Iron-yttrium garnet films with magnetic vortices

Submicron iron-yttrium garnet films have been studied. The effect of the switching of nonreciprocal resonance absorption, which is caused by the switching of the polarization of vortices, has been revealed.     

Switching characters of asymmetric ferroelectric films

The switching property of asymmetric ferroelectric films has been studied from Landau free energy and the Khalanikov mation. The polarization response and switching current under a step electric field and pulsed field have been investigated. Watching times, for both the up-switching and down-switching process, have been obtained and discussed.     

Polarization switching kinetics of ferroelectric nanostructures of vinylidene fluoride-trifluoroethylene copolymer

The polarization switching kinetics of ferroelectric polymer Langmuir-Blodgett films and copolymer nanocrystals (nanomesas) was investigated using piezoresponse force microscopy. The nanocrystals were prepared by self-organization from Langmuir-Blodgett films of a 70% vinylidene fluoride and 30% trifluoroethylene copolymer. The polarization switching time exhibits an exponential dependence on reciprocal voltage and decreases with temperature being consistent with the nucleation switching dynamics.     

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).     

Influence of fatigue processes on the switching currents in lead titanate and lead zirconate-titanate films

The processes of polarization fatigue in PbTiO₃ and Pb(Zr_0.5Ti_0.5)O₃ films on silicon substrates have been studied by the method of switching in rectangular pulsed fields. Dependences of the switchable polarization, the maximum switching current i _max, and the switching time ??s on the number of switching cycles have been obtained from the data on the switching currents. It has been shown that the mobility of domain walls is retained in the fatigue processes and the observed fatigue is connected with switching off a fraction of the switchable volume due to pinning of reverse domain nuclei by charged defects.     

Polarization switching properties of spray deposited CsNO3: PVA composite films

The ferroelectric and switching properties of spray deposited cesium nitrate: poly (vinyl alcohol) composite films at different substrate temperatures (T _s ) have been studied. The optimum value of remanent polarization was obtained in the film deposited at T _s =200°C, which may be due to larger structural distortion (c/a ratio) and less porosity as revealed by x-ray diffraction and field emission scanning electron microscope (FESEM) analysis. The switching current transients have been analyzed by nucleation limited switching model (NLS) with the Lorentzian distribution function. This model gives excellent agreement with the experimental polarization current throughout the whole time range. The switching parameters were determined in the composite films deposited at different T _s and found to be optimum at T _s =200°C. The effect of pulse amplitude on the domain switching properties has also been studied and analyzed. The peak value of polarization current exhibits an exponential dependence on the external applied field.     

Effect of pore size on ferroelectric properties of multiferroic BiFeO3 films prepared on porous silicon

The ferroelectric properties of BiFeO₃ (BFO) films spray deposited on porous silicon have been studied. The analysis of XRD and FESEM investigations show that the crystalline strain in the BFO films increases with pore size. The BFO films on porous silicon substrate showed improvement in ferroelectric fatigue behavior, remanent polarization and ferroelectric switching time. A maximum memory window of 5.54 V at 1 MHz and a large remanent polarization (P_r) of 13.1 μC/cm² have been obtained at room temperature. The improvement in the ferroelectric properties of these films has been correlated to the crystalline strain.     

Spreading resistance microscopy of polycrystalline and single-crystal ferroelectric films

Thin films based on lead zirconate titanate with stoichiometric composition near the morphotropic boundary have been studied using atomic-force microscopy methods. The dependence of the local conductivity on the local polarization direction has been observed for all samples, independently of substrate type, deposition method, and film thickness. It has been shown that the current response to the applied voltage exhibits a long current relaxation, about several tens of seconds, which is two to three orders of magnitude greater than the current relaxation time in an external circuit, associated with the ferroelectric domain switching. The conductivity features have been explained by recharging of traps localized at ferroelectric grain boundaries near electrodes and involved in polarization charge screening.     

Polarization switching without domain formation at the intrinsic coercive field in ultrathin ferroelectric PbTiO₃

Polarization switching in ferroelectrics has been thought to occur only through the nucleation and growth of new domains. Here we use in situ synchrotron x-ray scattering to monitor switching controlled by applied chemical potential. In sufficiently thin PbTiO? films, nucleation is suppressed and switching occurs by a continuous mechanism, i.e., by uniform decrease and inversion of the polarization without domain formation. The observed lattice parameter shows that the electric field in the film during switching reaches the theoretical intrinsic coercive field.     

Evolution of the switching current during the accumulation of fatigue in lead titanate and lead zirconate titanate thin films

The fatigue processes caused by repeated cyclic switching of polarization in lead titanate and lead zirconate titanate thin films on silicon substrates by applying rectangular pulse and sine wave fields are investigated. Upon an increase in the cycling number, an increase in the coercive and internal bias fields is observed, along with a reduction in the switching current maximum, the switching time, and the switchable polarization. At the same time, no change in the mobility of domain boundaries is observed.     

Analysis of ferroelectric switching in sodium nitrite:poly(vinyl alcohol) nanocomposite films

The Fourier transform infrared (FTIR) spectra and switching current response in sodium nitrite:poly(vinyl alcohol) nanocomposite films have been studied as a function of composition of NaNO₂. The switching current data fitted well to infinite-grain model (IGM) in the region t<t _s and to finite-grain model (FGM) in the region t≥t _s . The microscopic parameters like the dimensionality, the domain wall velocity, and the nucleation rate have been evaluated which provide more physical insight of the switching phenomena in the composite films. The polarization current and nucleation rate are optimum in 50 wt.% composite film and have been discussed in terms of grain size and strain variations with the composition. The effect of applied field and pulse width variation on the switching behavior of 50 wt.% composition has also been studied. The exponential field dependence of the domain wall velocity and the nucleation rate indicate that nucleation mechanism is responsible for switching phenomena in the composite films. The writing pulse width affects significantly on the switching behavior of the composite films.     

Fatigue of lead titanate and lead zirconate titanate thin films

The fatigue of lead titanate and lead zirconate titanate ferroelectric thin films, i.e., a change in the polarization as a function of the number of switching cycles in an external electric field, is investigated experimentally. The threshold numbers of switching cycles are determined to be 10¹⁰–10¹¹ for the lead titanate films and 10⁹–10¹⁰ for the lead zirconate titanate films. It is shown that a change in the temperature does not substantially affect the threshold number of switching cycles at which the switched polarization decreases drastically. However, an increase in the external field strength leads to a noticeable decrease in the threshold number of switching cycles. The process of fatigue is accompanied by an increase in the coercive field and the internal bias field. It is established that, as the number of switching cycles increases, the internal bias field changes more significantly as compared to the coercive field. The absence of a change in the phase composition in repeatedly switched samples indicates that the fatigue processes have a nonchemical nature. The anomaly observed in the frequency dependence of the permittivity in the frequency range 10⁶–10⁷ Hz due to the domain structure disappears after multiple switching cycles. This suggests that the observed fatigue phenomenon has a domain nature.     

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.     

Investigation of ferroelectric properties of bismuth ferrite films by the second optical harmonic generation technique

The specific features of the ferroelectric polarization switching in bismuth ferrite thin films doped with neodymium ions are investigated by the optical second harmonic generation technique. The structure and nonlinear optical properties of the samples prepared are studied in the course of ferroelectric polarization switching in planar geometry over wide ranges of film thicknesses and electric field frequencies.     

1/<Emphasis Type="Italic">E</Emphasis><Superscript>2</Superscript> Law: Solution of the Unsolved Problem in the Physics of Ferroelectrics

An unusual frequency dependence of a coercive field observed in polycrystalline ferroelectric films of Pb(Zr_{1 ? x}Ti_x)O₃ solid solutions in a wide frequency range has been explained within the model of surface sources emitting thin ferroelectric domains with reversed polarization. This model makes it possible to interpret experimental data as the manifestation of the coercivity paradox in polycrystalline films and predicts the existence of the limiting frequency for switching of domains in agreement with the experiment. The proposed mechanism of polarization switching explains the observed temperature dependence of the activation field E_a ∝ T^?1/2. Furthermore, it predicts an increase in the activation energy for the nucleation of domains with an increase in the size of the source, indicating that the coercive field increases with the degradation of small     

Fast polarization reversal in thin copolymer films of vinylidene fluoride-trifluoroethylene

In this paper we present new experiments on the switching of the polarization in thin (<1 m) ferroelectric copolymer films of P(VDF-TrFE). We observed an unexpectedly short switching time for these thin films: The polarization of a 0.2 m thick film could be reversed by poling fields of 600 MV/m in less than 100 ns. This result which may revive interest in ferroelectric data storage is discussed in terms of a switching process which also involves the ohmic conductivity of the sample.     

聚(偏氟乙烯-三氟乙烯)纳米薄膜极化反转与疲劳特性

采用旋涂法制备了厚度为140 nm的聚(偏氟乙烯-三氟乙烯)[P(VDF-TrFE)]纳米薄膜, 研究了不同退火温度以及环境相对湿度对薄膜的极化反转和疲劳性能的影响. 运用X射线衍射仪、扫描电子显微镜和傅里叶变换红外光谱仪等测试技术对薄膜的微结构进行了表征. 实验结果表明, 通过不同温度的退火处理, P(VDF-TrFE)铁电薄膜的结晶度随着退火温度的升高而不断提高, 并且一定的温度范围内的退火处理可以提高薄膜的极化性能; 此外, P(VDF-TrFE) 铁电薄膜性能还表现出一定的环境湿度的敏感特性, 这与薄膜的物理性能和结构特点密切相关; P(VDF-TrFE)铁电薄膜在不同的环境湿度条件下 表现出较好的电学特性, 其漏电流均保持在10 ^-7A/cm² 的较低水平. 本工作揭示了再退火过程对薄膜的极化反转速度和疲劳恢复特性的影响, 并结合薄膜二次疲劳结果, 探讨了薄膜可逆的内部疲劳恢复特性机理.     

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.     

Polarization fatigue in poly(vinylidene fluoride (78%)-trifluoroethylene (22%)) ferroelectric thin films: a pulse train study

This paper investigates the influences of polarization fatigue on remanent polarization and switching time by pulse train measurements in ferroelectric poly(vinylidene fluoride (78%) and trifluoroethylene (22%)) thin films. Fatigue was carried out by a series of bipolar switching pulses with constant pulse width (on-time) and various interval times between pulses (off-time). The experimental observations indicated that the off-time period showed no obvious influence on fatigue rate and the switching time increased with the increase of fatigue cycles. The origination of these phenomena was discussed according to the charge injection model.     

Polarization Switching Bistability and Dynamics in Vertical-Cavity Surface-Emitting Laser under Orthogonal Optical Injection

Bistability as well as dynamics associated to polarization switching in a vertical-cavity surface-emitting laser (VCSEL) submitted to orthogonal optical injection from an external master laser are investigated. Intensity-polarization switching bistability is studied by increasing and decreasing the intensity of the injected beam and for a fixed frequency detuning, i.e. the frequency offset between master and the free running VCSEL. Depending on the frequency detuning, bistable switching is experimentally found to occur with or without injection locking of the fundamental mode to the master laser frequency. For large positive detuning, injection locking of the first-order transverse mode with a strong depression of the fundamental mode has been observed. The case of frequency – polarization switching bistability, i.e. when the frequency detuning is first decreased and then increased for a fixed injected power, has also been theoretically analyzed. Qualitative comparison with previous experimental work is presented for the dependence of the width of the bistable switching region on the injection strength level. Finally we show an experimental result of complicated dynamics including period doubling route to chaos that accompany polarization switching and which motivates future theoretical investigations.