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Spontaneous Polarization (spontaneous + polarization)
Selected AbstractsDomain Engineering of Lead-Free Li-Modified (K,Na)NbO3 Polycrystals with Highly Enhanced PiezoelectricityADVANCED FUNCTIONAL MATERIALS, Issue 12 2010Ke Wang Abstract Aging and re-poling induced enhancement of piezoelectricity are found in (K,Na)NbO3 (KNN)-based lead-free piezoelectric ceramics. For a compositionally optimized Li-doped composition, its piezoelectric coefficient d33 can be increased up to 324 pC N,1 even from a considerably high value (190 pC N,1) by means of a re-poling treatment after room-temperature aging. Such a high d33 value is only reachable in KNN ceramics with complicated modifications using Ta and Sb dopants. High-angle X-ray diffraction analysis reveals apparent changes in the crystallographic orientations related to a 90° domain switching before and after the aging and re-poling process. A possible mechanism considering both defect migration and rotation of spontaneous polarization explains the experimental results. The present study provides a general approach towards piezoelectric response enhancement in KNN-based piezoelectric ceramics. [source] Photoresponsive Ferroelectric Liquid-Crystalline PolymersADVANCED FUNCTIONAL MATERIALS, Issue 1 2007P. Beyer Abstract The photoresponse of ferroelectric smectic side-chain liquid-crystalline (LC) polymers containing a photoisomerizable azobenzene derivative as a covalently linked photochromic side group is investigated. By static measurements in different photostationary states, the effect of trans,cis isomerization on the material's phase-transition temperatures and its ferroelectric properties (spontaneous electric polarization PS and director tilt angle ,) are analyzed. It turns out that the Curie temperature (transition SC* to SA) can be reversibly shifted by up to 17,°C. The molecular mechanism of this "photoferroelectric effect" is studied in detail using time-resolved measurements of the dye's optical absorbance, the director tilt angle, and the spontaneous polarization, which show a direct response of the ferroelectric parameters to the molecular isomerization. The kinetics of the thermal reisomerization of the azo dye in the LC matrix are evaluated. A comparison to the reisomerization reaction in isotropic solution (toluene) reveals a faster thermal relaxation of the dye in the LC phase. [source] Understanding the Nature of Ultrafast Polarization Dynamics of Ferroelectric Memory in the Multiferroic BiFeO3ADVANCED MATERIALS, Issue 28 2009Dhanvir Singh Rana The crystallographic anisotropy in electromagnetic terahertz radiation from multiferroic BiFeO3 (100), (110), and (111) films suggests the ultrafast depolarization of ferroelectric order as a new mechanism of terahertz emission. This implies that the spontaneous polarization in ferroelectric materials can be manipulated in time scale of picoseconds, thus paving the way to ultrafast data-storage devices based on nonvolatile ferroelectric memories. [source] Microstructure,Dielectric Properties Relationship in Ba0.6Sr0.4TiO3,Mg2SiO4,Al2O3 Composite CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2010Ying Chen 0.60Ba0.6Sr0.4TiO3(BST),(0.40,x)Mg2SiO4(MS),xAl2O3 (x=0, 0.5, 3, 5wt%) composite ceramics exhibit excellent characteristics suitable for tunable device applications. With increasing amount of Al, the dielectric peak can be quantitatively broadened and suppressed; the "phase transition temperature"Tc or (Tm) shifts to a lower temperature. Meanwhile, the tunability is still high in a wider temperature range. Far from Tc, pyroelectric effects are observed by using the Byer and Roundy technology and Slim polarization hysteresis loops are observed under high ac dielectric field at 10Hz. These proved the existence of spontaneous polarization in certain possible orientations in a broad temperature range above Tc in the paraelectric medium and reveal why 0.60BST,(0.40,x)MS,xAl2O3 have such remarkable dielectric nonlinearity. [source] Selective floating gate non-volatile paper memory transistorPHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 9 2009Rodrigo Martins Abstract Here we report the performance of a selective floating gate (VGS) n-type non-volatile memory paper field-effect transistor. The paper dielectric exhibits a spontaneous polarization of about 1 mCm,2 and GIZO and IZO amorphous oxides are used respectively as the channel and the gate layers. The drain and source regions are based in continuous conductive thin films that promote the integration of fibres coated with the active semiconductor. The floating memory transistor writes, reads and erases the stored information with retention times above 14500 h, and is selective (for VGS > 5 ± 0.1 V). That is, to erase stored information a symmetric pulse to the one used to write must be utilized, allowing to store in the same space different information. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] In-plane polarization of GaN-based heterostructures with arbitrary crystal orientationPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 10 2010Q. Y. Wei Abstract The total polarization fields of pseudomorphic InxGa1,xN/GaN and AlxGa1,xN/GaN heterostructures with 0,,,x,,,0.4 have been calculated as a function of the crystal orientation. Especial attention is placed on the direction and magnitude of in-plane piezoelectric polarization, which is not negligible for the non-polar and semi-polar growth. For an arbitrary crystal orientation, the piezoelectric polarization prevails in the InGaN/GaN system while the spontaneous polarization prevails in the AlGaN/GaN system. The in-plane potential due to polarization fields in non-polar epilayers is found to depend on the degree of planarity of the heterojunctions, and on the respective lateral dimensions. [source] Relaxor-based thin film memories and the depolarizing field problemPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 6 2007Manuel I. Marqués Abstract A simple model for a thin film memory based on a first neighbor interacting model is studied in detail. We have found that the minimum possible value for the thickness (D) as a function of the lateral size of the memory (L), the screening of the charges at the substrate (S) and the strength of the ferroelectric interaction (J), in order to obtain spontaneous polarization is D = SL /2J. We propose a new mechanism to obtain miniaturization of thin film memories to a single layer based on the use of relaxor ferroelectrics instead of regular ferroelectrics. Under the hypothesis of an internal organization of the random fields inside the nanofilm we show analytically how it should be possible to miniaturize the memory to a width as small as D = 1 for any value of L, J and S. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Photoreflectance studiesof N- and Ga-face AlGaN/GaN heterostructures confininga polarisation induced 2DEGPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003A. T. Winzer Abstract Photoreflectance measurements have been carried out in order to determine the electric field strength F within the topmost layers of Ga-face polarity AlxGa1,xN/GaN and N-face polarity GaN/AlxGa1,xN/GaN heterostructures containing high-mobile polarisation induced 2DEGs. For both types of samples F decreased from 400 kV/cm at room temperature up to 200 kV/cm when cooling down the structure to T = 5 K. Our results strongly emphasise the existence of surface donor and surface acceptor states of the Ga- and N-face samples, respectively. The temperature dependence of F is explained by the change of the piezoelectric and spontaneous polarization. From self-consistent conduction band calculations the bare surface potential was obtained. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electro-optic property of ZnO:Mn epitaxial filmsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2008T. Oshio Abstract Mn doping effect on the dielectric and electro-optic (E-O) property of ZnO epitaxial films was studied. The leakage current of the ZnO film is drastically decreased by doping Mn with the concentration of 3-5 at%. Although, ZnO: 4 at% Mn with the lowest leakage current has the largest frequency dispersion of birefringence shift (, n) at 1 kHz, and its E-O coefficient (rc) was larger than that of ZnO single crystal. It is presumably caused by another polarization effect but spontaneous polarization, such as space charge. To investigate the origin of the dispersion of , n, discharging property was evaluated. When the signal at 1 kHz was applied, ZnO: 3 and 5 at% Mn showed the discharging with relaxation time of 5 msec. Furthermore, the discharging property with same relaxation time was observed at 20 kHz at where the dispersion of , n was not observed. However, ZnO: 4 at% Mn does not show such a discharging behavior. Therefore, it is considered that the space charge is not the origin of the dispersion of , n. On the other hand, the dispersion of , n was minimum and no dielectric dispersion was observed at 20 kHz for all samples. The rc of ZnO: 3, 4 and 5 at% Mn measured at 20 kHz, which should be the rc originated from the spontaneous dipolar polarization, were derived to be 0.81, 0.81 and 0.62 pm/V, respectively. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Piezoelectric Field Influence on GaN/AlxGa1,xN Quantum Well Optical PropertiesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2003S. Fanget Abstract The absorption and luminescence properties of hetero-polarization GaN/AlxGa1,xN (x = 0.12 and 0.165) quantum well (QW) structures are studied by photoreflectivity, photoluminescence excitation spectroscopy (PLE), and photoluminescence at low temperature. The QW transition energy as a function of well thickness exhibits a quantum-confined Stark effect (QCSE) due to the presence of a strong built-in electric field (piezoelectricity and spontaneous polarization). An electric field strength of 120 kV/cm in the barrier and between 600 and 800 kV/cm in the well are obtained from the analysis of Franz-Keldysh oscillations in photoreflectivity spectra. These values are in good agreement with results from the fit of the QW transition energy versus the thickness, using the electric field as a parameter. [source] Simulation of polycrystalline ferroelectrics based on discrete orientation distribution functionsPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2005Ingo Kurzhöfer Ferroelectric materials exhibit a spontaneous polarization, which can be reversed by an applied electric field of sufficient magnitude. The resulting nonlinearities are expressed by characteristic dielectric and butterfly hysteresis loops. These effects are correlated to the structure of the crystal and especially to the axis of spontaneous polarization in case of single crystals. We start with a representative meso scale, where the domains consist of unit cells with equal spontaneous polarization. Each domain is modeled within a coordinate invariant formulation for an assumed transversely isotropic material as presented in [10], in this context see also [8]. In this investigation we obtain the macroscopic polycrystalline quantities via a simple homogenization procedure, where discrete orientation distribution functions are used to approximate the different domains. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Photorefractive effect of ferroelectric liquid crystalsTHE CHEMICAL RECORD, Issue 1 2006Takeo Sasaki Abstract This paper reviews our recent work on the photorefractive effect of ferroelectric liquid crystals (FLCs). The photorefractive effect is defined as the optical modulation of the refractive index of a medium as a result of a variety of processes. The interference of two laser beams in a photorefractive material establishes a refractive index grating. This phenomenon enables the creation of different types of photonic applications. FLCs exhibit fast electric field response, and the orientation of the molecular axis of FLCs changes its direction according to the change in direction of the spontaneous polarization (Ps). When two laser beams interfere in a photoconductive FLC, an orientational grating is formed. The mechanism of the formation of the grating is based on the response of the Ps to the photoinduced internal electric field. The time of formation of the refractive index grating is significantly shorter in FLC materials. © 2006 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 6: 43,51; 2006: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20071 [source] Monoclinic PZN-8%PT [Pb(Zn0.3066Nb0.6133Ti0.08)O3] at 4,KACTA CRYSTALLOGRAPHICA SECTION C, Issue 12 2007Jennifer S. Forrester The structure of the relaxor ferroelectric Pb(Zn0.3066Nb0.6133Ti0.08)O3 (lead zinc niobium titanium trioxide), known as PZN-8%PT, was determined at 4,K from very high resolution neutron powder diffraction data. The material is known for its extraordinary piezoelectric properties, which are closely linked to the structure. Pseudo-cubic lattice parameters have led to considerable controversy over the symmetry of the structure. We find the structure to be monoclinic in the space group Cm (No. 8), with the Zn, Nb and Ti cations sharing the octahedrally coordinated B site (site symmetry m, special position 2a) and Pb occupying the 12-coordinate A site (site symmetry m, special position 2a). O atoms occupy a disorted octahedron around the B site (site symmetry m and special position 2a, and site symmetry 1 and general position 4b). Atomic coordinates have been determined for the first time, allowing the direction of spontaneous polarization to be visualized. 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