Showing papers on "Ferroelectricity published in 2007"

Multiferroics: a magnetic twist for ferroelectricity

Abstract: Magnetism and ferroelectricity are essential to many forms of current technology, and the quest for multiferroic materials, where these two phenomena are intimately coupled, is of great technological and fundamental importance. Ferroelectricity and magnetism tend to be mutually exclusive and interact weakly with each other when they coexist. The exciting new development is the discovery that even a weak magnetoelectric interaction can lead to spectacular cross-coupling effects when it induces electric polarization in a magnetically ordered state. Such magnetic ferroelectricity, showing an unprecedented sensitivity to ap plied magnetic fields, occurs in 'frustrated magnets' with competing interactions between spins and complex magnetic orders. We summarize key experimental findings and the current theoretical understanding of these phenomena, which have great potential for tuneable multifunctional devices.

Applications of Modern Ferroelectrics

Abstract: Long viewed as a topic in classical physics, ferroelectricity can be described by a quantum mechanical ab initio theory. Thin-film nanoscale device structures integrated onto Si chips have made inroads into the semiconductor industry. Recent prototype applications include ultrafast switching, cheap room-temperature magnetic-field detectors, piezoelectric nanotubes for microfluidic systems, electrocaloric coolers for computers, phased-array radar, and three-dimensional trenched capacitors for dynamic random access memories. Terabit-per-square-inch ferroelectric arrays of lead zirconate titanate have been reported on Pt nanowire interconnects and nanorings with 5-nanometer diameters. Finally, electron emission from ferroelectrics yields cheap, high-power microwave devices and miniature x-ray and neutron sources.

Lead-free piezoelectric ceramics: Alternatives for PZT?

Abstract: Investigations in the development of lead-free piezoelectric ceramics have recently claimed comparable properties to the lead-based ferroelectric perovskites, represented by Pb(Zr,Ti)O3, or PZT In this work, the scientific and technical impact of these materials is contrasted with the various families of “soft” and “hard” PZTs On the scientific front, the intrinsic nature of the dielectric and piezoelectric properties are presented in relation to their respective Curie temperatures (T C) and the existence of a morphotropic phase boundary (MPB) Analogous to PZT, enhanced properties are noted for MPB compositions in the (Na,Bi)TiO3-BaTiO3 and ternary system with (K,Bi)TiO3, but offer properties significantly lower The consequences of a ferroelectric to antiferroelectric transition well below T C further limits their usefulness Though comparable with respect to T C, the high levels of piezoelectricity reported in the (K,Na)NbO3 family are the result of enhanced polarizability associated with the orthorhombic-tetragonal polymorphic phase transition being compositionally shifted downward As expected, the properties are strongly temperature dependent, while degradation occurs through the thermal cycling between the two distinct ferroelectric domain states Extrinsic contributions arising from domains and domain wall mobility were determined using high field strain and polarization measurements The concept of “soft” and “hard” lead-free piezoelectrics were discussed in relation to donor and acceptor modified PZTs, respectively Technologically, the lead-free materials are discussed in relation to general applications, including sensors, actuators and ultrasound transducers

Tunnel junctions with multiferroic barriers

Abstract: Multiferroics are singular materials that can exhibit simultaneously electric and magnetic orders. Some are ferroelectric and ferromagnetic and provide the opportunity to encode information in electric polarization and magnetization to obtain four logic states. However, such materials are rare and schemes allowing a simple electrical readout of these states have not been demonstrated in the same device. Here, we show that films of La0.1Bi0.9MnO3 (LBMO) are ferromagnetic and ferroelectric, and retain both ferroic properties down to a thickness of 2 nm. We have integrated such ultrathin multiferroic films as barriers in spin-filter-type tunnel junctions that exploit the magnetic and ferroelectric degrees of freedom of LBMO. Whereas ferromagnetism permits read operations reminiscent of magnetic random access memories (MRAM), the electrical switching evokes a ferroelectric RAM write operation. Significantly, our device does not require the destructive ferroelectric readout, and therefore represents an advance over the original four-state memory concept based on multiferroics.

Strain Tuning of Ferroelectric Thin Films

Abstract: Predictions and measurements of the effect of biaxial strain on the properties of epitaxial ferroelectric thin films and superlattices are reviewed. Results for single-layer ferroelectric films of biaxially strained SrTiO3, BaTiO3, and PbTiO3 as well as PbTiO3/SrTiO3 and BaTiO3/SrTiO3 superlattices are described. Theoretical ap- proaches, including first principles, thermodynamic analysis, and phase-field models, are applied to these biaxially strained materials, the assumptions and limitations of each technique are explained, and the predictions are compared. Measurements of the effect of biax- ial strain on the paraelectric-to-ferroelectric transition temperature (TC) are shown, demonstrating the ability of percent-level strains to shift TC by hundreds of degrees in agreement with the predic- tions that predated such experiments. Along the way, important ex- perimental techniques for characterizing the properties of strained ferroelectric thin films and superlattices, as well as appropriate sub- strates on which to grow them, are mentioned.

Giant sharp and persistent converse magnetoelectric effects in multiferroic epitaxial heterostructures.

Abstract: Magnetoelectric coupling between magnetic and electrical properties presents valuable degrees of freedom for applications. The two most promising scenarios are magnetic-field sensors that could replace low-temperature superconducting quantum interference devices, and electric-write magnetic-read memory devices that combine the best of ferroelectric and magnetic random-access memory. The former scenario requires magnetically induced continuous and reversible changes in electrical polarization. These are commonly observed, but the coupling constants thus obtained are invalid for data-storage applications, where the more difficult to achieve and rarely studied magnetic response to an electric field is required. Here, we demonstrate electrically induced giant, sharp and persistent magnetic changes (up to 2.3 x 10(-7) s m(-1)) at a single epitaxial interface in ferromagnetic 40 nm La(0.67)Sr(0.33)MnO(3) films on 0.5 mm ferroelectric BaTiO(3) substrates. X-ray diffraction confirms strain coupling via ferroelastic non-180( composite function) BaTiO(3) domains. Our findings are valid over a wide range of temperatures including room temperature, and should inspire further study with single epitaxial interfaces.

Very large spontaneous electric polarization in BiFeO3 single crystals at room temperature and its evolution under cycling fields

Abstract: Electric polarization loops are measured at room temperature on highly pure BiFeO3 single crystals synthesized by a flux growth method. Because the crystals have a high electrical resistivity, the resulting low leakage currents allow the authors to measure a large spontaneous polarization in excess of 100μCcm−2, a value never reported in the bulk. During electric cycling, the slow degradation of the material leads to an evolution of the hysteresis curves eventually preventing full saturation of the crystals.

Strain-gradient-induced polarization in SrTiO3 single crystals.

Abstract: Piezoelectricity is inherent only in noncentrosymmetric materials, but a piezoelectric response can also be obtained in centrosymmetric crystals if subjected to inhomogeneous deformation. This phenomenon, known as flexoelectricity, can significantly affect the functional properties of insulators, particularly thin films of high permittivity materials. We have measured strain-gradient-induced polarization in single crystals of paraelectric SrTiO3 as a function of temperature and orientation down to and below the 105 K phase transition. Estimates were obtained for all the components of the flexoelectric tensor, and calculations based on these indicate that local polarization around defects in SrTiO3 may exceed the largest ferroelectric polarizations. A sign reversal of the flexoelectric response detected below the phase transition suggests that the ferroelastic domain walls of SrTiO3 may be polar.

Unit-cell scale mapping of ferroelectricity and tetragonality in epitaxial ultrathin ferroelectric films

Abstract: Typically, polarization and strain in ferroelectric materials are coupled, leading to the generally accepted direct relation between polarization and unit-cell tetragonality. Here, by means of high-resolution transmission electron microscopy we map, on the unit-cell scale, the degree of tetragonality and the displacements of cations away from the centrosymmetry positions in an ultrathin epitaxial PbZr(0.2)Ti(0.8)O(3) film on a SrRuO(3) electrode layer deposited on a SrTiO(3) substrate. The lattice is highly tetragonal at the centre of the film, whereas it shows reduced tetragonality close to the interfaces. Most strikingly, we find that the maximum off-centre displacements for the central area of the film do not scale with the tetragonality. This challenges the fundamental belief in a strong polarization-tetragonality coupling in PbTiO(3)-based ferroelectrics, at such thicknesses. Furthermore, a systematic reduction of the atomic displacements is measured at the interfaces, suggesting that interface-induced suppression of the ferroelectric polarization plays a critical role in the size effect of nanoscale ferroelectrics.

Spiral Magnets as Magnetoelectrics

Abstract: Magnetoelectric multiferroics is an old but emerging class of materials that combine coupled electric and magnetic dipole order. In these materials, ferroelectric and magnetic (ferromagnetic or antiferromagnetic) states coexist or compete with each other. The interaction leads to a so-called magnetoelectric effect, which is the induction of magnetization by an electric field or electric polarization by a magnetic field. In the past few years, a new set of magnetoelectric multiferroics such as TbMnO3 and Ni3V2O8 has been discovered. In these magnetoelectric multiferroics, ferroelectric order develops upon a magnetic phase transition into a spiral magnetic ordered phase. In addition, these systems show large magnetoelectric effects accompanied by metamagnetic transitions. Noncollinear spiral magnetism is the key to understanding the magnetoelectric properties in these systems. Here I discuss the magnetoelectric coupling in spiral magnets and review recent advances in the understanding of ferroelectr...

Processing, Structure, Properties, and Applications of PZT Thin Films

Abstract: There has been a resurgence of complex oxides of late owing to their ferroelectric and ferromagnetic properties. Although these properties had been recognized decades ago, the renewed interest stems from modern deposition techniques that can produce high quality materials and attractive proposed device concepts. In addition to their use on their own, the interest is building on the use of these materials in a stack also. Ferroelectrics are dielectric materials that have spontaneous polarization in certain temperature range and show nonlinear polarization–electric field dependence called a hysteresis loop. The outstanding properties of the ferroelectrics are due to non-centro-symmetric crystal structure resulting from slight distortion of the cubic perovskite structure. The ferroelectric materials are ferroelastic also in that a change in shape results in a change in the electric polarization (thus electric field) developed in the crystal and vice versa. Therefore they can be used to transform acoustic wav...

Band gap and Schottky barrier heights of multiferroic BiFeO3

Abstract: BiFeO3 is an interesting multiferroic oxide and a potentially important Pb-free ferroelectric. However, its applications can be limited by large leakage currents. Its band gap is calculated by the density-functional based screened exchange method to be 2.8eV, similar to experiment. The Schottky barrier height on Pt or SrRuO3 is calculated in the metal induced gap state model to be over 0.9eV. Thus, its leakage is not intrinsic.

Theory of Polarization: A Modern Approach

Abstract: In this Chapter we review the physical basis of the modern theory of polarization, emphasizing how the polarization can be defined in terms of the accumulated adiabatic flow of current occurring as a crystal is modified or deformed. We explain how the polarization is closely related to a Berry phase of the Bloch wavefunctions as the wavevector is carried across the Brillouin zone, or equivalently, to the centers of charge of Wannier functions constructed from the Bloch wavefunctions. A resulting feature of this formulation is that the polarization is formally defined only modulo a “quantum of polarization” – in other words, that the polarization may be regarded as a multi-valued quantity. We discuss the consequences of this theory for the physical understanding of ferroelectric materials, including polarization reversal, piezoelectric effects, and the appearance of polarization charges at surfaces and interfaces. In so doing, we give a few examples of realistic calculations of polarization-related quantities in perovskite ferroelectrics, illustrating how the present approach provides a robust and powerful foundation for modern computational studies of dielectric and ferroelectric materials.

Modified (K0.5Na0.5)NbO3 based lead-free piezoelectrics with broad temperature usage range

Abstract: (K0.5Na0.5)NbO3 (KNN) based lead-free materials exhibit arguably, comparable piezoelectric properties to conventional Pb(Zr,Ti)O3 ceramics owing to an orthorhombic to tetragonal polymorphic phase transition (TO-T) occurring near room temperature. However, this transition correspondingly results in a strong temperature dependence of the dielectric and piezoelectric properties, being limited further by domain instability, during thermal cycling between the two ferroelectric phases. Analogous to BaTiO3 based piezoelectrics, the addition of CaTiO3 in KNN materials was found to shift the TO-T well below room temperature. Piezoelectric and electromechanical values of KNN–LiSbO3–CaTiO3 material were found to be d33∼210pC∕N, d15∼268pC∕N and k33∼61%, k15∼56%, respectively, with greatly improved temperature stability over the temperature range of −50–200°C, demonstrating practical potential for actuator and/or ultrasonic transducer applications.

Raman and infrared spectra of multiferroic bismuth ferrite from first principles

Abstract: The entire zone-center phonon spectrum of the $R3c$ ferroelectric antiferromagnetic phase of bismuth ferrite is computed using a first-principles approach based on density functional theory. Two phonon modes exhibiting eigendisplacement vectors that strongly overlap with the atomic distortions taking place at the ferroelectric structural phase transition are identified and give support to a transition with displacive character. Both Raman and infrared reflectivity spectra are also computed, providing benchmark theoretical results for the assignment of experimental spectra.

Phase structures and electrical properties of new lead-free (Na0.5K0.5)NbO3–(Bi0.5Na0.5)TiO3 ceramics

Abstract: Highly dense (1−x)(Na0.5K0.5)NbO3–x(Bi0.5Na0.5)TiO3 (NKN-BST) solid solution piezoelectric ceramics have been fabricated by ordinary sintering. All compositions show pure perovskite structures, showing room-temperature symmetries of orthorhombic at x⩽0.02, of tetragonal at 0.03⩽x⩽0.09, of cubic at 0.09 0.20. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal ferroelectric phases was identified in the composition range of 0.02

Ferroelectricity Induced by Proper-Screw Type Magnetic Order

Abstract: A novel type of magnetic origin of ferroelectric polarization is proposed. In crystals with triclinic, monoclinic, or trigonal (rhombohedral) symmetries, the proper screw type of magnetic ordering can induce ferroelectricity through the variation in the metal–ligand hybridization with spin–orbit coupling. The ferroelectric polarization in delafossite compounds Cu(Fe,Al)O 2 is successfully explained by the mechanism. The relation between the polarization direction and the magnetic modulation vector is also predicted.

Ferromagnetism in nanoscale BiFeO3

Abstract: A remarkably high saturation magnetization of ∼0.4μB∕Fe along with room temperature ferromagnetic hysteresis loop has been observed in nanoscale (4–40nm) multiferroic BiFeO3 which in bulk form exhibits weak magnetization (∼0.02μB∕Fe) and an antiferromagnetic order. The magnetic hysteresis loops exhibit exchange bias and vertical asymmetry which could be because of spin pinning at the boundaries between ferromagnetic and antiferromagnetic domains. Interestingly, both the calorimetric and dielectric permittivity data in nanoscale BiFeO3 exhibit characteristic features at the magnetic transition point. These features establish the formation of a true ferromagnetic-ferroelectric system with a coupling between the respective order parameters in nanoscale BiFeO3.

Electrically assisted magnetic recording in multiferroic nanostructures.

Abstract: We demonstrate the room-temperature control of magnetization reversal with an electric field in an epitaxial nanostructure consisting of ferrimagnetic nanopillars embedded in a ferroelectric matrix. This was achieved by combining a weak, uniform magnetic field with the switching electric field to selectively switch pillars with only one magnetic configuration. On the basis of these experimental results, we propose to use an electric field to assist magnetic recording in multiferroic systems with high perpendicular magnetic anisotropy.

Domain wall contributions to the properties of piezoelectric thin films

Abstract: In bulk ferroelectric ceramics, extrinsic contributions associated with motion of domain walls and phase boundaries are a significant component of the measured dielectric and piezoelectric response. In thin films, the small grain sizes, substantial residual stresses, and the high concentration of point and line defects change the relative mobility of these boundaries. One of the consequences of this is that thin films typically act as hard piezoelectrics. This paper reviews the literature in this field, emphasizing the difference between the nonlinearities observed in the dielectric and piezoelectric properties of films. The effect of ac field excitation levels, dc bias fields, temperature, and applied mechanical stress are discussed.

Ferroelectricity Induced by Proper-Screw Type Magnetic Order(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

Abstract: A novel type of magnetic origin of ferroelectric polarization is proposed. In crystals with triclinic, monoclinic, or trigonal (rhombohedral) symmetries, the proper screw type of magnetic ordering can induce ferroelectricity through the variation in the metal–ligand hybridization with spin–orbit coupling. The ferroelectric polarization in delafossite compounds Cu(Fe,Al)O 2 is successfully explained by the mechanism. The relation between the polarization direction and the magnetic modulation vector is also predicted.

Dual nature of improper ferroelectricity in a magnetoelectric multiferroic.

Abstract: Using first-principles calculations, we study the microscopic origin of ferroelectricity (FE) induced by magnetic order in the orthorhombic HoMnO3. We obtain the largest ferroelectric polarization observed in the whole class of improper magnetic ferroelectrics to date. We find that the two proposed mechanisms for FE in multiferroics, lattice and electronic based, are simultaneously active in this compound: a large portion of the ferroelectric polarization arises due to quantum-mechanical effects of electron orbital polarization, in addition to the conventional polar atomic displacements. An interesting mechanism for switching the magnetoelectric domains by an electric field via a 180 degrees coherent rotation of Mn spins is also proposed.

Large bulk polarization and regular domain structure in ceramic BiFeO3

Abstract: Regularly twinned domain structures are observed by scanning piezoforce microscopy on single crystalline grains of BiFeO3 ceramics being grown by a special low temperature sintering process. The domains are considerably larger than those observed in thin films. Their spontaneous polarization comes close to that predicted theoretically and overcomes restrictions hitherto being set to bulk single crystals. The observed ferroelastic twin domain structure resembles that of classic T domains in rhombohedrally distorted NiO, but is additionally superimposed by ferroelectric twin domain patterns.

Domain Control in Multiferroic BiFeO3 through Substrate Vicinality

Abstract: Control over ferroelectric polarization variants in BiFeO3 films through the use of various vicinal SrTiO3 substrates is demonstrated. The ferroelectric polarization variants in these films are characterized by piezoelectric force microscopy and the corresponding structural variants are carefully analyzed and confirmed by X-ray diffraction. Implementation of this approach has given us the ability to create single domain BiFeO3 films on (001), (110), and (111) surfaces. The piezo/ ferroelectric properties of the BiFeO3 films, in turn, can be tailored through this approach. Such results are very promising for continued exploration of BiFeO3 films and provide a template for detailed multiferroic-coupling studies in the magnetoelectric BiFeO3 system. Magnetoelectric coupling in multiferroic materials has attracted much attention because of the intriguing science underpinning this phenomenon. Additionally, there is an exciting potential for applications and devices that take advantage of these materials with multiple order parameters. BiFeO3 (BFO) is a room temperature, single-phase magnetoelectric multiferroic with a ferroelectric Curie temperature of ∼ 1103 K and an antiferromagnetic Neel temperature of ∼ 643 K. Recent studies of BFO thin films have shown the existence of a large ferroelectric polarization, as well as a small net magnetization of the Dzyaloshinskii-Moriya type resulting from a canting of the antiferromagnetic sublattice. The ferroelectric polarization in BFO can have orientations along the four cube diagonals ( ), and the direction of the polarization can be changed by ferroelectric and ferroelastic switching. Our previous studies have shown coupling between ferroelectricity and antiferromagnetism in BFO thin films resulting from the coupling of both antiferromagnetic and ferroelectric domains to the underlying ferroelastic domain switching events. Such a study was a crucial first step in the exploration of approaches to control and manipulate magnetic properties using an electric field. It was also noted, however, that these films exhibit a very complicated domain structure, which complicates the interpretation of the fundamental properties of this system as well as the interactions across hetero-interfaces. The lack of large single crystals of the desired crystallographic orientation provokes another motivation to explore approaches to create “single crystalline” epitaxial films that are free of ferroelectric/ferroelastic domains. Recent studies have explored the ability to control the ferroelectric domain configuration, which is formed after the phase transformation, through substrate engineering. In this study, we demonstrate an approach to control the ferroelectric domain structure in BFO films through the use of vicinal SrTiO3 (STO) substrates. This has enabled us to create thin films that “mimic” the primary crystal facets of the pseudo-cubic unit cell, namely single domain (100), (110), and (111) surfaces. The ferroelectric domain structure of an epitaxial BFO film on STO substrates with different orientations can be modeled using the phase-field method in which the spatial distribution of the polarization field and its evolution is described by the time dependent Ginzburg–Landau (TDGL) equations. For a BFO film grown on a (001)-oriented perovskite substrate, there are eight possible ferroelectric polarization directions corresponding to the four structural variants of the rhombohedral phase (Fig. 1a). In this case, the individual domains are energetically degenerate, and domain structures with twinning are expected in order to relax the elastic energy of the film. Experimentally, however, only the downward directed polarization variants are observed, indicating the existence of a self-poling effect (as a consequence of the bottom electrode). Figure 1b shows an in-plane (IP) piezoforce microscopy (PFM) image of a BFO film grown on (001) STO substrate. The three contrast levels observed in the IP-PFM images acquired along the two orthogonal directions, together with the uniform out-of-plane (OP) PFM contrast (not shown), indicate that the domain structure of the BFO films is characterized by four polarization variants. The C O M M U N IC A TI O N

Domain switching kinetics in disordered ferroelectric thin films.

Abstract: We investigated domain kinetics by measuring the polarization switching behaviors of (111)-preferred polycrystalline $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_{3}$ films, which are widely used in ferroelectric memories. Their switching behaviors at various electric fields and temperatures could be explained by assuming the Lorentzian distribution of logarithmic domain-switching times. We suggested that the local field variation due to dipole defects at domain pinning sites could explain the Lorentzian distribution.

Ferroelectric to relaxor crossover and dielectric phase diagram in the BaTiO3–BaSnO3 system

Abstract: The (1−x)BaTiO3–xBaSnO3 (0⩽x⩽0.30) perovskite solid solution ceramics were prepared by solid state reaction and studied by dielectric spectroscopy. The complex dielectric permittivity was measured as a function of frequency (0.1Hz–100kHz) in the temperature (T) range of 123–573K. The transition from the high-temperature paraelectric state where the dielectric constant obeys the Curie-Weiss law to the ergodic cluster state is found to occur at the same temperature of 485K in all the compositions of x⩾0.04 and at lower temperatures in those with a smaller x. For 0⩽x⩽xc=0.19, the temperature of the dielectric peak Tm, corresponding to the diffuse transition from the ergodic polar cluster state to the ferroelectric state, decreases with increasing x and does not depend on frequency. The diffuseness of the peak gradually increases. For x>xc, the permittivity exhibits relaxor behavior with the frequency-dependent Tm satisfying the Vogel-Fulcher law. The temperature variation of the permittivity on the high-temp...

Dielectric properties and relaxor behavior of rare-earth (La, Sm, Eu, Dy, Y) substituted barium zirconium titanate ceramics

Abstract: Based on the Ti-vacancy defect compensation model, (Ba1−xLnx)Zr0.2Ti0.8−x∕4O3 (Ln=La,Sm,Eu,Dy,Y) ceramics have been fabricated via the conventional solid-state reaction method. The microstructures, dielectric properties, and ferroelectric relaxor behavior of (Ba1−xLnx)Zr0.2Ti0.8−x∕4O3 ceramics have been investigated. The results indicate that rare-earth ions with various ionic radii enter the unit cell to substitute for A-site Ba2+ ions and inhibit the grain growth. The typical ferroelectric relaxor behavior is induced due to the rare-earth ions substitution. The diffuseness of the phase transition and the degree of ferroelectric relaxor behavior are enhanced, the TC is remarkably shifted to lower temperature, and the tunability is suppressed with the increase of x value and substituted ionic radius for (Ba1−xLnx)Zr0.2Ti0.8−x∕4O3 (x=0.005–0.04, Ln=La,Sm,Eu,Dy,Y) ceramics. Tunable ferroelectric materials with moderate dielectric constant and low dielectric loss can be obtained by manipulating the doping am...

AgNbO3: A lead-free material with large polarization and electromechanical response

Abstract: Polarization measurements reveal that AgNbO3 has an extremely large polarization, which can reach a value of 52μC∕cm2 in polycrystals. Experiments also show that the large internal atom distortion in AgNbO3 is also strongly coupled to the electric field, indicating that high piezoelectric performance can be realized in AgNbO3 system. This finding opens the way to designing a new class of lead-free, high-performance piezoelectric materials based on AgNbO3.

Dynamical magnetoelectric coupling in helical magnets.

Abstract: We develop a theory of collective mode dynamics in the helical magnets coupled to electric polarization via spin-orbit interaction. The low-lying modes associated with the ferroelectricity are not the transverse optical phonons, but are the spin waves hybridized with the electric polarization. This hybridization leads to the Drude-like dielectric function $\ensuremath{\epsilon}(\ensuremath{\omega})$ in the limit of zero magnetic anisotropy. There are two additional low-lying modes: phason of the spiral and rotation of helical plane along the polarization axis. Role of these low-lying modes in the neutron scattering and antiferromagnetic resonance is revealed, and a novel experiment to detect the dynamical magnetoelectric coupling is discussed.