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.     

Properties of ferroelectric thin films with a first-order phase transitions

A Green's function technique is used to investigate the properties of ferroelectric thin films with a first-order phase transitions on the basis of the transverse Ising model. Taking into account the four-spin interactions beside the two-spin interactions the dependence of the polarization on film thickness and temperature and the thickness dependence of the Curie temperature become more complicated.     

Phase diagrams of a ferroelectric superlattice: A Green’s function technique study

A Fermi-type Green’s function method has been used to investigate the phase transition properties of a ferroelectric superlattice with two alternating materials on the basis of the transverse Ising model. By performing a higher-order decoupling to the equations of motion for the Green’s functions, the eigenfrequencies of the infinite ferroelectric superlattice are obtained. Moreover, we discuss the dependence of the phase diagrams on the interface coupling strength, the transverse field, and the thicknesses of two slabs. The comparison between the Green’s function technique and the usual mean-field approximation is illustrated.     

Effects of the transverse Ising model parameters on the polarizations of multi-surface ferroelectric thin films

The layer polarizations and average polarizations of multi-surface ferroelectric thin films described by the transverse Ising model (TIM) have been studied within the framework of a Green’s function technique. The effects of the exchange interactions and transverse field parameters on the polarizations are discussed systemically. By performing a higher-order decoupling to the Fermi-type Green’s functions, the numerical results beyond those of the usual mean-field approximation are obtained. It is shown that the larger the exchange interaction, the larger the layer polarization and the average polarization; the larger the transverse field, the smaller the polarization.     

Phase transition properties of ferroelectric thin films with two surface layers

The phase diagrams of ferroelectric thin films with two surface layers described by the transverse Ising model have been studied under the mean-field approximation. We discuss the effects of the exchange interaction and transverse field parameters on the phase diagrams. The results indicate that the phase transition properties of the phase diagrams can be greatly modified by changing the transverse Ising model parameters. In addition, the crossover features of the parameters from the ferroelectric dominant phase diagram to the paraelectric dominant phase diagram are determined for ferroelectric thin films with two surface layers.     

参数修改对铁电薄膜相变性质的影响

在关联有效场理论的框架内, 利用微分算子技术, 详细地计算了基于横场伊辛模型描述的对称铁电薄膜系统的相变性质. 根据薄膜各层自旋平均值构成的一系列耦合方程, 推导出可以用来计算任意层的具有不同表面层的薄膜相图的解析通式方程, 讨论了参数修改对薄膜相互作用参数从FPD (铁电相占主导地位的相图)到PPD (顺电相占主导地位的相图)过渡值和参数空间中各相变区域的影响. 在与平均场近似进行比较的结果显示, 关联有效场理论所得到的铁电薄膜的铁电性在某种程度上比平均场近似下的结果减弱. 关键词： 铁电薄膜 横场伊辛模型 相图 居里温度     

Phase transition behaviors of a ferroelectric thin film with long-range interactions

Within the framework of the mean-field approximation, the transverse-field Ising model is used to investigate the surface parameter modification of interaction constants and transverse fields on phase transition behaviors of a ferroelectric thin film with long-range interactions. The variation of Curie temperature as a function of the surface transverse field for different surface interaction constants or thickness is discussed. Meanwhile, the variation of Curie temperature as a function of the surface exchange interaction for different surface transverse fields is examined. In addition, the dependence of Curie temperature on the film thickness for different surface exchange interactions and surface transverse fields is obtained.     

Study of Defect-Layers Effect in Ferroelectric Thin Film with TransverseIsing Model

By taking into account the two-spin interaction in the transverse Ising model (TIM), the influence of the defect layers (including J_B andΩ_B on the polarization and Curie temperature are calculated numerically, within the framework of the decoupling approximation under Green's function. The numerical results show that the polarization and Curie temperature will both become large sensitively due to the large values ofJ_B and the small value of Ω_B of the defect layers. Meanwhile, the dependence of the crossover values of the exchange interaction J_A, the transverse field Ω_Aof the bulk material on the exchange
interaction J_B and the transverse fieldΩ_B of the defect layers are shown in 3-Dimensional (3-D) figures for the first time. Moreover, the transition features of the ferroelectric thin film with defect layers are presented.     

Influence of Surface Transition Layers on Phase Transformation and Pyroelectric Properties of Ferroelectric Thin Film

Taking into account surface transition layers （STLs）, we study the phase transformation and pyroelectric properties of ferroelectric thin films by employing the transverse Ising model （TIM） in the framework of the mean field approximation. The distribution functions representing the intra-layer and inter-layer couplings between the two nearest neighbour pseudo-spins are introduced to characterize STLs. Compared with the results obtained by the traditional treatments for the thin films using only the single surface transition layer （SSL）, it is shown that the STL model reflects a more realistic and comprehensive situation of films. The effects of various parameters on the phase transformation properties have shown that STL can make the Curie temperature of the film higher or lower than that of the corresponding Sulk material, and the thickness of STL is a key factor influencing the film properties. For a film with definite thickness, there exists a critical STL thickness at which ferroelectricity will disappear when the intra-layer and inter-layer interactions are weak.     

多晶铁电薄膜的相变——应力和晶粒尺寸效应

用掠入射X射线衍射法观察到钛酸铅多晶铁电薄膜表面层与体内的相变温度不同,表面层的结构参数也有异于体内;唯象地把多晶铁电薄膜抽象为一个表面层为细晶粒、低应变层,体内为粗晶粒、高应变层的两层结构,根据应力和晶粒尺寸效应对铁电相变的影响,解释了钛酸铅多晶铁电薄膜的相变特征 关键词：     

Spin-Polarization of Ferroelectric Supperlattice with Spin-1/2 Transverse Ising Model

By using the simple decoupling approximation to Fermi-type Green's functionfor the transverse Ising model under pseudospin theory, we systemicallystudy the influence of different exchange interactions (transverse fields)of the two distinct materials on the polarization and Curie temperature offinite alternating superlattice. Meanwhile, we analyze the effect of thewhole parameters of the top surface, present their influence on thepolarization of each layer (including the mean polarization of the wholeferroelectric superlattice) and on the Curie temperature. The results showthe ratio of the exchange interactions (the transverse fields), which are ofthe two alternating materials have deeply impact on the polarization andCurie temperature of the supperlattice. Moreover, the top surface also hasgreat influence on the whole ferroelectric superlattice.     

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.     

Ferroelectric phase transition of a finite alternating superlattice

The phase transition properties of ferroelectric supperlattice with finite alternating layers have been investigated by using the transverse Ising model within the mean field approximation. The effects of surface modification are introduced on the assumption that the exchange interaction and transverse field parameters on the top surface are different from those in other layers of the superlattice. The phase diagrams are described in two different ways. The results indicate that the features of the phase diagrams can be greatly modified by changing the transverse Ising model parameters.     

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.     

On the theory of thermodynamic properties of geometrically confined disordered ferroelectrics

We propose a theoretical approach to calculate the thermodynamic properties of thin films fabricated from disordered ferroelectrics. To calculate the above thermodynamic properties, we use so-called random field method, modified for the case of thin films. The essence of modification is the altering of the interaction between impurity dipoles by geometrical confinement. We show that in thin films the ferroelectric phase transition is inhibited as compared to the case of bulk samples. Our theory is generalizable to ferroelectrics of other shapes as well as magnets and multiferroics.     

Spin-reorientation phase transition on the surface and in the bulk of α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> single crystals

Direct comparison of the properties of a thin surface layer and the bulk of macroscopic hematite (α-Fe₂O₃) crystals was used to study the magnetic structure of the surface layer and the bulk and the processes attendant on spin-reorientation phase transition (SRT). The investigation tool was simultaneous γ-ray, X-ray, and electronic Mössbauer spectroscopy, which enabled us to study the bulk and surface properties of macroscopic samples simultaneously and to compare them directly. Direct evidence of the existence of a surface “transition layer” on hematite crystals is obtained. The existence of this layer was suggested and described by Krinchik and Zubov [JETP 69, 707 (1975)]. The study in the SRT region showed that (1) the Morin SRT in the crystal bulk occurs in a jump (as a first-order phase transition), whereas in the surface layer of about 200 nm thick, some smoothness appears in the mechanism of magnetic-moment reorientation; (2) SRT in the surface layer, as in the bulk, involves an intermediate state in which low-and high-temperature phases coexist; and (3) SRT in the surface layer occurs at a temperature several degrees higher than in the bulk. Our experimental evidence on the SRT mechanism in the surface layer correlates with the inferences from phenomenological theory developed by Kaganov [JETP 79, 1544 (1980)].     

Optical measurements of field-induced phenomena of the magnetic phase transition in quasi 2D MnS layers grown by MBE

We used a sensitive optical method to study the magnetic phase transition of antiferromagnetic MnS layers. The method is applicable for very small numbers of spins, e.g., thin single layers. We studied the optical and magnetic properties of MnS layers using the internal optical transition of the manganese 3d-shell. The temperature dependence of the Mn-emission exhibits a pronounced minimum revealing the para- to anti-ferromagnetic phase transition. The MnS layers were grown by molecular beam epitaxy, embedded between diamagnetic ZnSe cladding layers on a (100)-GaAs substrate. It was found that the Néel-temperature itself is influenced by the biaxial strain and can be changed in an external magnetic field in case of quasi 2D MnS-layers. The phase diagram reveals a weak Ising like anisotropic contribution in case of a 1.8 nm thin layer, whereas a 8.6 nm thick layer behaves still like an ideal isotropic Heisenberg system.     

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.     

The effects of uniaxial stress distribution on the ferroelectric properties of thin films with first-order phase transition

Based on the Landau theory, the first-order phase transition properties of ferroelectric thin films have been studied by taking into account uniaxial stress distribution effects. The stress is supposed to decrease from interface to surface exponentially according to the experimental results. It is shown that tensile stress decreases the polarization and the Curie temperature while compressive stress increases the polarization and the Curie temperature. A stress-driven phase transition is found at the critical stress. Our prediction is compared with the available experiment results.