Dynamic hysteresis loop in a ferroelectric heterostructure

A Landau–Devonshire theory in combination with Landau–Khalatnikov dynamic equation has been firstly used to study the dynamic hysteresis loop of a ferroelectric heterostructure consisting of two different films. The surface transition layer within each component film and an antiferroelectric coupling at the interface between two films are considered. A parameter β is introduced to describe the differences of physical properties between two constituent films. The influence of parameterβ, surface transition layer, antiferroelectric coupling and electric field frequency on the dynamic hysteresis loop of the ferroelectric heterostructure is discussed in detail. The results show that the system can exhibit antiferroelectriclike behavior (i.e., multi-loop hysteresis) through tuning some critical factors.     

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.     

Ti-doping induced antiferroelectric to ferroelectric phase transition and electrical properties in Sm-PbZrO3 thin films

The antiferroelectric (Pb_0.985Sm_0.01) (Zr_1-xTi_x)O₃ (Ti-PSZO) thin films were synthesized on Pt(111)/Ti/SiO₂/Si substrates using a chemical solution deposition method. The films were crystallized in the perovskite phase with a preferential orientation along (111) direction. With Ti doping in PSZO, a gradual transformation from antiferroelectric to ferroelectric phase transition was noticed at room temperature owing to the Ti doping induced lattice distortion. The phase transition has been confirmed through the P - E hysteresis loops, X-ray diffraction (peak shifting), capacitance-voltage measurements, and Raman scattering analysis. The thin film with Ti = 0.15 doping displayed a ferroelectric behavior with high dielectric constant and large dielectric tunability of about 62%. Also, Ti doping altered the Curie temperature (T_c) and enhanced the order of dielectric diffuseness. It is believed that Ti-doping in PSZO is an effective way to induce an antiferroelectric - ferroelectric phase transition and to tailor the electrical characteristics of PSZO thin films.     

Dielectric susceptibility of a ferroelectric bilayer film with surface transition layers

Using the transverse Ising model within the framework of the mean-field theory, we investigate a ferroelectric bilayer film with the surface transition layer within each constituent slab and an antiferroelectric interfacial coupling between two slabs. The combined influence of the surface transition layer and antiferroelectric interfacial coupling on the dielectric susceptibility of a bilayer film is discussed in detail. The results show that the surface transition layer plays a crucial role in dielectric susceptibility of a bilayer film.     

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.     

Nonlinear dielectric properties of planar structures based on ferroelectric betaine phosphite films

Ferroelectric films of partly deuterated betaine phosphite are grown on NdGaO₃(001) substrates with an interdigitated system of electrodes on their surfaces by evaporation at room temperature. These films have a high capacitance in the ferroelectric phase transition range. The dielectric nonlinearity of the grown structures is studied in small-signal and strong-signal response modes and in the intermediate region between these two modes by measuring the capacitance in a dc bias field, dielectric hysteresis loops, and the Fourier spectra of an output signal in the Sawyer-Tower circuit. In the phase transition range, the capacitance control ratio at a bias voltage U _bias = 40 V is K ? 7. The dielectric nonlinearity of the structures in the paraelectric phase is described by the Landau theory of second-order phase transitions. The additional contribution to the nonlinearity in the ferroelectric phase is related to the motion of domain walls and manifests itself when the input signal amplitude is higher than U _st ～ 0.7–1.0 V. The relaxation times of domain walls are determined from an analysis of the frequency dependences of the dielectric hysteresis.     

Antiferroelectric films of deuterated betaine phosphate

Thin films of partially deuterated betaine phosphate have been grown by the evaporation on Al₂O₃(110) and NdGaO₃(001) substrates with a preliminarily deposited structure of interdigitated electrodes. The grown films have a polycrystalline block structure with characteristic dimensions of blocks of the order of 0.1–1.5 mm. The degree of deuteration of the films D varies in the range of 20–50%. It has been found that, at the antiferroelectric phase transition temperature T_c^afe = 100–114 K, the fabricated structures exhibit an anomaly of the electrical capacitance C, which is not accompanied by a change in the dielectric loss tangent tanδ. The strong-signal dielectric response is characterized by the appearance of a ferroelectric nonlinearity at temperatures T >T_c^afe, which is transformed into an antiferroelectric nonlinearity at T <T_c^afe. With a further decrease in the temperature, double dielectric hysteresis loops appear in the antiferroelectric phase. The dielectric properties of the films have been described within the framework of the Landau-type thermodynamic model taking into account the biquadratic coupling ξP²η² between the polar order parameter P and the nonpolar order parameter η with a positive coefficient ξ. The E–T phase diagram has been constructed.     

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.     

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.     

The hysteresis loops of a ferroelectric bilayer film with surface transition layers

Based on the transverse Ising model in the framework of the mean field approximation,this paper discusses a ferroelectric bilayer film with the surface transition layers within each constituent slab and an antiferroelectric interfacial coupling between two slabs.The hysteresis loop of a bilayer film is investigated.The results show that the surface transition layer in a ferroelectric bilayer film plays a significant role in realizing the multiple-state memory.     

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.     

Effect of dielectric layer on ferroelectric responses of P(VDF-TrFE) thin films

The dielectric layer in the sandwich structural device plays a very important role in determining the electrical properties of the ferroelectric film. In this paper, we investigate the effect of the dielectric layers with different thicknesses on switching performance of ferroelectric P(VDF-TrFE) thin films. The hysteresis loops become slanting with increasing thickness of the dielectric layer. A negative slope of the ‘real’ hysteresis loop is apparently observed which demonstrates negative capacitance effect caused by the dielectric layer. This behavior is simulated qualitatively by the Weiss mean field model considering an interfacial dielectric layer in series with a ferroelectric layer. The agreement between experiments and simulations supports that negative capacitance results from the positive feedback among electric dipoles. Furthermore, the switching time of the ferroelectric film increases with the increase of dielectric layer thickness. This study shows that the ferroelectric sandwich structure provides great potential towards low power negative capacitance devices.     

Surface effects and diffusive phase transition in ferroelectric thin films: A self-consistent approach

A self-consistent Landau phenomenological approach has been used to study the ferroelectric transition in films in the presence of various surface effects such as depolarization and strain. The polarization distribution of the film is computed and its variation with respect to temperature, thickness and strain is determined. The gradual decrease in polarization across the transition shows the diffusive behavior which is confirmed from the soft mode and the dielectric susceptibility analysis. The critical thickness below which ferroelectricity disappears is also computed. The degree of diffuseness in the transition is obtained from the susceptibility exponent which shows more and more diffusive behavior for smaller and smaller film thickness.     

Thermodynamic modeling of critical properties of ferroelectric superlattices in nano-scale

Modeling nano-scale ferroelectric superlattices using the Landau free-energy functional approach requires incorporating contributions from the interfacial and depolarization field effects. The choice of the order parameter then becomes a vital issue. In this paper, we compare the predictions of models using the spontaneous polarization as order parameter (SPOP approach) with models using the total polarization as order parameter (TPOP approach). We have comprehensively calculated the critical properties of nano-scale ferroelectric superlattices, such as the phase-transition temperature, critical thickness and Curie–Weiss-type relation using both approaches. We found that all the SPOP results are in excellent agreement with experimental measurements and first-principle calculations in all cases studied here. The TPOP approach, on the other hand, much overestimates the depolarization by underestimating the effect of the dielectric screening and produces results that deviate significantly from the experimental ones. Our results also traced the dependence of the critical properties on the thicknesses of the constituent layers of the ferroelectric superlattices to the interfacial and depolarization field effects.     

Dielectric studies of phase transitions in the ferroelectric CdTiO3 and the Sr1−x CdxTiO3 solid solution

Temperature dependences of the permittivity and of the dielectric hysteresis loops in ceramic samples of nominally pure CdTiO₃ and a Sr_1?x Cd_xTiO₃ solid solution were studied. At 76.5±0.5 K, CdTiO₃ was established to undergo a ferroelectric phase transition close to the tricritical point. The temperature dependence of spontaneous polarization of CdTiO₃ is described within the Landau theory of phase transitions with the critical order parameter exponent ≈0.25. The phase diagram of the Sr_1?x Cd_xTiO₃ solid solution was drawn in (T, x) coordinates, and the critical concentration x _c =0.002, above which an induced polar state sets in the solid solution, was determined.     

Lead titanate ferroelectric films on single-crystal silicon

This paper reports on the results of investigations into the phase transformations observed in Pb/Ti/Si and Ti/Pb/Si thin-film heterostructures upon layer-by-layer magnetron sputtering of lead and titanium onto a single-crystal silicon substrate and subsequent annealing in an oxygen atmosphere. It is shown that the dielectric properties of lead titanate films depend on the order of sputtering of lead and titanium metal layers onto the surface of single-crystal silicon. The ferroelectric properties are revealed in 3000-nm-thick lead titanate films prepared by two-stage annealing of the Pb/Ti/Si thin-film heterostructure (with the upper lead layer) at T ₁=473 K and T ₂=973 K for 10 min. These films are characterized by the coercive field E _c=4.8 kV/cm and the spontaneous polarization P _s=16.8 μC/cm². The lead titanate films produced by annealing of the Ti/Pb/Si thin-film heterostructure (with the upper titanium layer) do not possess ferroelectric properties but exhibit properties of a conventional dielectric. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 4, 2002, pp. 745–749. Original Russian Text Copyright ? 2002 by Sidorkin, Sigov, Khoviv, Yatsenko, Logacheva.     

Pyroelectric and electromechanical properties of the antiferroelectric liquid crystal MHPOBC

We study pyroelectric and electromechanical effects in the prototype antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl-phenyl)4′-octylbiphenil-4-carboxylate (MHPOBC). The linear electromechanical effect in the freely suspended liquid crystal films of MHPOBC has been detected in the broad temperature range inclusive of the antiferroelectric SmC _A ^* as well as paraelectric SmA. The anomalous behavior of the hysteresis loop of SmC _β ^* in the (pyroelectric coefficient, dc bias electric voltage) coordinates has been found.     

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.     

Au/PZT/BIT/p-Si异质结的制备与性能研究

采用脉冲激光沉积(PLD)工艺,制备了以Bi₄Ti₃O₁₂(BIT)为过渡阻挡层的Au/PZT/BIT/p-Si异质结.研究了BIT铁电层对Pb(Zr_0.52Ti_0.48)O₃(PZT)薄膜晶相结构、铁电及介电性能的影响,对Au/PZT/BIT/p-Si异质结的导电机制进行了讨论.氧气氛530℃淀积的PZT为多晶铁电薄膜,与直接淀积在Si基片上相比,加入BIT铁电层后PZT铁 关键词： 铁电薄膜 异质结构 脉冲激光沉积(PLD)     

Geometrical effects in nanodimensional epitaxial films of barium strontium titanate

The effect of the thickness of 6- to 950-nm-thick Ba_0.8Sr_0.2TiO₃ films epitaxially grown on (001)MgO on their ferroelectric properties is investigated. Raman scattering and X-ray diffraction data indicate a structural phase transition taking place at a film thickness of ≈70 nm, which changes drastically the lattice parameters of the film. Raman spectra taken of the films confirm that they are in the ferroelectric state and that their symmetry changes in going over a critical thickness, as revealed by a sharp displacement of the peaks corresponding to the A ₁(TO) and E(TO) components of the soft mode. At film thicknesses of <100 nm, the permittivity versus thickness dependence exhibits two peaks at thicknesses of ～18 and ～36 nm. Near the first peak, the dielectric nonlinearity is considerably higher than near the second one.