Polarization switching in perforated ferroelectric films

The polarization switching in ferroelectric films of barium strontium titanate with a planar structure of interdigitated electrodes has been studied experimentally. The local polarization distribution in an interelectrode gap of these films in an applied electric field has been investigated using nonlinear optical microscopy. The polarization switching parameters of unperforated and perforated films have been compared.     

Thermodynamic,optical and switching parameters of a ferroelectric liquid crystalline material having SmA*-SmC*-SmBh* phase sequence

Thermodynamic and electro-optical characterization of a ferroelectric liquid crystalline material, namely ((S) (+) 4-(1-methylheptyloxy) phenyl 4′-octyloxybiphenyl-4-carboxylate) possessing paraelectric SmA*, ferroelectric SmC*, hexatic SmB_h* and SmI* phases has been carried out. Phase identification has been done by optical and thermodynamic studies. Switching parameters viz. spontaneous polarization, switching time and rotational viscosity have been determined. The spontaneous polarization has been found to increase with decreasing temperature in SmC* phase. The switching time is found of the order of few milliseconds.     

Investigation of a ferroelectric/manganite heterostructure by second optical harmonic generation

Metal/ferroelectric/manganite tunnel structures and ferroelectric/manganite heterostructures connected in series via a manganite in the conductive phase were investigated. It was shown that the source of optical nonlinearity is a current arising in a tunnel structure due to the tunneling of electrons via a ferroelectric barrier. The resulting current flowing in the manganite of the heterostructure breaks the central symmetry of the manganite and/or causes ferroelectric polarization switching.     

Tuning band alignment and optical properties of 2D van der Waals heterostructure via ferroelectric polarization switching

Favourable band alignment and excellent visible light response are vital for photochemical water splitting. In this work, we have theoretically investigated how ferroelectric polarization and its reversibility in direction can be utilized to modulate the band alignment and optical absorption properties. For this objective, 2D van der Waals heterostructures (HTSs) are constructed by interfacing monolayer MoS₂ with ferroelectric In2Se3. We find the switch of polarization direction has dramatically changed the band alignment, thus facilitating different type of reactions. In In₂Se₃/MoS₂/In₂Se₃ heterostructures, one polarization direction supports hydrogen evolution reaction and another polarization direction can favour oxygen evolution reaction. These can be used to create tuneable photocatalyst materials where water reduction reactions can be selectively controlled by polarization switching. The modulation of band alignment is attributed to the shift of reaction potential caused by spontaneous polarization. Additionally, the formed type-II van der Waals HTSs also significantly improve charge separation and enhance the optical absorption in the visible and infrared regions. Our results pave a way in the design of van der Waals HTSs for water splitting using ferroelectric materials.     

Investigation of the fractal features of the polarization reversal processes in ferroelectric crystals under injection conditions

The features of polarization reversal processes of ferroelectric crystals are analyzed under the condition that electron beams of a scanning electron microscope are injected into the subsurface layer of a sample. The proposed mathematical model describes the dynamics of the polarization switching of a ferroelectric crystal using the injection contact technique. The simulation relies on the basic principles of scale invariance in domain structure evolution. The polarization switching current of ferroelectric crystals is investigated as the dynamic characteristic of a finite medium exhibiting fractal behavior. The application of fractal and multifractal analysis of a time series enables us to calculate the fractal dimension of the polarization reversal process under injection conditions.     

Three‐Dimensional,Time‐Resolved Profiling of Ferroelectric Domain Wall Dynamics by Spectral‐Domain Optical Coherence Tomography

We apply here spectral‐domain optical coherence tomography (SD‐OCT) for the precise detection and temporal tracking of ferroelectric domain walls (DWs) in magnesium‐doped periodically poled lithium niobate (Mg:PPLN). We reproducibly map static DWs at an axial (depth) resolution down to ～ 0.6 μm, being located up to 0.5 mm well inside the single crystalline Mg:PPLN sample. We show that a full 3‐dimensional (3D) reconstruction of the DW geometry is possible from the collected data, when applying a special algorithm that accounts for the nonlinear optical dispersion of the material. Our OCT investigation provides valuable reference information on the DWs’ polarization charge distribution, which is known to be the key to the electrical conductivity of ferroelectric DWs in such systems. Hence, we carefully analyze the SD‐OCT signal dependence both when varying the direction of incident polarization, and when applying electrical fields along the polar axis. Surprisingly, the large backreflection intensities recorded under extraordinary polarization are not affected by any electrical field, at least for field strengths below the switching threshold, while no significant signals above noise floor are detected under ordinary polarization. Finally, we employed the high‐speed SD‐OCT setup for the real‐time DW tracking upon ferroelectric domain switching under high external fields.     

Switching time dispersion and retention of bistable states in Langmuir-Blodgett ferroelectric films

Polarization switching and retention of each of the two polarized states in Langmuir—Blodgett (LB) ferroelectric films are studied using nonlinear dielectric spectroscopy. It is found that polarized states can be preserved for a long time, but the polarization dynamics in 10–40-nm-thick LB films is characterized by a considerable switching-time dispersion. In addition, ferroelectric LB films exhibit clearly manifested asymmetry of switching to states with opposite directions of polarization. To explain the experimental results, a polarization-switching model is proposed that takes into account the energy of interaction of a ferroelectric polymer with boundary surfaces. The effect of inhomogeneity of the LB film structure on the ferroelectric switching dynamics is also discussed.     

Determining the influence of ferroelectric polarization on electrical characteristics in organic ferroelectric field-effect transistors

Organic ferroelectric field-effect transistors(OFeFETs) are regarded as a promising technology for low-cost flexible memories. However, the electrical instability is still a critical obstacle, which limits the commercialization process. Based on already established models for polarization in ferroelectrics and charge transport in OFeFETs, simulation work is performed to determine the influence of polarization fatigue and ferroelectric switching transient on electrical characteristics in OFeFETs. The polarization fatigue results in the decrease of the on-state drain current and the memory window width and thus degrades the memory performance. The output measurements during the ferroelectric switching process show a hysteresis due to the instable polarization. In the on/off measurements, a large writing/erasing pulse frequency weakens the polarization modulation and thus results in a small separation between on- and off-state drain currents. According to the electrical properties of the ferroelectric layer, suggestions are given to obtain optimal electrical characterization for OFeFETs.     

Analysis of polarization switching in ferroelectric crystals in the injection mode

We report on the results of experiments on polarization switching in a ferroelectric TGS crystal during injection of electron beams from a scanning electron microscope under a surface layer. A series of models reflecting the polarization switching dynamics of a ferroelectric crystal under the action of an injected charge is constructed. The implementation of these models is based on the principles of evolution of domain structures taking into account analysis of possible polarization switching mechanisms for ferroelectric samples. A mathematical model developed using these principles demonstrates qualitative similarity of model current pulses and those obtained experimentally in the injection mode.     

Polarization reversal behavior of an asymmetric ferroelectric thin film

Using the Landau–Khalatnikov equation of motion, the polarization reversal behavior in an asymmetric ferroelectric thin film has been studied. Our model first introduces a third power of polarization to describe the asymmetry of a ferroelectric thin film with surface transition layer, which originates from the difference between the surfaces. Interestingly, vertical drift of polarization switching behaviors was found in this system. The properties consisting of hysteresis loop, spontaneous polarization, switching current of an asymmetric ferroelectric thin film with surface transition layer are discussed.     

Study of the domain structure evolution in single crystals of relaxor ferroelectric Sr0.61Ba0.39Nb2O6:Ce1

Optical visualization with simultaneous recording of the switching current was used for studying of domain structure evolution in single crystals relaxor ferroelectric strontium barium niobate (Sr_0.61Ba_0.39Nb₂O₆) doped by cerium (0.22 mol %) (SBN61:Ce). It was shown that the maze domain structures can be observed during polarization reversal due to local change of refractive index induced by formation of microscale domains with charged domain walls. It was proposed, that optical inhomogeneities occur in the switching regions, while the optical uniform areas correspond to regions where the switching either has not started yet or has already completed. The original mathematical treatment of the obtained images and switching currents has been used for quantitative characterization of the polarization reversal process.     

Field-induced vortices in weakly anisotropic ferroelectrics

In microscale and nanoscale ferroelectric samples, the formation and growth of domains are the usual stages of the polarization switching mechanism. By assuming weak polarization anisotropy and by solving the Ginzburg–Landau–Khalatnikov equation we have explored an alternative mechanism which consists in ferroelectric switching induced by vortex formation. We have studied the polarization dynamics inside a ferroelectric circular capacitor where switching leads to the formation of a metastable vortex state with a rotational motion of the polarization. Our results are consistent with recent first-principle simulations [I.I. Naumov, H.X. Fu, Phys. Rev. Lett. 98, 077603 (2007)] and with experiments on PbZr_0.2Ti_0.8O₃ [A. Gruverman, D. Wu, H.J. Fan, I. Vrejoiu, M. Alexe, R.J. Harrison, J.F. Scott, J. Phys. Condens. Matter 20 342201(2008)] and demonstrate that vortex-induced polarization switching can be an effective mechanism for circular nanocapacitors.     

铁电双层膜的反转动力学行为

基于Landau-Khalatnikov运动方程,本文研究了含有表面过渡层和铁电界面耦合的反转动力学行为(包括平均极化、反转时间、反转电流和矫顽场).研究结果表明:在铁电双层膜系统中存在一个竞争的机理,即表面过渡层与界面耦合的竞争作用.我们发现在双层膜反转过程中出现了反常行为,这些反常行为归因于表面过渡层与界面耦合之间的竞争.表面过渡层与界面耦合的共同行为对铁电双层膜的动力学特性起到了决定性的作用.     

Dynamic fatigue during polarization switching of lead zirconate titanate ceramics with variable ferroelectric hardness

The remanent polarization of lead zirconate titanate ceramics with variable ferroelectric hardness is experimentally studied in a strong ac field. The results obtained suggest that the main mechanism of multiplication of crystal defects that controls the fatigue rate of soft ferroelectric ceramics is the process of twinning and untwinning in crystallites during polarization switching. In hard ferroelectric ceramics, the formation of a hysteresis loop is accompanied by the formation of stable configurations of mechanical twins in crystallites due to secondary twinning. Polarization switching in these configurations does not induce twin-wall motion, and the fatigue rate is low.     

Bilateral switching of the modulated electrooptic contrast in PLZTN ceramics

Spatial distribution of ferroelectric polarization in Pb(Zr,Ti)O₃ ceramics doped with lanthanum and niobium has been investigated using scanning electrooptic confocal microscopy. Measurement of the modulated electrooptic signal in presence of a sinusoidal electric field reveals ferroelectric domains at microscopic scale not accessible to conventional polarized optical imaging. Both electrooptic images and local electrooptic loops have been observed after applying a bias electric field to the ceramic samples, being initially non-ferroelectric macroscopically. Bilateral reversal of the electrooptic contrast during the poling cycle has been detected and explained taking into account non-180° switching processes in ferroelectric grains of arbitrary orientation.     

A viscoelastic model of polarization switching in polymer ferroelectrics

The theoretical aspects of the viscoelastic model for polarization switching in ??soft?? organic ferroelectrics have been considered. The model describes the amplitude-frequency dependences of the hysteresis loops obtained upon polarization switching in thin films of the ferroelectric copolymer poly(vinylidene fluoride-trifluoroethylene).     

Nonlinear optical and electrostatic force microscopy for ferroelectric polarization imaging

Second-harmonic generation (SHG)-based nonlinear optical microscopy is used for spatially resolved imaging of the polarization switching in lead zirconium titanate ferroelectric thin films. The local SHG hysteresis loops reveal a strong dependence on film composition and structure. The SHG microscopy results are in good agreement with the efficiency of electrostatic force microscopy writing and allow us to predict the microscopic dielectric memory efficiency, both in contact and contact-less ways. Received: 15 January 2001 / Revised version: 27 March 2002 / Published online: 6 June 2002     

Potential distribution in ferroelectric ceramic during pulsed exoelectron emission

The potential and polarization distributions in a planar emitting layer of PLZT-9/65/35 ferroelectric ceramic with a set of conductive strip electrodes on the emitting side and a continuous electrode on the opposite side are studied by numerical methods. The state arising immediately after polarization switching at the leading edge of an applied voltage pulse (i.e., before the polarization charges are screened by free charges) is considered. When the pulsed field strength far exceeds the double coercive field, regions with alternating polarization are found to form in the surface layer between the strips. The normal component of the polarization at its maxima is close to saturation. The electric field on both sides of the surface varies as the polarization vector and reaches 200 kV/cm. At surface microirregularities, the electric field strength is much higher. This means that field emission is responsible for electron escape from the ferroelectric ceramic during pulsed polarization switching.     

Ferroelectric polarization reversal tuned by magnetic field in a ferroelectric BiFeO3/Nb-doped SrTiO3 heterojunction

Interfacial resistive switching of a ferroelectric semiconductor heterojunction is highly advantageous for the newly developed ferroelectric memristors. Moreover, the interfacial state in the ferroelectric semiconductor heterojunction can be gradually modified by polarization reversal, which may give rise to continuously tunable resistive switching behavior. In this work, the interfacial state of a ferroelectric BiFeO₃/Nb-doped SrTiO₃ junction was modulated by ferroelectric polarization reversal. The dynamics of surface screening charges on the BiFeO₃ layer was also investigated by surface potential measurements, and the decay of the surface potential could be speeded up by the magnetic field. Moreover, ferroelectric polarization reversal of the BiFeO₃ layer was tuned by the magnetic field. This finding could provide a method to enhance the ferroelectric and electrical properties of ferroelectric BiFeO₃ films by tuning the magnetic field.     

Thermally induced jump-like pulse processes in a strontium barium niobate crystal doped with metal impurities

The specific features of jump-like pulse processes of spontaneous polarization switching in relaxor ferroelectric crystals of strontium barium niobate are investigated. Based on the fixed parameters of the jump-like switching pulses, three possible mechanisms of their formation are revealed.