Equations for domain walls in a coordinate system of the ferroelectric phase

Two methods are suggested for writing equations for domain walls in a coordinate system of the ferroelectric phase in ferroelastics and multiaxial ferroelectrics. The equations for domain walls in ferroelectric barium titanate and ferroelastic lead orthophosphate are derived. It is shown that suborientation states are possible in these crystals. The suggested methods make it possible to find the matrices of the transformation from the coordinate system of the paraelectric phase to a coordinate system of the ferroelectric phase for each orientation state.     

Fractal dimension and size scaling of domains in thin films of multiferroic BiFeO3

Domains in ferroelectric films are usually smooth, stripelike, very thin compared with magnetic ones, and satisfy the Landau-Lifshitz-Kittel scaling law (width proportional to square root of film thickness). However, the ferroelectric domains in very thin films of multiferroic BiFeO3 have irregular domain walls characterized by a roughness exponent 0.5-0.6 and in-plane fractal Hausdorff dimension H||=1.4+/-0.1, and the domain size scales with an exponent 0.59+/-0.08 rather than 1/2. The domains are significantly larger than those of other ferroelectrics of the same thickness, and closer in size to those of magnetic materials, which is consistent with a strong magnetoelectric coupling at the walls. A general model is proposed for ferroelectrics, ferroelastics or ferromagnetic domains which relates the fractal dimension of the walls to domain size scaling.     

On the mechanism of the photodomain effect in ferroelectrics

The influence of nonequilibrium electrons on the domain structure and switching of ferroelectrics (photodomain effect) has been investigated in ferroelectric crystals and ceramics. In the present paper, this effect is observed and explained as a result of the domain walls screening in frame ofYshibashi-Takagi theory.     

Conduction of topologically protected charged ferroelectric domain walls

We report on the observation of nanoscale conduction at ferroelectric domain walls in hexagonal HoMnO(3) protected by the topology of multiferroic vortices using in situ conductive atomic force microscopy, piezoresponse force microscopy, and Kelvin-probe force microscopy at low temperatures. In addition to previously observed Schottky-like rectification at low bias [Phys. Rev. Lett. 104, 217601 (2010)], conductance spectra reveal that negatively charged tail-to-tail walls exhibit enhanced conduction at high forward bias, while positively charged head-to-head walls exhibit suppressed conduction at high reverse bias. Our results pave the way for understanding the semiconducting properties of the domains and domain walls in small-gap ferroelectrics.     

Effect of epitaxial strain on the spontaneous polarization of thin film ferroelectrics

Epitaxial strain can substantially enhance the spontaneous polarizations and Curie temperatures of ferroelectric thin films compared to the corresponding bulk materials. In this Letter we use first principles calculations to calculate the effect of epitaxial strain on the spontaneous polarization of the ferroelectrics , , and , and the multiferroic material . We show that the epitaxial strain dependence of the polarization varies considerably for the different systems, and in some cases is, in fact, very small. We discuss possible reasons for this different behavior and show that the effect of epitaxial strain can easily be understood in terms of the piezoelectric and elastic constants of the unstrained materials. Our results provide a computational tool for the quantitative prediction of strain behavior in ferroelectric thin films.     

Permissible domain walls in ferroelectric species

A previously developed method makes it possible to determine the orientations of domain walls in ferroelectrics which are permissible under the condition that neighbouring domains must be mechanically compatible. In this paper, the method has been applied to all 88 ferroelectric species in which the polarization is the parameter of the transition. Types of permissible walls and their orientations are listed in form of tables. Cases where no walls fulfil the compatibility conditions are shown.     

Diffuse ferroelectric phase transitions in cubically sabilized perovskites

Typical examples of ferroelectrics with diffuse phase transitions (relaxor ferroelectrics), like Pb(Mg_1/3Nb_2/3)O₃ are actually non transforming. The paraelectric phase is fully stablized against a ferroelectric phase transition in this case. A phase transition can be induced, however, by an electric field with appropriate orientation below the temperature of the dielectric constant maximum. The analogy with stress-induced martensitic phase transitions in metallic alloys is pointed out. Pecularities of the properties and of the polarization reversal of such systems are demonstrated. Actual diffuse ferroelectric phase transitions in disordered solid solutions and mixed compounds with a partially stabilized parent phase are compared with athermal martensitic transformations. With particular regard to technical ceramics based on PZT, the influence of interfaces between transformed regions and remnants of the parent phase which have to distinguished from domain walls, and of the reduced stability of the ferroelectric phase on the properties of these systems is discussed. Some effects usually explained solely by domain processes may be understood also from this point of view.

Stabilization of a parent phase against an order-disorder-type phase transition is supposed to be caused by glass-like freezing caused by inelastic cooperative interactions between disordered molecular groups.     

Ferroelectricity in spiral magnets

It was recently observed that the ferroelectrics showing the strongest sensitivity to an applied magnetic field are spiral magnets. We present a phenomenological theory of inhomogeneous ferroelectric magnets, which describes their thermodynamics and magnetic field behavior, e.g., dielectric susceptibility anomalies at magnetic transitions and sudden flops of electric polarization in an applied magnetic field. We show that electric polarization can also be induced at domain walls and that magnetic vortices carry electric charge.     

铁电体的光伏效应

介绍了铁电光伏效应的发展历史和现状,通过与传统半导体p-n结光伏器件比较,旨在阐述铁电光伏器件非比寻常的优点和重要的应用前景.铁电光伏效应分为体光伏效应和反常光伏效应,多种物理机制已被发现,无疑为铁电光伏效应的提高指明了方向.还对钙钛矿氧化物、卤化物和双钙钛矿结构氧化物等铁电体中的光伏效应进行了阐述,讨论了通过引入新的自由度实现多功能性光伏器件的可能性.     

Ferroelectric domain structure of SrBi2Nb2O9 epitaxial thin films

The domain structure in a ferroelectric with well-defined crystallography and negligible ferroelastic distortion (<0.002%) is reported. In contrast to prototypical ferroelectrics in which long-range elastic strain dictates the domain structure, in SrBi2Nb2O9 the elastic term is insignificant, allowing dipole-dipole interactions and domain wall energies to dominate in determining the domain structure. Electron microscopy reveals ferroelectric domains that are irregularly shaped and highly curved. Out-of-phase boundary defects are shown to be weakly correlated with 90 degrees ferroelectric domain structure.     

On the helical domain structure of the liquid ferroelectrics

The case when a nematic liquid crystalline phase is ferroelectric is considered. Such “liquid ferroelectric” is, surely, instable and transforms into a helical cholesteric-type structure which for this case is a variety of the ferroeltric domain structure. One can think that some cholesteric phases are liquid ferroelectrics with the helical domain structure.     

Domain wall roughness in epitaxial ferroelectric PbZr0.2Ti0.8O3 thin films

The static configuration of ferroelectric domain walls was investigated using atomic force microscopy on epitaxial PbZr(0.2)Ti(0.8)O(3) thin films. Measurements of domain wall roughness reveal a power-law growth of the correlation function of relative displacements B(L) alpha L(2zeta) with zeta approximately 0.26 at short length scales L, followed by an apparent saturation at large L. In the same films, the dynamic exponent mu was found to be approximately 0.6 from independent measurements of domain wall creep. These results give an effective domain wall dimensionality of d = 2.5, in good agreement with theoretical calculations for a two-dimensional elastic interface in the presence of random-bond disorder and long-range dipolar interactions.     

Giant flexoelectric effect in ferroelectric epitaxial thin films

We report on nanoscale strain gradients in ferroelectric HoMnO(3) epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.     

Abrupt appearance of the domain pattern and fatigue of thin ferroelectric films

We study the domain structure in ferroelectric thin films with a "passive" (nonferroelectric) layer at the interface between the film and electrodes. An abrupt transition from a monodomain to a polydomain state has been found with the increase of the passive layer thickness d. The domain width changes very quickly at the transition (exponentially with d(-2)). The slope of the hysteresis loop is in agreement with experiment, assuming realistic parameters of the layer. The slope scales as 1/d, involving only the properties of the layer. We believe that specific properties of the domain structure in ferroelectrics with a passive layer can resolve the long-standing "paradox of the coercive field."     

强电场作用下反铁电锆钛酸铅的介电行为研究

采用电滞回线方法和偏置直流电场中叠加小交变电场方法研究了锆钛酸铅反铁电陶瓷材料在强电场作用下的介电行为.测量结果显示，锆钛酸铅反铁电材料的介电常数随外加电场强度呈非线性变化，在反铁电 铁电转变的电场区间形成介电峰.表征极化强度随电场强度变化率的微分介电常数εd峰值出现在反铁电 铁电转换电场强度处，最高达到41000.随着偏置电场增加反铁电向铁电体转变过程中，小信号介电常数εc减小；在电场降低铁电回复成反铁电过程中，小信号介电常数εc增大，小信号介电常数εc峰先于微分介电常数εd峰出现.根据电场作用下反铁电 关键词： 锆钛酸铅反铁电陶瓷 介电行为 强电场条件     

Shedding light on nanoscale ferroelectrics

Light matter interactions in nanoscale ferroelectric materials have received growing interest over the last years due to new developments in scientific instrumentation and novel materials that allow for the study of so far scarcely investigated and/or hidden nanoscale phenomena. Here we provide an overview of these developments and investigations that have been carried out over a broad range of frequencies spanning from the X-ray to the THz regime. Of interest are the physical quantities and fingerprints stemming from nanoscale regions in ferroelectrics, including domain walls, upon light illumination.     

存储器用铁电薄膜及电极材料的研究

铁电体独特的自发电极化双稳性质和非线性光学性质使其在光电子器件中得到广泛应用．为了实现器件的小型化和与微电子、光电子工艺兼容，铁电薄膜已成为一个研究热点．自发电极化的大小和取向以及外电场、缺陷和铁电薄膜／电极界面与自发电极化的交互作用决定了铁电薄膜的性质和服役行为．文章以铁电存储器和光电子器件应用为背景，选择了具有重大应用前景的Bi4-xLaxTi3O12（BLT）、SrBi2Ta2O9（SBT）、PbZrxTi1-xO3（PET）和LiNbO3（LN）铁电薄膜以及相关的La（Sr，Co）O3（LSCO）和LaNiO3（LNO）等电极材料为研究对象，研究了缺陷电荷和电畴的交互作用和它们在交变外电场中的动力学行为，探明了铁电薄膜疲劳现象的物理本质；从晶格结构与缺陷的观察研究入手，探索了材料铁电性质的起源和优化材料铁电性质的途径；从铁电薄膜／电极界面结构与性质的研究入手，寻找更有效、更稳定的电极材料与结构，从而为器件应用打下了基础；在研究外电场对铁电薄膜生长机制影响的基础上，找到了利用外电场调控铁电薄膜结构的新途径，发展了新的、与半导体器件和光电子器件工艺兼容的制膜方法．     

Epitaxially strained SnTiO_3 at finite temperatures

By combining the effective Hamiltonian approach and direct ab initio computation, we obtain the phase diagram of SnTiO_3 with respect to epitaxial strain and temperature. This demonstrates the complex features of the phase diagram and provides an insight into this system, which is a presumably simple perovskite. Two triple points, as shown in the phase diagram, may be exploited to achieve high-performance piezoelectric effects. Despite the inclusion of the degree of freedom related to oxygen octahedron tilting, the ferroelectric displacements dominate the structural phases over the whole misfit strain range. Finally, we show that SnTiO_3 can change from hard to soft ferroelectrics with the epitaxial strain.     

Relaxor ferroelectric behavior in high-energy electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymers

This paper presents experimental results showing that in a certain composition range and under a proper electron-irradiation treatment, a normal ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer can be converted into a material exhibiting many typical features of relaxor ferroelectrics, suggesting that this is a new class of relaxor ferroelectric material. Furthermore, the irradiated copolymer can generate giant electrostriction (ƽ%) with a high elastic energy density. The X-ray diffraction results obtained from the irradiated copolymer under electric field, indicate that the observed polarization and strain responses are mainly due to the local phase transformation from a non-polar phase to a polar phase.     

Slow polarization processes in relaxor ferroelectrics

The general features of the slow polarization kinetics in relaxor ferroelectrics, observed and studied by us in the specific example of crystals of barium-strontium niobate (SBN) solid solutions of various compositions and of lead magnesium niobate (PMN), are considered. The dielectric hysteresis loops and spectra of the polarization relaxation time distribution in quasi-static and static electric fields were found to reveal characteristic anomalies related to a random internal electric field in the bulk of a relaxor ferroelectric. Such a field caused by structural disorder accounts for the anomalously broad spectrum of potential barriers for domain walls. The part played by free charge carriers in the formation of giant barriers is demonstrated. The paper presents some quantitative data characterizing the specific features of the structure and physical properties of the relaxors.