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.     

铁电薄膜漏电流的应变调控

基于密度泛函理论的第一性原理并结合非平衡格林函数, 探讨了应变对 BaTiO₃ 铁电薄膜漏电流的影响规律.研究表明,压应变能有效地抑制BaTiO₃ 铁电薄膜漏电流, 特别是当压应变为4%时,其漏电流相对无应变状态降低了近10 倍.通过考察体系的透射系数和电子态密度发现: 一方面压应变状态下铁电隧道结的透射几率要比张应变时小,特别是在费米面附近;另一方面随着张应变过渡至压应变时,价带的位置逐渐向低能区移动而导带向高能区移动,导致了其带隙的增大, 从而有效抑制了漏电流. 本研究为寻找抑制铁电薄膜漏电流,提高铁电薄膜及铁电存储器的性能提供合适的方法. 关键词： 铁电薄膜 双轴应变 漏电流 第一性原理     

Review on the properties of the ferrorelaxor polymers and some new recent developments

It has been found that by introducing defects into the P(VDF-TrFE) copolymers, it is possible to convert the polymer from a normal ferroelectric to a relaxor ferroelectric. A new class of ferroelectric polymers, i.e., the terpolymers of P(VDF-TrFE-CFE) or of P(VDF-TrFE-CTFE), was developed from the normal ferroelectric PVDF-TrFE polymer by employing proper defect modifications which eliminate detrimental effects associated with a normal first order F?CP transition while maintaining high material responses. Relevant studies show that this class of electroactive polymers offers unique properties in comparison with other ferroelectric polymers. The syntheses of these relaxor ferroelectric polymers have been done by a combination of the suspension polymerization process and an oxygen-activated initiator at a temperature of 40°C. Films from cast solution can be made in different lengths and thicknesses. Stretching of these films increases the performance as well as the mechanical properties. These relaxor-ferroelectric terpolymers P(VDF-TrFE-CFE), P(VDF-TrFE-CTFE) are multifunctional, i.e., electrostrictive material, dielectric for electric energy storage. The terpolymer exhibits high electrostrictive strain (>7%) with relatively high modulus (>0.4?GPa). Examples of devices applications using unimorphe systems are presented. Micropump and Optical device concerning a liquid-filled varifocal lens on a chip are described.     

Memory element based on ferroelectric field-effect transistor with use of ZnO:Li/LaB6 heterostructures

Ferroelectric field-effect transistors using ZnO:Li films simultaneously as a transistor channel and as a ferroelectric active element have been prepared and studied. We show an opportunity of using the ferroelectric field-effect transistor based on ZnO:Li films in ZnO:Li/LaB₆ heterostructure as a bistable memory element for information recording. The proposed structure of a ferroelectric memory cell does not possess the fatigue under repeated readout of single recorded information that will allow increasing the resource of storage devices essentially.     

Dielectric,ferroelectric and non-linear optical behavior of crystalline erbium tartrate dihydrate

The dielectric, ferroelectric and non-linear optical behavior of erbium tartrate dihydrate crystals is studied. The dielectric constant has been measured as a function of temperature and frequency. The crystals of erbium tartrate dihydrate show an anomalous dielectric behavior as a function of temperature: the dielectric constant increases with temperature, attains a peak around 235 °C and then decreases as the temperature goes beyond 235 °C. The dielectric anomaly at 235 °C is suggested to be due to ferroelectric phase transition brought about in the material. The experiment on polarization versus electric field shows a hysteresis loop thereby establishing the ferroelectric nature of erbium tartrate dihydrate. The results of thermal study show that the material is thermally stable up to a temperature of about 240 °C. The non-linear optical study shows that erbium tartrate dihydrate has second harmonic generation efficiency comparable to that of potassium dihydrogen phosphate (KDP).     

Combined effect of the transition layer and interfacial coupling on the properties of ferroelectric bilayer film

Within the framework of modified Ginzburg--Landau--Devonshire phenomenological theory, a ferroelectric bilayer film with a transition layer within each constituent film and an interfacial coupling between two materials has been studied. Properties including the Curie temperature and the spontaneous polarization of a bilayer film composed of two equally thick ferroelectric constituent films are discussed. The results show that the combined effect of the transition layer and the interfacial coupling plays an important role in explaining the interesting behaviour of ferroelectric multilayer structures consisting of two ferroelectric materials.     

Effect of electrode on dielectric susceptibility and pyroelectric properties of a ferroelectric thin film capacitor using landau-khalatnikov theory

The influence of electrodes on the dielectric susceptibility and pyroelectric properties of a ferroelectric thin film with surface transition layers has been investigated within the framework of Landau-Khalatnikov dynamic theory. The contribution of the electrodes is reflected by the depolarization field in the free-energy function. The large electrode effect implies the strong depolarization field in ferroelectric thin films. The results show that the electrode materials can greatly impact the dynamic dielectric and pyroelectric properties of a ferroelectric thin film.     

单体浓度对聚合物稳定铁电液晶稳定性的影响

制备了不同单体浓度的聚合物稳定铁电液晶器件,测试了震动实验和热稳定实验前后样品的排列织构、电光特性和对比度.用原子力显微镜研究了各样品的聚合物形貌,表明在器件内部形成了沿摩擦方向延伸的聚合物网络,并且随着单体含量的增加聚合物网络更加致密.由于聚合物网络体锚定的引入,使得铁电液晶分子在外力和温度场下的运动受到限制,聚合物体锚定的增强使聚合物稳定铁电液晶的稳定性也逐渐提高.实验结果表明,单体含量为4％时,聚合物稳定铁电液晶的抗震性和热稳定性良好,电光曲线为无阈值“V”字型,对比度达150∶1.     

PZT铁电材料的总剂量辐照效应实验研究

 采用传统固相反应法制备PZT铁电材料,并制作成平行平板无源电容器结构,在ELV-8电子直线加速器上进行了总剂量效应辐照实验。结果表明：样片经过不同强度高能高速直流电子束辐照后的电滞回线随着辐照强度的增加,电滞回线所包围的面积逐渐减小,饱和极化强度、剩余极化强度和矫顽场呈线性减小。其中当辐照剂量为1×10⁸rad（Si）时,饱和极化强度、剩余极化强度和矫顽场的衰减幅度分别为14.1%,15.0%和2.7%,样片抗总剂量辐照能力可达1×10⁸rad（Si）。     

Atomic-scale compensation phenomena at polar interfaces

The interfacial screening charge that arises to compensate electric fields of dielectric or ferroelectric thin films is now recognized as the most important factor in determining the capacitance or polarization of ultrathin ferroelectrics. Here we investigate using aberration-corrected electron microscopy and density-functional theory to show how interfaces cope with the need to terminate ferroelectric polarization. In one case, we show evidence for ionic screening, which has been predicted by theory but never observed. For a ferroelectric film on an insulating substrate, we found that compensation can be mediated by an interfacial charge generated, for example, by oxygen vacancies.     

Narrow Waveguide Based on Ferroelectric Domain Wall

Ferroelectric materials are spontaneous symmetry breaking systems that are characterized by ordered electric polarizations.Similar to its ferromagnetic counterpart,a ferroelectric domain wall can be regarded as a soft interface separating two different ferroelectric domains.Here we show that two bound state excitations of electric polarization(polar wave),or the vibration and breathing modes,can be hosted and propagate within the ferroelectric domain wall.In particular,the vibration polar wave has zero frequency gap,thus is constricted deeply inside ferroelectric domain wall,and can even propagate in the presence of local pinnings.The ferroelectric domain wall waveguide as demonstrated here offers a new paradigm in developing ferroelectric information processing units.     

Thermal behavior of Jonscher’s formalism in relaxor ferroelectrics showing diffuse phase transitions

A theoretical study of the thermal behavior of the exponent s of Jonscher’s power law in ferroelectric materials showing diffuse phase transitions (DPTs) is presented. The proposed model is applied to the experimental results of lanthanum-doped lead zirconate titanate ferroelectric ceramics. Both the proposed model and the experimental results show a critical point (a maximum) around the maximum of the dielectric loss factor, which moves to higher temperatures when the frequency increases, the typical behavior of a relaxor material with DPTs. PACS 77.22.Gm; 77.80.Bh; 77.84.Dy     

Multiferroic tunnel junctions

Magnetic tunnel junctions with ferroelectric barriers, often referred to as multiferroic tunnel junctions, have been proposed recently to display new functionalities and new device concepts. One of the notable predictions is that the combination of two charge polarizing states and the parallel and antiparallel magnetic states could make it a four resistance state device. We have recently studied the ferroelectric tunneling using a scanning probe technique and multiferroic tunnel junctions using ferromagnetic La_0.7Ca_0.3MnO₃ and La_0.7Sr_0.3MnO₃ as the electrodes and ferroelectric (Ba, Sr)TiO₃ as the barrier in trilayer planner junctions. We show that very thin (Ba, Sr)TiO₃ films can sustain ferroelectricity up till room temperature. The multiferroic tunnel junctions show four resistance states as predicted and can operate at room temperatures.     

Investigation of the seeding-layer effect for a ferroelectric thin film with the transverse Ising model

The transition feature of a ferroelectric thin film with a seeding layer is studied based on the transverse Ising model. The influence of the seeding layer on the transition behavior of a ferroelectric thin film is investigated systemically, and the effect of the interaction parameters for the seeding layer on the phase diagram is also obtained. Meanwhile, the polarization and Curie temperature of the ferroelectric thin film are calculated for different seeding-layer structures. The results show that the polarization and Curie temperature of the film will be obviously modified on adding a seeding layer.     

Study of Phase Transition Properties in Epitaxial Ferroelectric Film

Based on the transverse Ising model and using decoupling approximation to the Fermi-type Green's function, we study the phase transition properties of the epitaxial ferroelectric film with one substrate. A general recursive equation of the ferroelectric thin film with two n-layer materials is obtained, which enables us to study the phase transition properties for any number layers forepitaxial ferroelectric thin film. With the help of this equation, we analyze the effect of the exchange interaction and the transverse field in the phase diagram, the influence to the polarizations and Curie temperature numerically. The results show that epitaxial ferroelectric film are able to induce a strong increase or decrease of Curie temperature to different exchange interactions and transverse fields within the epitaxial film layers. The theoreticalresults are in reasonable accordance with experimental data of different ferroelectric thin film.     

Magnetic inversion symmetry breaking and ferroelectricity in TbMnO3

TbMnO3 is an orthorhombic insulator where incommensurate spin order for temperature T(N)<41 K is accompanied by ferroelectric order for T<28 K. To understand this, we establish the magnetic structure above and below the ferroelectric transition using neutron diffraction. In the paraelectric phase, the spin structure is incommensurate and longitudinally modulated. In the ferroelectric phase, however, there is a transverse incommensurate spiral. We show that the spiral breaks spatial inversion symmetry and can account for magnetoelectricity in TbMnO3.     

Compositional analysis of ferroelectric films coated with carbon layer using laser-induced plasmas spectroscopy

An experimental investigation of the effect of different thickness of carbon layer coated on ferroelectric films on the atomic emission intensity using laser-induced plasmas spectroscopy technique with charge-coupled device (CCD) experimental system has been conducted. The experimental results show that the intensity of the spectra emitted with the carbon layer thickness of 15 μm is much higher than that of pure ferroelectric films. By using this best experimental condition, the atomic concentration ratios of ferroelectric films are evaluated by rationing the integrated intensities of selected spectral emission lines of the plasma produced from the films. And the experimental results show that N_La/N_Co and N_Sr/N_Co atomic concentration ratios are almost in agreement with the corresponding values obtained by traditional compositional analysis techniques of inductively coupled plasma (ICP).     

Nanoscale electrostatic manipulation of magnetic flux quanta in ferroelectric/superconductor BiFeO3/YBa2Cu3O(7-δ) heterostructures

Using heterostructures that combine a large-polarization ferroelectric (BiFeO3) and a high-temperature superconductor (YBa2Cu3O(7-δ)), we demonstrate the modulation of the superconducting condensate at the nanoscale via ferroelectric field effects. Through this mechanism, a nanoscale pattern of normal regions that mimics the ferroelectric domain structure can be created in the superconductor. This yields an energy landscape for magnetic flux quanta and, in turn, couples the local ferroelectric polarization to the local magnetic induction. We show that this form of magnetoelectric coupling, together with the possibility to reversibly design the ferroelectric domain structure, allows the electrostatic manipulation of magnetic flux quanta.     

质子对BaTiO3薄膜辐照损伤的计算机模拟

本文先应用分子动力学模拟BaTiO₃体系在初级击出原子(primary knock-on atom, PKA)轰击下缺陷产生和复合的动力学过程, 模拟结果表明：PKA的方向和能量对缺陷数目有重要影响, 并计算了Ba, O和Ti原子的平均位移阈能分别为69 eV, 51 eV和123 eV, 远大于SRIM程序默认的位移阈能25 eV. 然后应用蒙特卡罗软件包SRIM, 模拟质子在BaTiO₃薄膜中的能量损失过程, 比较位移阈能对模拟结果的影响, 分析质子能量和入射角度对空位数量以及分布的影响. 结果表明空位数量随着质子能量增加而增加, 增加的速率随能量的增加是降低的;当入射角度大于60°, 空位数量随入射角增大而明显减少.