric dipole equivalent of ordered magnetic spin lattices driven by the Dzyaloshinskii-Moriya interaction (DMi)9,10 has not been experimentally observed. Here we examine a domain structure in a single PbTiO3 epitaxial layer sandwiched between SrRuO3 electrodes. We observe periodic clockwise and anticlockwise ferroelectric vortices that are modulated by a second ordering along their toroidal core. The resulting topology, supported by calculations, is a labyrinth-like pattern with two orthogonal periodic modulations that form an incommensurate polar crystal that provides a ferroelectric analogue to the recently discovered incommensurate spin crystals in ferromagnetic materials11-13. These findings further blur the border between emergent ferromagnetic and ferroelectric topologies, clearing the way for experimental realization of further electric counterparts of magnetic DMi-driven phases.

Path-dependent Vortex Switching in Ferroelectric Nanoplate Junctions Toward a Memory Device Concept

Ferroelectric vortex has attracted much attention as a promising candidate for memories with high density and high stability. It is a crucial problem to precisely manipulate the vortex chirality in order to utilize it to store information. Nevertheless, so far, a practical and direct strategy for vortex switching is still lacking. Moreover, the strong coupling of chirality between neighboring vortices in continuous systems like superlattices limits the application of ferroelectric-vortex-based memories. Here, we design a ferroelectric nanoplate junction to break the strong coupling between neighboring vortices. Phase-field simulation results demonstrate that the vortex chirality of the nanoplates could be efficiently tuned by sweeping local electric and thermal fields in the nanoplate junction. More importantly, the weak coupling between two neighboring nanoplates through the intermediate junction brings a deterministic vortex switching behavior. Based on this, we propose a concept of vortex memory devices. Our study provides an effective way to control the vortex chirality and suggests an opportunity for designing new memory devices based on ferroelectric vortex.

Electric field control of chirality.

Polar textures have attracted substantial attention in recent years as a promising analog to spin-based textures in ferromagnets. Here, using optical second-harmonic generation–based circular dichroism, we demonstrate deterministic and reversible control of chirality over mesoscale regions in ferroelectric vortices using an applied electric field. The microscopic origins of the chirality, the pathway during the switching, and the mechanism for electric field control are described theoretically via phase-field modeling and second-principles simulations, and experimentally by examination of the microscopic response of the vortices under an applied field. The emergence of chirality from the combination of nonchiral materials and subsequent control of the handedness with an electric field has far-reaching implications for new electronics based on chirality as a field-controllable order parameter.

Control of the chirality of a vortex in a ferroelectric nanodot by uniform electric fields mediated by inhomogeneous surface screening

The manipulation of a ferroelectric vortex under the boundary condition of inhomogeneous screening is simulated by phase-field calculations. It is revealed that the vortex chirality can be tuned by utilizing uniform electric fields under inhomogeneous screening boundary conditions in which the switching mechanism is distinct from that under boundary conditions of asymmetric screening previously reported. The influence of the screening inhomogeneity and the ambient temperature on the vortex switching is further studied. The results indicate that inhomogeneous screening conditions are of vital importance to the switching behaviors of vortex chirality in a ferroelectric nanodot under the influence of a uniform electric field. Furthermore, the critical electric field is inextricably linked to the screening inhomogeneity and the ambient temperature.