Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S20: Focus Session: Nanoscale Ferroelectrics, Switching, and Domains |
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Sponsoring Units: DMP Chair: Ho Nyung Lee, Oak Ridge National Laboratory Room: Colorado Convention Center 105 |
Wednesday, March 7, 2007 2:30PM - 3:06PM |
S20.00001: Probing nanoscale ferroelectricity by ultraviolet Raman spectroscopy Invited Speaker: Conventional vibrational spectroscopies operating in visible and infrared range fail to measure the phonon spectra of nanoscale ferroelectric structures because of extremely weak signals and the overwhelming substrate contribution. In this talk, application of ultraviolet (UV) Raman spectroscopy for studies of lattice dynamics and ferroelectric phase transitions in nanoscale ferroelectrics will be presented. We demonstrate that UV Raman spectroscopy is an effective technique allowing the observation of phonons and determination of the ferroelectric phase transition temperature ($T_{c})$ in nanoscale ferroelectrics, specifically, BaTiO$_{3}$/SrTiO$_{3}$ superlattices having the ferroelectric BaTiO$_{3}$ layers as thin as 1 unit cell, and single BaTiO$_{3}$ layers as thin as 4~nm. BaTiO$_{3}$/SrTiO$_{3}$ superlattices and ultrathin BaTiO$_{3}$ films studied were grown by molecular beam epitaxy on SrTiO$_{3}$ as well as GdScO$_{3}$ and DyScO$_{3}$ substrates. Excellent epitaxial quality and atomically abrupt interfaces are evidenced by X-ray diffraction and high resolution transmission electron microscopy. UV Raman results show that one-unit-cell thick BaTiO$_{3}$ layers in BaTiO$_{3}$/SrTiO$_{3}$ superlattices are ferroelectric with the $T_{c}$ as high as 250 K, and induce the polarization in much thicker SrTiO$_{3}$ layers adjacent to them. The $T_{c}$ in superlattices was tuned by hundreds of degrees from $\sim $170 to 650 K by varying the thicknesses of BaTiO$_{3}$ and SrTiO$_{3}$ layers. Using scandate substrates enables growth of superlattices with systematically changed coherent strain, thus allowing studying the stress effect on the ferroelectric phase transitions. UV Raman data are supported by the thermodynamic calculations of polarization in superlattices as a function of temperature. The work was done in collaboration with A. Soukiassian, W. Tian, D.G. Schlom, Y.L. Li, L.-Q. Chen, X.X. Xi (Pennsylvania State University), A. Bruchhausen, A. Fainstein (Centro Atomico Bariloche {\&} Instituto Balseiro, Argentina), R. S. Katiyar (University of Puerto Rico), A. Cantarero (University of Valencia, Spain), K.J. Choi, D.M. Kim, C.-B. Eom (University of Wisconsin), H.P. Sun, X.Q. Pan (University of Michigan), S.M. Nakhmanson; K.M. Rabe (Rutgers University), Q.X. Jia (Los Alamos National Laboratory) [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S20.00002: Abnormal retention behavior of Bi$_{3.25}$La$_{0.75}$Ti$_{3}$O$_{12}$ thin films observed by electrostatic force microscopy T.Y. Kim, J.H. Lee, H.R. Yoon, Y.J. Oh, M.R. Choi, W. Jo We report charge retention in c-axis oriented and preferentially (117) oriented ferroelectric Bi$_{3.25}$La$_{0.75}$Ti$_{3}$O$_{12}$ thin films by electrostatic force microscopy. Core-level electronic states and local ordering were examined by x-ray photoelectron spectroscopy and extended x-ray absorption fine structure, which are useful to interpret the unique retention behaviors in the films. Raman scattering spectroscopic studies were also used to look into phonon modes of the materials, which are occasionally difficult to understand due to other phases. Surface charges of the films were observed as a function of time in a selected area which consists of a single-poled region and a reverse-poled region. The (117) oriented film shows the extended exponential decay with characteristic scaling exponents, \textit{n $\sim $ }1.5. The preferentially $c$-axis oriented film shows retained behaviors regardless of the poling. Decay and retention mechanisms of the regions are explained by space-charge redistribution and trapping of defects in the films. [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S20.00003: Chemical Control of Ferroelectric Switching in PbTiO$_3$ Films Ruey-Ven Wang, Stephen Streiffer, Fan Jiang, Paul Fuoss, Dillon Fong, Jeffrey Eastman, G. Brian Stephenson, Kujtim Latifi, Carol Thompson Stabilization of monodomain polarization in ultrathin ferroelectric films can be accomplished via surface-adsorbed ions [Fong, D. D. \emph{et al}., \emph{Phys. Rev. Lett.} \textbf{96}, 127601/1-4 (2006)]. Here, we use in-situ grazing-incidence synchrotron x-ray scattering to study the ferroelectric polarization and surface structure of PbTiO$_3$ ferroelectric thin films as a function of vapor environment above the film surface. Coherent PbTiO$_3$ films of 10 nm thickness were grown on conducting SrRuO$_3$ on (001) SrTiO$_3$ substrates. We observe that the polarization direction in the PbTiO$_3$ film can be reversed by changing between oxidizing and reducing atmospheres, and the piezoelectric butterfly loop can be traced out as a function of oxygen partial pressure. Additionally, a new surface reconstruction is observed under reducing conditions. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S20.00004: Difference in ferroelectric aging between A-site and B-site acceptor doped BaTiO$_{3}$ crystals. Lixue Zhang, Xiaobing Ren Aging, the time-dependent changing of material properties, has been widely found in acceptor-doped ABO$_{3}$ ferroelectrics. The origin is usually ascribed to gradual domain stabilization by acceptor-dopant-generated oxygen vacancies. As in ABO$_{3}$ systems both A-site and B-site acceptor doping can induce oxygen vacancies, they are expected to cause similar aging effect. However, here we report that there exists a significant difference in aging effect between A-site (K-doped) and B-site (Mn-doped) acceptor-doped BaTiO$_{3}$ crystals. The B-site acceptor doping has much stronger aging effect. This new phenomenon can be fully explained by a semi-quantitative model based on the defect symmetry principle$^{1-4}$. According to this model, the ``strength'' of aging is determined by a symmetry-conforming force of the defect symmetry to crystal symmetry. This model may also have potential applications in predicting and understanding the strength of the aging effect in other systems. [1] X. Ren, Nat. Mater., 3:91, 2004; [2] L.X. Zhang, W. Chen and X. Ren, Appl. Phys. Lett., 85:5658, 2004; [3-4] L.X. Zhang and X. Ren, Phys. Rev. B, 71:174108, 2005; Phys. Rev. B 73:094121, 2006. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S20.00005: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S20.00006: High Speed Nanoscale Ferroelectric Domain Reading and Writing Ramesh Nath, Ramamoorthy Ramesh, Bryan Huey Piezo Force Microscopy is commonly employed for nanoscale studies of ferroelectric thin films, providing images of local domain orientation and piezoelectric properties. However, applications of PFM to dynamic studies are limited because image acquisition times are long (typically $>$100 sec.). A recent variation in Atomic Force Microscopy, High Speed Scanning Property Measurements, overcomes this challenge by allowing image frame rates on the order of one second, for image sizes from nanometers to tens of micrometers. For epitaxial thin films of PZT and BiFeO3, domain nucleation and growth is statistically studied at the nanoscale based on hundreds of images acquired at time steps of one second. For uniform films homogeneous exponential domain growth is observed, while heterogeneous domain growth is detected at epitaxial PZT grain boundaries. Finally, individual domain reading and writing is achieved at the highest tip speeds reported, beyond 1 centimeter/second, revealing a two-stage relationship between domain size and tip speed discussed experimentally and theoretically. [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S20.00007: Switching domain dynamics in ferroelectric thin films. Alexei Grigoriev, Dal-Hyun Do, Rebecca Sichel, Paul Evans, Bernhard Adams, Eric Dufresne Polarization switching in ferroelectric materials is governed by the microscopic details of the nucleation and growth of polarization domains. The electric-field dependence of the density of domain nucleation and the domain wall velocity are largely unknown. Using time-resolved x-ray microdiffraction, we have explored the switching dynamics of thin ferroelectric films over a wide range of applied electric fields, starting from the coercive field and ranging up to the maximum field allowed by the thin film capacitors. By separating dynamics of nucleation and domain wall motion we can study the relationship between these two phenomena and their relative contributions to the polarization switching process. [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S20.00008: Coexistence of polar order and local domain dynamics in ferroelectric SrTi$^{18}$O$_3$ Annette Bussmann-Holder, Helmut Buettner, Alan Bishop Perovskite oxide ferroelectrics show classical soft mode behaviour typical for the onset of a homogeneous long-range polar state and a displacive phase transition. Besides these long wave length properties, local effects are observed by different probes which reveal that dynamical symmetry breaking already takes place far above the actual instability. It is shown here that displacive mean-field type dynamics can indeed \underline{coexist} with local dynamical symmetry breaking. [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S20.00009: Ferroelastic domain dynamics in polydomain, epitaxial BaTiO$_{3}$ thin films Anthony Meier, Bruce Wessels The dynamics of 90$^{o}$-domain switching in polydomain, epitaxial barium titanate thin films were studied using the linear electro-optic effect. Co-planar electrodes were deposited on the film surface and bias was applied in the plane of the film for poling. Upon application of a bias pulse, the E-field driven electro-optic response increased to its saturation value within the 28 ns rise time of the measurement system. Upon removal of the bias pulse, a slow decay of the electro-optic response due to strain-driven relaxation of the ferroelastic domains was observed. Measured relaxation time constants ranged from 5 to 17 ms, exhibiting a power law dependence on the applied E-field amplitude given by $\tau $ = AE$^{m }$ with m = 0.98-1.23. X-ray diffraction measurements indicated that under a steady state in-plane bias voltage, the $a$-domain fraction increased while both the $c$- and $a$-domain surface normal lattice parameters increased due to the in-plane compressive strains that result from the 90$^{o}$ flipping of $c$-domains. X-ray diffraction measurements after removal of the bias voltage were indistinguishable from those prior to application of the bias voltage indicating that the ferroelastic domains had relaxed back to their original state. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S20.00010: Ferroelectric domain dynamics under an external field Andrew Rappe, Young-Han Shin, Ilya Grinberg, I-Wei Chen Ferroelectric oxides with the perovskite structure are promising materials for nonvolatile random access computer memories. PbZr$_{1-x}$Ti$_x$O$_3$ is currently used for this purpose. In these materials, storage of a bit involves the reorientation of polarization, or the movement of a ferroelectric domain wall. However, the intrinsic properties of the polarization reversal process of ferroelectrics at the microscopic level still have not been revealed, either by experiments or computations. In this talk, I will show how this problem can be studied with a multi-scale approach. First, an interatomic potential is parameterized to first-principles calculations, and molecular dynamics (MD) simulations are performed. Second, stochastic Monte Carlo simulations are conducted, with nucleation and growth rates extracted from the MD simulations. For PbTiO$_{3}$, we find that while the overall domain-wall speed from our calculation is in good agreement with the recent experiments, the size of the critical nucleus is much smaller than predicted from the Miller-Weinreich model. We think that this discrepancy can be explained by a diffuse-boundary model and by the fact that the overall wall motion is controlled by both the nucleation and growth processes. [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S20.00011: Ferroelectric instabilities in CaTiO3 nanoparticles from first principles Shen Li, Karin Rabe Ferroelectric instabilities of nanoparticles are expected to be markedly different from those of the bulk material. In many cases, ferroelectricity could be weakened or suppressed, although there is no clear reason why this should always be the case. Previous first-principles studies have shown that in bulk cubic CaTiO3, the polar instability is suppressed by the stronger oxygen octahedron-rotational instabilities, yielding a nonpolar ground state. To investigate the possibility that the nanoparticle configuration could weaken or eliminate the octahedron rotation, we performed first-principles calculations for a single-unit-cell cluster, containing one oxygen octahedron, using a real-space pseudopotential density-functional-theory method (PARSEC). For an electrically isolated cluster, the symmetric nonpolar state is found to be stable. However, if the depolarization field produced by a polar distortion of the cluster is screened, a lower-symmetry polar distorted structure becomes more favorable, so that the cluster can be considered ferroelectric. Our results are consistent with the recent findings regarding the central importance of compensation of the depolarization field in the ferroelectricity of perovskite oxide thin films and nanostructures. [Preview Abstract] |
Wednesday, March 7, 2007 5:06PM - 5:18PM |
S20.00012: A single electric relaxation time in $Ba_{1-x}Sr_{x}TiO_{3}$ nanoparticles at low temperatures Liyuan Zhang, Jun Zhou, Zhonglin Wang, Dragomir Davidovic It is shown that the dielectric response of $Ba_{0.77}Sr_{0.23}TiO_{3}$ nanoparticles at temperatures below 200K has a frequency and temperature dependence in agreement with the Debye theory with a single relaxation time, which exhibits the Arrhenius law. By contrast, at temperature above 210K the dielectric response exhibits broad range of relaxation times characteristic of relaxor-ferroelectrics. We suggest that the single relaxation time at low temperature indicates frustrated ferroelectricity, analogous to frustrated antiferromagnetism. [Preview Abstract] |
Wednesday, March 7, 2007 5:18PM - 5:30PM |
S20.00013: Size Effect on Ferroelectric Transitions in Nanograin Barium Titanate Polycrystals I-Wei Chen, Yudi Wang, Tieyu Sun, Xiaohui Wang Data of dielectric constants and polarization of $<$100 nm BaTiO$_{3}$ are now available which allows a definitive assessment of the origin of the size effect in these ceramics of multiple polarization transitions. There are three effects to be considered. First, when the grain size is below 500 nm, the ferroelectric transition is not accompanied by the formation of multiple domain walls, implicating a large residual stress that causes an increase in the temperatures of subsequent tetragonal/orthorhombic and orthorhombic/rhombohedral transitions. Second, when the grain size of BaTiO$_{3}$ is below 50 nm, screening of grain boundary charge due to defect segregation is ineffective, implicating a large internal field even above T$_{c}$. Such a field shifts the temperatures of different transitions differently, favoring the phase of a larger polarization. Third, the dead layer at the grain boundary that clamps the polarization is significant when the grain size decreases to a few nm. It uniformly lowers all the transitions by the same temperature. These effects on transition temperatures and dielectric constant will be compared with the experimental data to assess their relative importance. [Preview Abstract] |
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