Ferroelectric Domain Wall Motion Research Articles

Magnetic Phase Transition-Induced Modulation of Ferroelectric Properties in Hexagonal RFeO3 (R = Tb and Ho).

Hexagonal rare-earth iron oxides (h-RFeO3) exhibit spontaneous magnetization and room-temperature ferroelectricity simultaneously. However, achieving a large magnetoelectric coupling necessitates further exploration. Herein, we report the impact of the magnetic phase transition on the ferroelectric properties of epitaxial h-RFeO3 (R = Tb and Ho) films prepared by pulsed laser deposition. The metastable h-RFeO3 phase is successfully stabilized with high crystallinity and low leakage current due to the ITO buffer layer, making it possible to investigate the ferroelectric properties. The h-TbFeO3 film exhibits a magnetic-field-induced transition from antiferromagnetic (AFM) to weak ferromagnetic (wFM) phases below 30 K, while also exhibiting ferroelectricity at 300 K. The dielectric constants change with the magnetic phase transition, demonstrating hysteresis in the magnetocapacitance. In contrast, the h-HoFeO3 film exhibits antiferroelectric-like behavior and an AFM-wFM phase transition. Notably, the h-HoFeO3 film shows a rapid increase in the remnant polarization during the AFM-wFM phase transition accompanied by an increase in the ferroelectric component. Considering the strong connection between the antiferroelectric behavior in the h-RFeO3 system and the ferroelectric domain wall motion, this considerable modification of ferroelectric properties during the magnetic phase transition is probably due to the faster movement of the ferroelectric domain walls in the wFM phase induced by the clamping effect. Our findings indicate the effectiveness of magnetic phase transitions in enhancing the magnetoelectric coupling, particularly when utilizing domain wall clamping properties.

Microscopic insights on field induced switching and domain wall motion in orthorhombic ferroelectrics

Microscopic insights on field induced switching and domain wall motion in orthorhombic ferroelectrics

Concentration-driving pinning effect in lead-free Mn-substituted BCZT ferroelectric ceramics

Domain wall motion in ferroelectrics, closely related to electric characteristics and amenable to tuning through doping, remains incompletely understood, particularly with regard to the pinning defect in the (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT) system, which has attracted significant interest. Here, we investigate the defect dipole-assisted pinning effect in Mn-substituted BCZT (abbreviated as BCZT-xMn) ceramics with Mn concentrations ranging from x = 0 to 0.04. As x increases, we observe a gradual decrease in the maximum permittivity, accompanied by a leftward shift in temperature, leading to a diffuse phase transition. Analysis of the dielectric spectrum reveals a consistent trend in the activation energy within a specific concentration range, suggesting the incorporation of oxygen vacancies to maintain electric neutrality. The resulting oxygen vacancies, combined with Mn dopants, form defect dipoles Mn4++e′− VO⋅⋅ −Mn4++e′, attaining sufficient concentration at x = 0.02, and enabling precise establishment of the pinning effect. This phenomenon is extensively documented by the observation of pinched polarization-electric field loops and double current peaks in the current-electric field curves, visually corroborated by experimental data and a 3D phase-field simulation model. Consequently, this study not only elucidates the concentration-driven pinning effect in the BCZT system but also provides valuable insights into doping effects in other lead-free ferroelectrics.

Light-driven motion of charged domain walls in isolated ferroelectrics

Light-induced ferroelectric domain wall motion turns out to be a promising phenomenon to develop new photocontrolled devices. However, the physical origin of this light-matter coupling when material is irradiated with visible light remains unclear. Here, a phenomenological model predicting the motion of charged domain walls (CDWs) is developed. The photoinduced electronic reconstruction mechanism is proposed as the primary absorption mechanism, leading to a linear dependence for the polarization perturbation with the light intensity. Domain wall motion is then driven by the energetic difference between domains in a CDW array, such that the macroscopic polarization can be easily tuned.

Structures and velocities of noisy ferroelectric domain walls

Ferroelectric domain wall motion is fundamental to the switching properties of ferroelectric devices and is influenced by a wide range of factors including spatial disorder within the material and thermal noise. We build a Landau-Ginzburg-Devonshire (LGD) model of ${180}^{\ensuremath{\circ}}$ ferroelectric domain wall motion that explicitly takes into account the presence of both spatial disorder and thermal noise. We demonstrate both creep flow and linear flow regimes of the domain wall dynamics by solving the LGD equations in a Galilean frame moving with the wall velocity $v$. Thermal noise plays a key role in the wall depinning process at small fields $E$. We study the scaling of the velocity $v$ with the applied DC electric field $E$ and show that noise and spatial disorder strongly affect domain wall velocities. We also show that domain walls develop a local, metastable paraelectric region and widen significantly in the presence of thermal noise in materials with ``multiwell'' potentials, representative of ferroelectrics at temperatures $T$ just below a first-order phase transition (Curie) temperature ${T}_{c}$. These calculations point to the potential of noise and disorder to become control factors for the switching properties of ferroelectric materials, for example for advancement of microelectronic applications.

Structural, magnetic, electrical and optical properties of Aurivillius phase Bi6Fe1.5Co0.5Ti3-W O18 (0 ≤ x ≤ 0.07) ceramics

In this paper, we investigated the structural, multiferroic, dielectric and optical properties of Aurivillius Bi6Fe1.5Co0.5Ti3-xWxO18 (0 ≤ x ≤ 0.07) ceramics. The doping of W6+ ions promoted the grain’s growth and induced c-axis orientation. The remnant magnetization Mr monotonously decreased from 0.49 to 0.13 emu/g with increasing the doping level from 0 to 0.07, which was primarily attributed to the reduced Fe2+-O-Fe3+ exchange and the improved orientation growth. The remnant polarization Pr increased from 4.5μC/cm2 for x = 0–8.3μC/cm2 for x = 0.05 and then decreased to 7.2μC/cm2 for x = 0.07, the enhanced ferroelectricity was mainly related to the suppressed oxygen vacancy and larger grain size, while the weakened ferroelectricity was ascribed to the improved c-axis orientation. The Mr (0.23 emu/g) along c axis was larger 0.13 emu/g in ab plane while the Pr (7.2μC/cm2) along c axis was smaller than 9.3μC/cm2 in ab plane in x = 0.07, which demonstrated the anistropy of multiferroicity. In addition, all samples exhibited two different dielectric relaxations. The first one with activation energy ranging from 0.47 to 0.53 eV was assigned to the hopping process of electron, while the second one with activation energy varied from 1.45 to 3.63 eV was associated with the viscous motion of ferroelectric domain walls. Two direct absorption edges were demonstrated by Tauc’s law in all samples. The band gap 1 increased from 1.93 to 1.95 eV due to the lower t2 g state of Fe2+. The band gap 2 varied between 2.21 and 2.28 eV, which was proposed to the competition between TiO6 distortion and Ti4+ content. These results indicate that doping donor W6+ in Aurivillius compounds is a feasible way to regulate the multiferroic and optical properties.

Domain Wall Acceleration by Ultrafast Field Application: An Ab Initio‐Based Molecular Dynamics Study

Optimizing ferroelectrics for contemporary high‐frequency applications asks for the fundamental understanding of ferroelectric switching and domain wall (DW) motion in ultrafast field pulses while the microscopic understanding of the latter is so far incomplete. To close this gap in knowledge, ab initio‐based molecular dynamics simulations are utilized to analyze the dynamics of 180° DWs in the prototypical ferroelectric material BaTiO3. How ultrafast field application initially excites the dipoles in the system and how they relax to their steady state via transient negative capacitance are discussed. Excitingly, a giant boost of the DW velocity related to the nonequilibrium switching of local dipoles acting as nucleation centers for the wall movement is found. This boost may allow to tune the local ferroelectric switching rate by the shape of an applied field pulse.

Dynamic response and roughening of ferroelectric domain walls driven at planar electrode edges

Understanding and controlling the motion, stability, and equilibrium configuration of ferroelectric domain walls is key for their integration into potential nanoelectronic applications, such as ferroelectric racetrack memories. Using piezoresponse force microscopy, we analyze the growth and roughness of ferroelectric domains in epitaxial thin film Pb(Zr0.2Ti0.8)O3, driven by the electric fields at straight edges of planar electrodes at two different temperatures. This device relevant geometry allows us to confirm that the domain walls are well described as one-dimensional monoaffine elastic interfaces driven in random-bond disorder. However, we observe a progressive increase in roughness as initially flat domain walls move through the disorder landscape, which could prove a significant limiting factor for racetrack-type memories using ferroelectrics.

Unexpectedly low barrier of ferroelectric switching in HfO2 via topological domain walls

Fluorite-structure ferroelectrics — in particular the orthorhombic phase of HfO2 — are of paramount interest to academia and industry because they show unprecedented scalability down to 1-nm-thick size and are compatible with Si electronics. However, their polarization switching is believed to be limited by the intrinsically high energy barrier of ferroelectric domain wall (DW) motions. Here, by unveiling a new topological class of DWs, we establish an atomic-scale mechanism of polarization switching in orthorhombic HfO2 that exhibits unexpectedly low energy barriers of DW motion (up to 35-fold lower than given by previous conjectures). These findings demonstrate that the nucleation-and-growth-based mechanism is feasible, challenging the commonly held view that the rapid growth of the oppositely polarized domain is impossible. Building on this insight, we describe a strategy to substantially reduce the coercive fields in HfO2-based ferroelectric devices. Our work is a crucial step towards understanding the polarization switching of HfO2, which could provide a means to solve the key problems associated with operation speed and endurance.

Electric-Field Control of Propagating Spin Waves by Ferroelectric Domain-Wall Motion in a Multiferroic Heterostructure.

Magnetoelectric coupling in multiferroic heterostructures offers a promising platform for electric-field control of magnonic devices based on low-power spin-wave transport. Here, electric-field manipulation of the amplitude and phase of propagating spin waves in a ferromagnetic Fe film on top of a ferroelectric BaTiO3 substrate is demonstrated experimentally. Electric-field effects in this composite material system are mediated by strain coupling between alternating ferroelectric stripe domains with in-plane and perpendicular polarization and fully correlated magnetic anisotropy domains with differing spin-wave transport properties. The propagation of spin waves across the strain-induced magnetic anisotropy domains of the Fe film is directly imaged and it is shown how reversible electric-field-driven motion of ferroelectric domain walls and pinned anisotropy boundaries turns the spin-wave signal on and off. Furthermore, linear electric-field tuning of the spin-wave phase by altering the width of strain-coupled stripe domains is demonstrated. The results provide a new route toward energy-efficient reconfigurablemagnonics.

Direct observation of domain wall motion and lattice strain dynamics in ferroelectrics under high-power resonance

Domain wall motion and lattice strain dynamics of ferroelectrics at resonance were simultaneously measured by combining high-power burst excitation and in situ high-energy x-ray diffraction. The increased loss at high vibration velocity was directly related to the increased domain wall motion, driven by dynamic mechanical stress. A general relationship between the microstructural strain contributions and macroscopic electromechanical behavior was established, allowing the prediction of high-power stability of ferroelectric materials. The results indicate that the materials' stability during high-power drive is predominantly related to the basic chemical composition, while the piezoelectric hardening mechanisms mainly influence the small-signal behavior.

Manipulating ferroelectric behaviors via electron-beam induced crystalline defects.

Ferroelectric nanoplates are attractive for applications in nanoelectronic devices. Defect engineering has been an effective way to control and manipulate ferroelectric properties in nanoscale devices. Defects can act as pinning centers for ferroelectric domain wall motion, altering the switching properties and domain dynamics of ferroelectrics. However, there is a lack of detailed investigation on the interactions between defects and domain walls in ferroelectric nanoplates due to the limitation of previous characterization techniques, which impedes the development of defect engineering in ferroelectric nanodevices. In this study, we applied in situ biasing transmission electron microscopy to explore how dislocation loops, which were judiciously introduced into barium titanate nanoplates via electron beam irradiation, affect the motion of ferroelectric domain walls. The results show that the motion was dramatically suppressed by these localized defects, because of the local strain fields induced by the defects. The pinning effect can be further enhanced by multiple domain walls embedded with defect arrays. These results indicate the possibility of manipulating domain switching in ferroelectric nanoplates via the electron beam.

Effect of ferroelectric domain walls on the dielectric properties of PbZrO3 thin films

In antiferroelectric PbZrO3 thin films, a weak residual ferroelectric phase is often observed on the double hysteresis loop and it is important to know its impact on the dielectric properties. To study this residual phase, a low and homogeneous electric field can be used because antiferroelectric domain walls are not sensitive to homogeneous fields; thus, contributions of ferroelectric domain wall motions to permittivity and dielectric losses can be isolated. In this paper, the hyperbolic law characterization is used on lead zirconate thin films, which present a residual ferroelectric phase. The study shows that domain wall contributions of the ferroelectric phase are small (less than 2% of the total permittivity), but their impacts are very important in the overall dielectric losses (≈26%). These losses are, however, lower than those obtained in pure ferroelectric materials due to a residual state composed of well distributed ferroelectric clusters of small size with no interactions between domain walls.

Dynamic dielectric-response model of flexoelectric polarization from kHz to MHz range in an ordered assembly of BaTiO3 nanocubes

Due to the strain gradient near each surface of a BaTiO3 nanocube in their ordered assembly, electric polarization appears due to flexoelectric effect. The magnitude of the flexoelectric polarization could be one order of magnitude larger than that of ferroelectric spontaneous polarization of BaTiO3. Thus, dielectric response of an assembly could be dominated by that of the flexoelectric polarization if there is no ferroelectric domain-wall motion. Numerical simulations of the dielectric response of a BaTiO3 nanocube in an ordered assembly are performed from kHz to MHz range based on a dynamic model of flexoelectric polarization assuming anharmonic potential. The calculated temperature dependence of the dielectric constant is consistent with the experimental data of high dielectric constant with nearly-flat temperature dependence. It is suggested that high dielectric constant with nearly-flat temperature dependence is not originated in ferroelectric nature of BaTiO3 nanocubes but originated in flexoelectric polarization in nanocubes which is also seen in non-ferroelectric materials.

The atomic-scale domain wall structure and motion in HfO2-based ferroelectrics: A first-principle study

The HfO2-based ferroelectrics have aroused considerable attention due to their potential application in silicon process-compatible memory devices. However, the ferroelectricity origin and the domain evolution have not yet been well understood. It is now generally accepted the orthorhombic Pca21 phase is one of ferroelectric phases for HfO2. In this work, by performing density functional theory calculations, we systematically studied the domain wall structures and evolution based on the Pca21 ferroelectric phase. More specifically, the atomic structures of ten types of possible 180° and 90° ferroelectric domain walls are predicted and explored. And the motion of certain domain walls is expected as the microscopic mechanisms of the polarization switching and ferroelectricity activation of Pca21 HfO2 under external electric field. Our results are in good agreement with the recent experimental results on ferroelectric domain walls in HfO2-based epitaxial thin-film and are helpful to understand the ferroelectricity origin of the HfO2-based ferroelectrics.

Modeling the Motion of Ferroelectric Domain Walls with the Classical Stefan Problem

With advances in nanotechnology, ferroelectric switching by individual domain walls (DWs) has become a subject of broad interest. Conventional models consider DW motion in a fixed homogeneous or inhomogeneous electric field. However, it is clear that the electric field commonly evolves in time due to the redistribution of bound charges and screening free charges on the ferroelectric surface, particularly due to surface conductance. Taking this effect into account remains a serious challenge. Here we propose a simple concept to describe simultaneously the evolution of the electric field and the DW motion in a ferroelectric sample. The approach is based on a full analogy between charge transport during ferroelectric switching and heat transport in a moving melting front: the classical Stefan problem. The analogy helps in the establishment of control of DW motion in thin films. Experimentally, DWs are displaced by voltage pulses under electrodes with a limited conductivity and detected using piezoresponse force microscopy. In the frame of a combined theoretical and experimental approach, DW-friction mechanisms and the impact of the DW energy on its shape are discussed. The study facilitates the development of nanoelectronic devices.

Dynamical Magnetic Field Accompanying the Motion of Ferroelectric Domain Walls.

The recently proposed dynamical multiferroic effect describes the generation of magnetization from temporally varying electric polarization. Here, we show that the effect can lead to a magnetic field at moving ferroelectric domain walls, where the rearrangement of ions corresponds to a rotation of ferroelectric polarization in time. We develop an expression for the dynamical magnetic field, and calculate the relevant parameters for the example of 90° and 180° domain walls, as well as for polar skyrmions, in BaTiO_{3}, using a combination of density functional theory and phenomenological modeling. We find that the magnetic field reaches the order of several μT at the center of the wall, and we propose two experiments to measure the effect with nitrogen-vacancy center magnetometry.

Effects of Nd-doping on multiferroic properties of Bi<sub>6−x</sub>Nd<sub>x</sub>Fe<sub>1.4</sub>Ni<sub>0.6</sub>Ti<sub>3</sub>O<sub>18</sub> polycrystalline

Single phase polycrystalline Nd-modified BNFNT-<i>x</i> series samples are obtained from the precursors of the same chemical formula, and prepared by using the citric acid-nitrate method. The X-ray photoelectron spectroscopy measurement indicates that a slight Nd modification does not exert significant influence on the stability of the octahedral FeO<sub>6</sub>, nor NiO<sub>6</sub> nor TiO<sub>6</sub>. When the molar concentration of Nd exceeds 0.25, the stability of BiO layer is cemented and conducive to the insulating role of BiO layer. It is seen that a small quantity of Nd substitution for bismuth can improve the ferroelectric polarization (2<i>P</i><sub>r</sub>) of ～ 19.7 <inline-formula><tex-math id="Z-20190127014418-3">\begin{document}$ \mu {\rm C/cm }^2$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20181287-e-lijj-revised_Z-20190127014418-3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20181287-e-lijj-revised_Z-20190127014418-3.png"/></alternatives></inline-formula>. The room-temperature magnetization (2<i>M</i><sub>s</sub>) can reach a maximal value of ～ 4.132 emu/g (1 emu/g = 10<sup>−3</sup> A·m<sup>2</sup>/g)in the BNFNT-0.20 sample. Two anomalies are observed in the temperature-dependent dielectric loss spectrum: one is situated in the temperature range from 200 K to 400 K and the other is located in the vicinity of 900 K. It is considered that the loss anomaly found near 900 K might be associated with the viscous motion of ferroelectric domain walls. In addition, the loss peak shown in a temperature range from 200 K to 400 K shifts toward the higher temperature with measuring frequency increasing, indicating the characteristics of dielectric relaxor behavior. The activation energy is evaluated to be 0.287−0.366 eV, which suggests that the relaxor is associated with the electrons transfer and hop between Fe<sup>3+</sup> and Fe<sup>2+</sup>. The room-temperature magnetization (2<i>M</i><sub>s</sub>) has reached a maximal value of ～ 4.132 emu/g in the BNFNT-0.20 sample. The lattice distortion due to the introduction of Nd changes the angle of such antiferromagnetic coupling bonds as Fe<sup>3+</sup>—O—Fe<sup>3+</sup>, Fe<sup>3+</sup>—O—Ni<sup>3+</sup> and Ni<sup>3+</sup>—O—Ni<sup>3+</sup>, which leads the AFM spin states to break, and thus increases the magnetic properties. While with further modification of Nd, the drastic lattice distortion reduces the occupation of the B-sites of the magnetic ions, which might be responsible for further deteriorating the magnetic properties.

Discovering minimum energy pathways via distortion symmetry groups

Physical systems evolve from one state to another along paths of least energy barrier. Without a priori knowledge of the energy landscape, multidimensional search methods aim to find such minimum energy pathways between the initial and final states of a kinetic process. However in many cases, the user has to repeatedly provide initial guess paths, thus ensuring that the reliability of the final result is heavily user-dependent. Recently, the idea of "distortion symmetry groups" as a complete description of the symmetry of a path has been introduced. Through this, a new framework is enabled that provides a powerful means of classifying the infinite collection of possible pathways into a finite number of symmetry equivalent subsets, and then exploring each of these subsets systematically using rigorous group theoretical methods. The method is shown to lead to the discovery of new, previously hidden pathways for the case studies of bulk ferroelectric switching and domain wall motion in proper and improper ferroelectrics, as well as in multiferroic switching. This approach is applicable to a wide variety of physical phenomena ranging from structural, electronic and magnetic distortions, diffusion, and phase transitions in materials.

Strain generation and energy-conversion mechanisms in lead-based and lead-free piezoceramics

Abstract