The effect of stresses on self-polarization of thin ferroelectric films

The effect of stresses, appearing due to a difference between the temperature coefficients of linear expansion of a substrate and ferroelectric film, on the self-polarization is discussed using thin films of lead zirconate-titanate PbZr_xTi_1?xO₃ (PZT) of different compositions as an example. It is assumed that the nature of self-polarization is connected with internal polarizing electric fields caused by the different density of charged surface states at the ferroelectric-layer interfaces, while tensile or compressive stresses are able only to change the polarization orientation, which causes the self-polarization to increase or decrease in magnitude. The problem of improving the efficiency of PZT films in infrared radiation detectors and memory devices is considered.     

应力作用下EuTiO3铁电薄膜电热效应的唯象理论研究

基于Laudau-Devonshire的热动力学模型, 计算了EuTiO₃铁电薄膜材料的电热效应. 结果显示在外加应力的调控下, 电极化、电热系数以及绝热温差都会随之变化. 外加垂直于表面的张应力加大, 薄膜的相变温度升高, 绝热温差增加, 最大绝热温差所对应的工作温度向高温区移动. 对于二维平面失配应变u_m =-0.005的薄膜, 当外加张应力σ₃ = 5 GPa时, 其最大电热系数为1.75×10^-3 C/m²·K, 电场变化200 MV/m 时室温下绝热温差ΔT 的最大值可达到14 K 以上, 绝热温差ΔT ≥13 K 的工作温区超过120 K, 表明可以通过调控外部应力来获取室温时较大的绝热温差. 此结果预示着铁电EuTiO₃ 薄膜在室温固态制冷方面可能具有较好的应用前景.     

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.     

Dielectric Properties of Nanoporous Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Films Filled with Ferroelectric SC(NH<Subscript>2</Subscript>)<Subscript>2</Subscript>

Temperature dependences of permittivity ε′ and third harmonic amplitude γ_3ω of nanocomposites obtained by embedding ferroelectric SC(NH₂)₂ in porous alumina films with pore sizes of 60 and 100 nm are studied. A substantial increase in the temperatures of ferroelectric phase transition T_c1 and T_c2 and that of phase transition T_i from incommensurate phase to paraphase are also observed. The temperatures of all phase transitions are found to rise as pore diameters shrink.     

Effects of oxygen partial pressure on the ferroelectric properties of pulsed laser deposited Ba0.8Sr0.2TiO3 thin films

The Ba_0.8Sr_0.2TiO₃ thin films were grown on the Pt–Si substrate at 700 °C by using a pulsed laser deposition technique at different oxygen partial pressure (PO₂) in the range of 1–20 Pa and their properties were investigated. It is observed that the PO₂ during the deposition plays an important role on the tetragonal distortion ratio, surface morphology, dielectric permittivity, ferroelectric polarization, switching response, and leakage currents of the films. With an increase in PO₂, the in-plane strain for the BST films changes from tensile to compressive. The films grown at 7.5 Pa show the optimum dielectric and ferroelectric properties and also exhibit the good polarization stability. It is assumed that a reasonable compressive strain, increasing the ionic displacement, and thus promotes the in-plane polarization in the field direction, could improve the dielectric permittivity. The butterfly features of the capacitance–voltage (C–V) characteristics and the bell shape curve in polarization current were attributed to the domain reversal process. The effect of pulse amplitude on the polarization reversal behavior of the BST films grown at PO₂ of 7.5 Pa was studied. The peak value of the polarization current shows exponential dependence on the electric field.     

Anomalies in the electrophysical, thermal, and elastic properties of layered ferroelectric semiconductor TlGaSe2: Instability in the electronic subsystem

The temperature dependences of the permittivity, dark current, and current-voltage characteristics of the layered ferroelectric semiconductor TlGaSe₂ were measured over the range 77–300 K. The ε(T) curve measured perpendicular to the layers at low frequencies is discovered for the first time to exhibit an anomalous dip with clearly defined boundaries at ～150 and ～200 K. Electrical instability in the form of low-frequency oscillations of current in current-voltage characteristics of a sample in the same temperature range are also observed experimentally for the first time. It is shown that this instability occurs only if an electric field is applied to a sample using potential-controlling contactless electrodes in the form of thin mica spacer layers. The nature of the instability and its influence on various physical properties of the layered TlGaSe₂ crystal are discussed.     

Effect of an electric field on the fracture of ferroelectric ceramic materials

This paper reports on the experimental results of measuring the time elapsed between the loading and the fracture of ferroelectric ceramic specimens under the action of a static electric field and mechanical stresses that differ in magnitude. The dependence of the durability of the specimens on the applied stress is determined for electric fields in the range from 0 to 5 MV/m. It is shown that, in the time range 1–10³ s, the durability of the ferroelectric ceramic material substantially increases in weak electric fields (the hardening effect) and significantly decreases in strong electric fields. The results obtained can be explained in terms of the fact that the load and the electric field affect the same defects (fracture nuclei) in the ferroelectric ceramics.     

铁电薄膜的多外场调控

铁电薄膜在非易失随机存储器、压电器件、热释电器件和电光器件中有着广泛的应用。由于极化—晶格—电荷自由度的强烈耦合,铁电薄膜的特性不仅可以被电场调控,也可以被多种其他外场调控。文章分别阐述了电场、力场和光场对铁电薄膜极化和导电特性的调控作用及物理机制,并展示了电驱动、机械驱动和光驱动等新型铁电器件的应用潜力。多种外场的调控作用可以突破传统电场调控在电路接入和击穿、漏电等失效行为方面的限制,为铁电器件的设计提供新思路。     

Pyroelectric and photoelectric responses of capacitors based on thin PZT films

The influence of 0.63-μm laser radiation and an external electric field on the photo-and pyroelectric response of thin ferroelectric Pb(Zr_0.54Ti_0.46)O₃ (PZT) films is studied at various temperatures. The films are grown by RF magnetron sputtering of a PZT ceramic target on platinum-coated substrates. It is found that the photosensitivity of PZT films increases after switching of the applied static electric field, as well as after an increase in temperature.     

Evolution of domain processes during the transition from classical ferroelectric to relaxor ferroelectric

Dependences of the relative permittivity on the external bias electric field of ceramics of the yPZN-mPMN-nPNN-xPT (x = 0.25?C0.40) system have been studied. The evolution of domain processes during the concentration transition from classical ferroelectric to relaxor ferroelectric has been shown. Tentative conclusions on the mechanisms of electric field-induced transitions from the relaxor state to the ??normal?? ferroelectric state have been drawn.     

Electric-field-induced unwinding of ferroelectric helix in thin smectic C* layers with soft and rigid anchoring of molecules

The unwinding of a helical structure in thin films of a ferroelectric smectic liquid crystal (LC) by an external electric field has been theoretically studied using a discrete model in which every LC layer is characterized by a two-dimensional vector ξ _i (describing the orientation of molecules) and by the polarization P _i . It is established that the unwinding of the LC helix in thin films significantly differs from the well-known behavior of thick samples. In particular, discrete intermediate states (differing by an integer or half-integer number of turns) are formed in thin films for both weak and strong anchoring of molecules to a substrate surface. The physical factor responsible for this behavior is the presence of near-surface regions with thicknesses below the helix pitch and the corresponding uncompensated polarization.     

Room temperature ferroelectricity of tetragonally strained SrTiO3 thin films on single crystal Rh substrates

Epitaxial SrTiO₃ thin films were deposited on single crystalline Rh substrates by pulsed laser deposition. The tetragonally stained structure of the SrTiO₃ thin films with a c/a ratio of 1.04 was confirmed by x-ray diffraction experiments. The SrTiO₃ thin films exhibited good ferroelectric properties with a high remanent polarization (2P_r) of 8 μC/cm² and a canonical ferroelectric piezoresponse hysteresis loop at room temperature. We estimated a high activation electric field of about 6.4 MV/cm for domain wall creeping. This activation electric field is higher than that of typical ferroelectric materials such as PbTiO₃.     

Evolution of the optical properties of Pb<Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>Sc<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> (PBSN-6) solid solution single crystals caused by a static electric field

The electric-field-induced variation of the optical properties (small-angle light scattering, birefringence) of PBSN-6 solid solutions was studied. It was found that in the absence of an electric field, the cubic nonpolar matrix contains, at temperatures below the dielectric permittivity maximum, spontaneously polarized regions of the ferroelectric phase not less than 10⁴ Å in size. It was shown that a weak electric field (～0.4 kV/cm) is capable of inducing a kinetic phase transition to the ferroelectric state, with the temperature of this transformation depending on the sample heating rate. The destruction of the induced state was accompanied by a sharp peak in the temperature dependence of the small-angle light scattering intensity (indicating the percolation nature of the transition) and was independent of the sample heating rate. The boundaries of stability of the induced state in various modes of application of an external electric field were determined, and the E-T phase diagram was constructed.     

Optical, electrical, and photophysical properties of films of polycomplexes of azobenzene derivatives with cobalt

The absorption spectra, photoconductivity, and electrical conductivity of films of new polycomplexes of 4-methacyloyloxy-(4′-carboxy-3′-oxy)azobenzene and 4-methacryloyloxy-2-nitro-(4′-carboxy-3′-oxy)azobenzene with cobalt were investigated with switched-off and switched-on external electric fields. The effect of an external electric field on the transmission of linearly polarized light through films near the long-wavelength absorption edge is revealed. It is shown that the change in the light transmission under the action of an external electric field is due to the spatial reorientation of the dipole moments of azobenzene groups photoinduced by polarized light in an external electric field. The effect of an electric field on the light transmission is explained by the formation of forces acting on the dipole moments of azobenzene groups and on metal ions. The increase in the dipole moments of azobenzene groups as a result of introduction of nitro groups into their composition decreases the effect of cobalt ions on the electro-optical properties of polycomplex films.     

Electric-field-induced phase transition in the relaxor ceramics based on PMN-PT

The mechanisms of formation of a plateau in dependences of the reversible permittivity on the external electric field strength in a ceramic sample of the yPZN-mPMN-nPNN — xPT (x = 0, 3; y = 0.0982; m = 0.4541; n = 0.1477) system have been analyzed using X-ray diffraction and atomic force microscopy. It has been shown that, in electric fields corresponding to the permittivity anomaly, there occurs an induced phase transition from the heterophase (tetragonal + pseudocubic) state to the single-phase (tetragonal) state. This leads to significant strains of the tetragonal unit cell (up to 0.1%) and the termination of 90° domain switching. The proposed qualitative model of the formation of the observed dielectric anomaly is based on two processes occurring with an increase in the electric field strength: the growth of polar regions of the tetragonal phase and their switching.     

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.     

Equal-permittivity states induced in a ferroelectric

Conditions under which a ferroelectric subjected to an electric field acquires equal-permittivity paraelectric states at different temperatures are studied. The temperature dependences of the control (inducing) field intensity and feasible permittivity interval (ɛ_min-ɛ_max) are obtained. The effect of intersection of the ɛ(E, T _i ) characteristics of the ferroelectric in the paraelectric state is analyzed. With the control field and temperature varying consistently, the permittivity ɛ_i of strontium titanate films and films of the barium titanate-strontium titanate solid solution may be kept constant in a wide (200–320 K) temperature range and, at the same time, changed by more than twofold by varying the electric field.     

Electroclinic effect in free-standing smectic elastomer films

The coupling of the mesogenic tilt in smectic liquid crystals to external electric fields in the layer plane (electroclinic effect) provides an opportunity to control the smectic layer thickness with electric fields. In ordered ferroelectric smectic elastomers it is possible to achieve a macroscopic electromechanical response. The effect is particularly pronounced near the smectic A-to-smectic C^* phase-transition temperature. In this work, the electrostriction of weakly cross-linked smectic elastomers is studied by means of optical interferometry of thin films. Its observed magnitude corresponds to an induced tilt of approximately 7.7°, yielding a layer contraction of 1%, in a 3 MV/m electric field. This value is in agreement with earlier measurements of tilt susceptibility in a structurally similar polymer, but is considerably smaller than previously reported data on elastomers with a comparable chemical structure. PACS 61.30.Vx; 83.80.Va; 77.65.Bn     

外加电场对铁电薄膜相变的影响

采用热力学非线性理论,研究了外加电场对立方基底Pb(Zr_0.3Ti_0.7)O₃(PZT)铁电薄膜相变的影响.通过数值计算,得到了"失配应变-外加电场"相图,及外加电场与极化强度的关系.当外加电场达到186 kV/cm时,能使生长在SrTiO₃ 基底上PZT铁电薄膜从单斜r相转变为c相.在实验上,采用扫描探针显微镜通过对PZT薄膜施加不同的极化电场来研究了它的电畴翻转.从得到的压电响应相图可以看出,绝大多数的电畴是清晰可 关键词： 铁电薄膜 相变 扫描探针显微镜 失配应变     

Room temperature ferroelectric effect and enhanced dielectric permittivity in Rochelle salt/PVA percolative composite films

Rochelle salt (RS) filled polyvinyl alcohol (PVA) composite films have been prepared via a simple solution casting technique. The transport, dielectric and ferroelectric properties of the samples have been studied. The dielectric permittivity decreases slowly with increasing frequency and rise gradually with increasing temperature and RS contents in the composites. As the volume fraction of the RS reaches to percolation threshold (f_c ～0.0538), an abrupt increase in the dielectric permittivity (～403 almost 80 times higher compared to pure PVA with low loss ～0.18 at 1 kHz and room temperature) occurs in the RS/PVA composite film, which is attributed to the formation of the conductive network in the matrix. Ferroelectric loops up to room temperature (300 K) and the slight increase in Curie temperature from 297 to 300 K have also been observed for percolative composite film. The developed composite material with low loss high dielectric permittivity and room temperature ferroelectric behaviors might be applied in the technological fields.