Effect of electromechanical boundary conditions on the properties of epitaxial ferroelectric thin films

The effects of internal stresses and depolarization fields on the properties of epitaxial ferroelectric perovskite thin films are discussed by employing the dynamic Ginzburg-Landau equation (DGLE). The numerical solution for BaTiO₃ film shows that internal stress and the depolarization field have the most effects on ferroelectric properties such as polarization, Curie temperature and susceptibility. With the increase of the thickness of the film, the polarization of epitaxial ferroelectric thin film is enhanced rapidly under high internal compressively stress. With the thickness exceeding the critical thickness for dislocation formation, the polarization increases slowly and even weakens due to relaxed internal stresses and a weak electrical boundary condition. This indicates that the effects of mechanical and electrical boundary conditions both diminish for ferroelectric thick films. Consequently, our thermodynamic method is a full scale model that can predict the properties of ferroelectric perovskite films in a wide range of film thickness.     

Size-induced appearance of ferroelectricity in thin antiferroelectric films

In the paper we consider size effects on phase transitions and polar properties of thin antiferroelectric films. We extend the phenomenological approach proposed by Kittel for thin films allowing for gradient (correlation) energy and depolarization field energy. Surface piezoelectric effect as well as misfit strain appear due to lattice constants mismatch between the film and its substrate. Direct variational method is used to derive the free energy with renormalized coefficients depending on the film thickness. Obtained free energy expression allows the calculation of phase diagrams and all electro-physical properties by a conventional minimization procedure. Approximate analytical expressions for the paraelectric–antiferroelectric–ferroelectric transition temperature dependences on film thickness, polarization gradient coefficient, and extrapolation lengths were obtained. The thickness dependence of the electric field critical value that causes antiferroelectric–ferroelectric phase transition was calculated. Under favorable conditions the antiferroelectric phase at first transforms into ferroelectric one and then into paraelectric phase with the decrease of the film thickness. Proposed theoretical consideration explains the experimental results obtained in antiferroelectric PbZrO₃ thin films.     

Enhancement of dielectric and ferroelectric properties in ferroelectric superlattices

I studied theoretically the enhancement of remanent polarization and dielectric permittivity of interfacial-coupled ferroelectric superlattices based on the Landau–Ginzburg theory. Our model adopts the Landau–Khalatnikov equation to describe hysteresis behavior and takes the time-dependent space-charge-limited conductivity into account to investigate the ferroelectric and dielectric properties of ferroelectric superlattices. The results are in good agreement with recent experimental observations on the enhancement of remanent polarization and permittivity of BaTiO₃/SrTiO₃ superlattices and heterolayered Pb(Zr,Ti)O₃ thin films.     

外场作用下铁电/铁磁双层膜的极化磁化性质

建立了铁电/铁磁双层膜模型,铁电层的电矩用连续标量描述,而铁磁层的自旋应用经典矢量描述.利用蒙特卡罗方法模拟了体系的热力学性质和极化、磁化行为.给出了零场下体系的内能、比热、极化和磁化随温度变化的关系,并分别研究了体系在外磁场和外电场下的极化和磁化行为.模拟结果表明,双层膜体系的内能、比热、极化和磁化性质因层间耦合系数的不同而明显不同,当界面耦合较弱时,双层膜表现出各自的热力学性质,当层间耦合增强到一定程度时,双层膜耦合为一个整体,表现出统一的热力学性质.该双层膜在外场中形成电滞回线和磁滞回线,并表现出偏置特性,界面耦合强度和温度影响滞后回线和偏置现象.     

Unusual behavior of the ferroelectric polarization in PbTiO3/SrTiO3 superlattices

Artificial PbTiO3/SrTiO3 superlattices were constructed using off-axis rf magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO3 to SrTiO3 was changed. For PbTiO3 layer thicknesses larger than the 3-unit cell SrTiO3 thickness used in the structure, the polarization is found to be reduced as the thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO3 to SrTiO3 of less than one, a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.     

Depolarization in modeling nano-scale ferroelectrics using the Landau free energy functional

We derived thermodynamically from first principles the free energy functional of a ferroelectric under an applied field E that contains the depolarization field E _d. We found that it is crucial to recognize the relationship between the total polarization and the applied field due to the depolarization field when expressing the free energy functional in the Landau form. Ad hoc formulations of ferroelectric models that overlooked this relationship effectively double-count the depolarization field, and thus cause serious overestimation of its effects. Examples are given in which the Curie temperature and the critical thickness of ferroelectricity, etc., can be overestimated this way by orders of magnitude. PACS 77.80.-e; 77.80.Bh     

General features of the intrinsic ferroelectric coercive field

The value of the intrinsic ferroelectric coercive field is obtained independently, from general energy considerations and from the predictions of several models of the ferroelectric state. All predictions yield a value of the order of the depolarization field, which is equal to the spontaneous polarization divided by the dielectric permittivity, and are consistent with the recent measurements of the intrinsic ferroelectric coercive field in ultrathin Langmuir-Blodgett films of copolymers of polyvinylidene fluoride with trifluoroethylene. Prior studies succeeded only in measuring the much smaller extrinsic coercive fields, which are limited by nucleation processes and domain motion.     

铁电超晶格的一个唯象模型

引入了一个描述铁电超晶格行为模式的唯象模型.在模型中,同时考虑体与体之间的多种耦合方式.通过数值模拟,得出了超晶格的极化强度、居里温度、临界尺寸和耦合系数之间的关系.认为耦合参数c是一个与温度、晶格尺寸相关的函数,进而提出了一个关于耦合参数c的经验公式,解释了一个近期的关于KTaO₃/KNbO₃超晶格材料的实验结果. 关键词：     

Depolarization field and properties of thin ferroelectric films with inclusion of the electrode effect

The influence of metallic electrodes on the properties of thin ferroelectric films is considered in the framework of the Ginzburg-Landau phenomenological theory. The contribution of the electrodes with different screening lengths l _s of carriers in the electrode material is included in the free-energy functional. The critical temperature T _cl , the critical thickness of the film, and the critical screening length of the electrode at which the ferroelectric phase transforms into the paraelectric phase are calculated. The Euler-Lagrange equation for the polarization P is solved by the direct variational method. The results demonstrate that the film properties can be calculated by minimizing the free energy, which has a standard form but involves the coefficient of the term P². This coefficient depends not only on the temperature but also on the film thickness, the surface and correlation effects, and the electrode characteristics. The calculations of the polarization, the dielectric susceptibility, the pyroelectric coefficient, and the depolarization field show that the ferroelectric state of the film can be destroyed using electrodes from a material whose screening length exceeds a critical value. This means that the electrodes being in operation can induce a transition from the ferroelectric phase to the paraelectric phase. The quantitative criteria obtained indicate that the phase state and properties of thin ferroelectric films can be controlled by choosing the appropriate electrode material.     

Domain-enhanced interlayer coupling in ferroelectric/paraelectric superlattices

We investigate the ferroelectric phase transition and domain formation in a periodic superlattice consisting of alternate ferroelectric (FE) and paraelectric (PE) layers of nanometric thickness. We find that the polarization domains formed in the different FE layers can interact with each other via the PE layers. By coupling the electrostatic equations with those obtained by minimizing the Ginzburg-Landau functional, we calculate the critical temperature of transition Tc as a function of the FE/PE superlattice wavelength Lambda and quantitatively explain the recent experimental observation of a thickness dependence of the ferroelectric transition temperature in KTaO3/KNbO3 strained-layer superlattices.     

铁电材料光催化活性的研究进展

光催化技术被认为是最有前景的环境污染处理技术,这就使得光催化剂材料备受瞩目.近年来,铁电材料作为新型光催化剂材料受到人们越来越多的关注,其原因在于铁电材料特有的自发极化有望解决催化反应过程中的电子-空穴对复合问题,进而提高光催化活性.本文从两个方面对铁电极化如何影响光催化进行综述:一方面,从铁电极化入手归纳总结其对电子-空穴对分离的影响,进而更深入地从极化引发的退极化场和能带弯曲两个部分来阐述具体的影响机理;另一方面,为了消除静电屏蔽,分别从温度、应力(应变)、电场三个外场因素调控极化入手,归纳总结外场调控极化对电子-空穴对分离的影响,进而影响光催化活性.最后对该领域今后的发展前景进行了展望.     

Study of Depolarization Field Influence on Ferroelectric Films Within Transverse Ising Model

An improved transverse Ising model is proposed by taking the depolarization field effect into account. Within the framework of mean-field theory we investigate the behavior of the ferroelectric thin film. Our results show that the influence of the depolarization field is to flatten the spontaneous polarization profile and make the films more homogeneous, which is consistent with Ginzburg-Landau theory. This fact shows that this model can be taken as an effective model to deal with the ferroelectric film and can be further extended to refer to quantum effect. The competition between quantum effect and depolarization field induces some interesting phenomena on ferroelectric thin films.     

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.     

Polarization relaxation induced by a depolarization field in ultrathin ferroelectric capacitors

Time-dependent polarization relaxation behavior induced by a depolarization field E(d) was investigated on high-quality ultrathin SrRuO3/BaTiO3/SrRuO3 capacitors. The E(d) values were determined experimentally from an applied external field to stop the net polarization relaxation. These values agree with those from the electrostatic calculations, demonstrating that a large E(d) inside the ultrathin ferroelectric layer could cause severe polarization relaxation. For numerous ferroelectric devices of capacitor configuration, this effect will set a stricter size limit than the critical thickness issue.     

The polarization and dielectric susceptibility of a ferroelectric bilayer film

A ferroelectric bilayer film consisting of two different ferroelectric constituent films with a transition layer within each constituent film and interfacial coupling between two materials is investigated based on the Ginzburg-Landau-Devonshire phenomenological theory. A parameter α, which describing the differences between physical properties of two constituent films is first introduced in our paper, and reflects a more realistic situation compared to the previous treatments assuming the same two constituent films. We study the polarization and dielectric susceptibility properties of the ferroelectric bilayer film with two different constituent films. The results present some interesting phenomena due to the introduction of parameter α.     

New antiferroelectric liquid crystal for use in LCD

In this work, the physical properties of newly synthesized liquid crystalline compound exhibiting two liquid crystalline phases (ferroelectric and antiferroelectric) were studied. Based on the results of differential scanning calorimetry, polarizing microscopy, and photoelastic modulator methods, the temperature dependences of spontaneous polarization, tilt angle, switching time, and birefringence in the ferroelectric, as well as antiferroelectric phases were determined. Furthermore, the influence of the external electric field on the liquid crystalline textures was studied and the phase sequences at heating and cooling were revealed. The temperature dependence of spontaneous polarization was analysed by means of Landau mean-field theory, and the critical parameter β obtained for ferroelectric liquid crystalline and isotropic liquid transition was 0.21 which is close to 0.25, the value characteristic for tri-critical point.     

铁电相变中极化与介电性的随机场效应

基于随机场Ising模型描述的有序无序相变中偶极子在电场作用下的反转运动, 研究了有序无序相变过程中电场与极化强度的关系. 认为tanh(x) 的函数关系与位移型二阶铁电相变极化强度随电场变化的实验结果完全相同. 由此得出基本结论: 偶极子的集体转向造成了极化强度的增大并等同于内电场的增加. 通过区分光学模和偶极子对介电隔离率的贡献, 考虑偶极子极化对介电常数复数形式的频率色散关系, 发现从高斯分布的居里温度可以导出二阶铁电相变过程中介电常数与温度和频率的色散关系. 关键词： 铁电相变 极化强度 随机场     

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.     

Interfaces in perovskite heterostructures

Recent advances in film synthesis have made it possible to investigate the properties of well-controlled interfaces in perovskite metal-oxides. A review of published experimental data and computational results indicate that so far most interfaces that have been analyzed in ferroelectric materials—while necessary to impose large lattice strain on the polar material—contribute little to the ferroelectricity and may instead be detrimental to the desired properties. In contrast, a very different situation arises at interfaces that show changes in the electronic configuration as a consequence of a compositional discontinuity. Data is shown for LaMnO₃/SrTiO₃ superlattices as an example of electronic effects that produce enhanced properties, further illustrating the richness of interfacial properties that can be obtained at interfaces (as shown in numerous published results for different but related interfaces).     

Quantum Monte Carlo simulation of the ferroelectric or ferrielectric nanowire with core shell morphology

By using the Quantum Monte Carlo simulation; the electric properties of a nanowire, consisting of a ferroelectric core of spin-1/2 surrounded by a ferroelectric shell of spin-1/2 with ferro- or anti-ferroelectric interfacial coupling have been studied within the framework of the Transverse Ising Model (TIM). We have examined the effects of the shell coupling J_s, the interfacial coupling J_Int, the transverse field Ω, and the temperature T on the hysteresis behavior and on the electric properties of the system. The remanent polarization and the coercive field as a function of the transverse field and the temperature are examined. A number of characteristic behavior have been found such as the appearance of triple hysteresis loops for appropriate values of the system parameters.