Antiferroelectric Coupling Research Articles

Effects of Electric Fields on Bilayer Ferroelectric Films

The Landau-Devonshire free energy expression for a bilayer ferroelectric film and Landau Khalatnikov equation of motion are used to study the polarization reversal of bilayer ferroelectric film. Antiferroelectric interlayer coupling is considered in the free energy expression of the bilayer film. The free energy in each layer of the bilayer film is considered to be constant and surface imperfection is ignored for simplicity of calculations in this preliminary study. In the application of a step field, effects of layer thicknesses, temperature and the interfacial coupling constant J of the bilayer film on the evolution of polarization, switching current and switching time are investigated.

Effects of Depolarization Field and Interfacial Coupling on the Polarization of Ferroelectric Bilayers

A ferroelectric bilayer model considering depolarization field and interfacial coupling is proposed and the expression of the depolarization field is derived. The spatial profiles of spontaneous polarization and hysteresis loops are calculated using the numerical method with and without considering the depolarization field. The effects of the depolarization field and interfacial coupling on the polarization of second-order ferroelectric bilayers are studied systematically. When interfacial coupling is ferroelectric coupling, the interface spontaneous polarization increases and the area of hysteresis loop becomes larger with increasing coupling. When interfacial coupling is antiferroelectric coupling, the depolarization field makes the central loop become smaller and the shape of the hysteresis loop becomes steep. Meanwhile, as interfacial coupling increases, the outer loops stretch further out horizontally and the size of the central loop widens.

An electron diffraction and Monte Carlo simulation study of an incommensurate antiferroelectric state in the relaxor ferroelectric Pb2ScTaO6

Incommensurate satellite reflections modulating along the 〈110〉* directions have been observed in the electron diffraction patterns of single crystals of the relaxor ferroelectric Pb2ScTaO6(PST) recordedviatransmission electron microscopy. The satellites occur characteristically within a specific temperature range and display differing or variable modulation vectors relative to their primary reflections. The satellites represent a weak frustrated antiferroelectric state in PST, termed the incommensurate antiferroelectric (IAFE) state. The observed IAFE state coexists with the ferroelectric and paraelectric states within a specific temperature regime and is dynamic in nature, meaning that the dispositions of the satellites can be altered by varying the temperature applied to the crystal,in-situin the transmission electron microscope. The observed satellites are associated with thin, needle-shaped, closely packed striated domains of about 5–15 nm in width. The satellites appear exclusively in crystals of PST with an advanced degree of 1:1 chemical long-range order, exceeding 90%. The satellites and their domains are interpreted as originating from a displacive, antiferroelectric coupling of the ions, driven in particular by the Pb ions. The Monte Carlo (MC) method was used exhaustively to evaluate the structural regimes that lead to the occurrence of the IAFE state. In the MC simulations, the displacements were correlated with the ferroelectric and antiferroelectric couplings, resulting in the IAFE domains and their associated satellites of differing dispositions or modulation vectors. The results of the MC simulations agree well with the electron diffraction observations, supporting the model of an antiferroelectric displacement with an incommensurate modulation of the Pb ions in the 〈110〉* directions.

Frustration of Magnetic and Ferroelectric Long-Range Order in Bi2Mn4/3Ni2/3O6

The slight incommensurate modulation of the structure of Bi(2)Mn(4/3)Ni(2/3)O(6) is sufficient to suppress the electrical polarization which arises in commensurate treatments of the structure, due to antiferroelectric coupling of local polar units of over 900 A(3). The incommensurate structure is produced by the competition between ferroelectric Bi lone pair-driven A site displacement, chemical order of Mn and Ni on the B site, and both charge and orbital order at these transition metals. The interplay between the frustrated polar Bi displacements and the frustrated spin order at the B site, induced by positional disorder, produces magnetodielectric coupling between the incommensurately modulated lattice and the spin-glass-like ground state with an unusual relationship between the magnetocapacitance and the applied field.

Hysteresis loops of first-order ferroelectric bilayers or superlattices and their size effect

Based on Landau–Ginzburg phenomenological theory, a first-order ferroelectric bilayer or superlattice with an antiferroelectric interface coupling has been studied by taking into account the spatial variation of polarization within each constituent film and the surface effect. The hysteresis loops are obtained and the size effect of a bilayer is discussed. The loop patterns vary between typically antiferroelectric and typically ferroelectric depending on the thickness ratio, the coupling constant, the thickness and the extrapolation length. The antiferroelectric coupling has a great effect on the phase transition of a bilayer. It is shown that the size-driven phase transition cannot be observed in a ferroelectric bilayer in the case of strong antiferroelectric coupling.

Antiferroelectriclike polarization behavior in compositionally varying (1−x) Pb(Mg1∕3Nb2∕3)O3–(x) PbTiO3 multilayers

Compositionally varying multilayers of (1−x) Pb(Mg1∕3N2∕3)O3–(x) PbTiO3 were fabricated using pulsed laser ablation technique. An antiferroelectriclike polarization hysteresis was observed in these relaxor based multilayer systems. The competition among the intrinsic ferroelectric coupling in the relaxor ferroelectrics and the antiferroelectric coupling among the dipoles at the interface gives rise to an antiferroelectriclike polarization behavior. An increment in the coercive field and the applied field corresponding to the polarization flipping at low temperatures, provide further insight on the competition among the long-range ferroelectric interaction and the interfacial interaction in the polarization behavior of these relaxor multilayers.

The ferroelectric and cubic phases in BaTiO3 ferroelectrics are also antiferroelectric.

Using quantum mechanics (QM, Density Functional Theory) we show that all four phases of barium titanate (BaTiO(3)) have local Ti distortions toward 111 (an octahedral face). The stable rhombohedral phase has all distortions in phase (ferroelectric, FE), whereas higher temperature phases have antiferroelectric coupling (AFE) in one, two, or three dimensions (orthorhombic, tetragonal, cubic). This FE-AFE model from QM explains such puzzling aspects of these systems as the allowed Raman excitation observed for the cubic phase, the distortions toward 111 observed in the cubic phase using x-ray fine structure, the small transition entropies, the heavily damped soft phonon modes, and the strong diffuse x-ray scattering in all but the rhombohedral phase. In addition, we expect to see additional weak Bragg peaks at the face centers of the reciprocal lattice for the cubic phase. Similar FE-AFE descriptions are expected to occur for other FE materials. Accounting for this FE-AFE nature of these phases is expected to be important in accurately simulating the domain wall structures, energetics, and dynamics, which in turn may lead to the design of improved materials.

Polarization behavior in systems with competing ferroelectric and antiferroelectric interactions

AbstractThe dielectric hysteresis loop constitutes the main feature indicative of the properties of a ferroelectric material, but in complex systems with ferro‐ and antiferroelectric interactions, the hysteresis loops are often multiple. In the present work, we introduce a model based on the coexistence of ferro‐ and antiferroelectric regions, which can describe the aforementioned multiple loops. Based on our model, simulated multiple hysteresis loops are in good agreement with experimental results. Hysteresis loops obtained from our model are sensitive to the relative values of the ferro‐ and antiferroelectric coupling constants, domain size, and temperature. The model describes well systems without the presence of frustration as it is the case of BAxBP1–x. In the case of systems exhibiting frustration like e.g. Rbx(NH4)1–xH2PO4, we present a straightforward extension of our model. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pyroelectric and dielectric properties of a ferroelectric bilayer with an antiferroelectric interfacial coupling

The pyroelectric coefficient and susceptibility of a ferroelectric bilayer with an antiferroelectric material coupling are investigated using the transverse Ising model within the effective field theory with a probability distribution technique. The effects of the magnitude interfacial coupling and the transverse field on the pyroelectric coefficient and susceptibility of the bilayer are discussed. It is shown that there exists a negative peak of pyroelectric coefficient which has not been found in the bilayer or the superlattice with ferroelectric interfacial coupling. We find that the pyroelectric coefficient and the susceptibility increase with the decrease of the magnitude of the interfacial coupling before the phase transition. We also find that a large transverse field and weak interfacial coupling can lead to the disappearance of some of the peaks of the pyroelectric coefficient and susceptibility of the bilayer.

Ferroelectric and antiferroelectric coupling in superlattices of paraelectric perovskites at room temperature

Results from dielectric and structural measurements on epitaxial SrTiO$_3$/BaZrO$_3$ superlattices reveal properties that cannot be explained simply in terms of those measured on single films of the constituent materials. For large superlattice periodicities (20/20 and 38/38 structures, i.e. samples in which each SrTiO3 and BaZrO3 layer are 20 or 38 unit cells thick, respectively), the capacitance-voltage curves indicate room-temperature ferroelectricity. For smaller periodicities (7/7 and 15/15), anti-ferroelectric-type behavior is observed, suggesting strong coupling between individual polar layers. This is consistent with recent second-harmonic generation results [A.Q. Jiang et al., J. Appl. Phys. 93, 1180 (2003)] of ordering in SrTiO$_3$/BaTiO$_3$ superlattices. However, both constituents of the structures investigated here are paraelectric. Strain-induced room-temperature ferroelectricity in SrTiO$_3$ and distance-dependent coupling between these layers are proposed as mechanisms leading to the observed behavior.

Dielectric hysteresis loops of first-order ferroelectric bilayers and antiferroelectrics

We consider a ferroelectric bilayer or superlattice with an antiferroelectric interface coupling between two materials which have first-order phase transitions in the bulk. The system is described by a Landau free-energy expression including a bilinear coupling. A detailed discussion is given for the case when the component materials are the same. For equal thicknesses the free energy is the same as for a bulk first-order antiferroelectric. For this, analytical expressions are given for the supercooling, thermodynamic and superheating critical temperatures. Dielectric hysteresis loops are calculated numerically, and it is shown that they can be classified according to a simple scheme. A qualitative discussion is given of the effect of including biquadratic interface coupling. For a general thickness ratio in the bilayer, an analytical result is found for the supercooling temperature and the other two are calculated numerically. Typical dielectric hysteresis loops are shown, and a classification scheme is presented.

Intrinsic hysteresis loops in ferroelectric film systems

With the increasing emphasis on the applications of ferroelectric films in memory devices, it is relevant to calculate hysteresis loops for film systems. Here we present two calculations based on the Landau Devonshire free energy. First we deal with a ferroelectric bilayer with an assumed antiferroelectric coupling across the interface. The calculations apply equally to a superlattice with this coupling. We show that by variation of the ratio L 1/L 2 of the film thicknesses it is possible to engineer the hysteresis loop between a form similar to that of a bulk antiferroelectric for L 1/L 2 ≈ 1 and a ferroelectric form for L 1/L 2 ≫ 1. The bilayer model includes the simplifying assumption that the polarization is constant in each film. We go on to discuss inclusion in the hysteresis calculations of polarization variation P(z) within a film; for simplicity we deal with a single film. We show that the numerical scheme used for first-order films (Ong et al., these Proceedings) can be extended to include an applied field so that hysteresis loops can be calculated; some examples are given.

Hysteresis loops of ferroelectric bilayers and superlattices

A ferroelectric superlattice with an antiferroelectric interfacial coupling is considered; the same model describes a bilayer with antiferroelectric coupling. By mapping minimum points in the Landau free energy expression and plotting them against the applied electric field, a triple hysteresis loop pattern is obtained. The loop patterns vary between typically ferroelectric and typically antiferroelectric depending on the layer thicknesses and the magnitude of the interfacial-coupling constant. This work suggests the possibility of designing multilayer elements for computer memories with four or more different storage states.

Simulation of Switching in Antiferroelectric Liquid Crystals

We are simulating the dynamic response of surface stabilised antiferroelectric liquid crystal displays, based on a one-dimensional approach. This leads to more detailed modelling than before1. The evolution of director profiles and the resulting optical transmission under applied voltages are studied. The different switching modes between the alternating state and the two uniform states, observed in antiferroelectric test cells, can be understood fairly well. The model incorporates a chevron geometry. The different terms in the bulk energy function can be rationally interpreted. The alignment layers are modelled by a polar and a non-polar surface energy term. The influence of parameters, such as the antiferroelectric coupling parameter, the polar and non-polar anchoring strength, the elastic constant etc. are discussed.

A COMPARATIVE STUDY OF DIRECTOR PROFILES IN SSFLCS AND SSAFLCS

Abstract A theoretical model computing the equilibrium distribution and internal voltage of anti-ferroelectric and ferroelectric LCs in the surface stabilized geometry has been developed. All relevant physical parameters have been included. The behavior is different in either case. The internal structure of both liquid crystal phases as a function of the applied voltage, the compression modulus, and the spontaneous polarization has been studied for different values of the antiferroelectric coupling constant.

Homogeneous switching in antiferroelectric liquid crystal displays

Abstract We use a very simple bookshelf model to describe the behaviour of antiferroelectric liquid crystals in an electric field. Starting from the energy expression of the LC, which comprises antiferroelectric coupling, coupling with the electric field and surface anchoring, we can derive the dynamic equations. We carry out numerical simulations in order to solve these equations in different cases. We consider the three different switching modes: switching from the alternating state to one of the uniform states, switching from a uniform state to the alternating state, and switching between the two uniform states.

Grey level control through thickness variations in antiferroelectric LCD'S

Abstract Using a uniform bookshelf model, we can describe the behaviour of antiferroelectric liquid crystals in an electric field. In the integrated LC energy density, we make a distinction between two contributions: Bulk energy, which consists of antiferroelectric coupling and coupling with the electric field, and surface energy, comprising the area anchoring. As soon as the ratio of these contributions varies over the surface of a pixel, grey levels are possible. This can be achieved by a varying anchoring constant or a varying thickness. In this paper, we discuss the influence of thickness variations of the LC layer on the grey scale control.

Simulation of dynamic response to an electric field of surface stabilized antiferroelectric liquid crystals

Abstract Responses of surface stabilized antiferroelectric liquid crystals (SSAFLCs) to an electric field are simulated based on a model of the AFLC phase. The apparent tilt angle, the polarization reversal current and the optical transmission under applied triangular wave voltages are calculated. The results obtained systematically explain the experimental observations fairly well. The effects of parameters such as amplitude and frequency of applied voltages, an antiferroelectric coupling parameter and surface anchoring strength on these phenomena are discussed.

Noise in beta "-alumina solid electrolytes: Theory and experiment.

Measurements of noise in \ensuremath{\beta}'' aluminas (Na, Pb, Ag, and Ca) showing diffusion noise spectra, with high-frequency asymptote ${\mathrm{\ensuremath{\omega}}}^{\mathrm{\ensuremath{-}}3/2}$, have been reported by Brophy and co-workers since 1985, both for ceramics and single crystals. Comparison with the standard formulas by Burgess, Lax and Mengert, and Van Vliet and Fassett, showed that the observed noise was 9 to 12 orders of magnitude too high. In the present paper this discrepancy is addressed and removed. A detailed analysis is presented based on the two-dimensional planar diffusive motion of ions and defects in the Beevers-Ross (BR) and anti-Beevers-Ross sites of the conduction plane. Coulomb interaction in the plane is shown to have little effect due to screening; however, the antiferroelectric coupling, which gives rise to superlattice ordering as observed by Collin et al., causes correlated jumps within a coherence area, involving ambipolar motion of vacancy anti-BR sites, cation BR sites, and cation anti-BR sites. In addition, electrostatic induction via the spinel blocks couples the fluctuations in adjacent conduction planes. For Na-\ensuremath{\beta}'' and Pb-\ensuremath{\beta}'' single crystals quantitative agreement is obtained and the essential features of x-ray- and neutron-scattering data are confirmed.

Critical dielectric relaxation of highly deuterated betaine arsenate at microwave frequencies

The dielectric behavior of highly deuterated betaine arsenate in the frequency range up to 11 GHz is described by a Cole-Davidson relaxation. The characteristic dynamics show a critical slowing down near the para-antiferroelectric phase transition. The temperature dependence of the dielectric constant is analyzed in terms of a pseudo-one-dimensional Ising model. Evidence is given for a dependence of antiferroelectric coupling on deuteration which can explain the change from a ferro- to an antiferroelectric ordering on deuteration. Measurements of the dielectric relaxation under a dc bias field parallel to the critical axis elucidate the unusual phase transition behavior at low temperatures. A multicritical point appears in theE-T-phase diagram. Frustration effects due to competing ferro- and antiferroelectric interactions are not seen.