Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C21: Focus Session: Domains, Switching, and Memristors |
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Sponsoring Units: DMP Chair: James Scott, University of Cambridge Room: 323 |
Monday, March 18, 2013 2:30PM - 2:42PM |
C21.00001: Phenomenological study of switching in strongly coupled multiferroics Kuntal Roy, Craig J. Fennie An ongoing challenge in the field of multiferroics is to understand new mechanisms and to realize new materials in which an electric field can deterministically switch the magnetization by 180$^{\mathrm{o}}$ at room temperature. One mechanism that has recently become of renewed interests~is that of ferroelectric-induced weak ferromagnetism, for which several new classes of materials have been proposed and found to be realizations of from first principles.~An open and challenging question concerning these systems is that of polarization-magnetization dynamics such as switching. In this talk we will discuss our initial work addressing this question. We apply a phenomenological approach, e.g., the Landau-Lifshitz-Gilbert equation for magnetization dynamics and dynamical systems equations for polarization to study the switching dynamics in single-phase multiferroic materials with strongly coupled polarization and magnetization. [Preview Abstract] |
Monday, March 18, 2013 2:42PM - 2:54PM |
C21.00002: Polarization switching dynamics in thin-film BaTiO$_{3}$/PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ bilayer capacitors Pavel Salev, Alexei Grigoriev In this work, we compare polarization switching and dielectric properties of single- (PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ (PZT)) and bi-layer (BaTiO$_{3}$/PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ (BTO/PZT)) ferroelectric thin-film materials. The ferroelectric films were grown by radio-frequency magnetron sputtering on SrRuO$_{3}$/SrTiO$_{3}$ (001) substrates. Pt top electrodes ranging in diameter from 50 um to 200 um were fabricated on top of ferroelectric films. Electrical measurements of switching dynamics and dielectric response revealed a significant difference in polarization switching between single- and bi-layer capacitors. Average remnant polarization in the bilayer was reduced to 60 uC/cm$^{2}$ from 90 uC/cm$^{2}$ polarization in a single layer capacitor, and the switching speed was reduced significantly. In this presentation, we will discuss effects of interfaces and polarization coupling on polarization dynamics and on the dielectric response in ferroelectric multilayers. [Preview Abstract] |
Monday, March 18, 2013 2:54PM - 3:06PM |
C21.00003: Domain Switching and Interaction with Misfit Dislocation in Bismuth Ferrite thin films: Phase-Field Simulation Khalid Ashraf, Sayeef Salahuddin Previously, we reported a massively parallel 3D phase-field model that can simulate micron scale ferroelectric thin films with arbitrary electrical and mechanical boundary conditions [1,2]. Using this model, we explained a number of recent experimental results of domain switching on various surfaces of BiFeO3 [1,2]. Here, we study the inhomogeneous strain distribution and its impact on domain nucleation and switching in the multi-ferroic material BiFeO3. We calculate the inhomogeneous strain induced in a thin film BiFeO3 due to misfit dislocations. We simulate the domain growth mechanism in films with and without misfit dislocation. We find that a compressively strained region in the film due to dislocation act as nucleation center whereas a tensile strained region act as pinning centers for lateral electric fields on the (001) surface of BFO. Both domain growth and relaxation speed are considerably higher close to a compressively strained region when the domain propagates perpendicular to the dislocation. An opposite trend is observed for domains relaxing along the dislocation. Our calculated domain propagation velocities are in excellent agreement with recently reported experiments.\\[4pt] [1] J. Heron, M Trassin, K Ashraf, et al., PRL, 89, 153504 (2011)\\[0pt] [2] K Ashraf, S. Salahuddin JAP,111(10), 103904 (2012).\\[0pt] [3] K Ashraf, S. Salahuddin JAP,112, 074102 (2012). [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:18PM |
C21.00004: Ferroelectric and multiferroic domain imaging by Laser-induced photoemission microscopy Anke Hoefer, Michael Fechner, Klaus Duncker, Ingrid Mertig, Wolf Widdra The ferroelectric as well as multiferroic surface domain structures of BaTiO3(001) and BiFeO3(001) are imaged based on photoemission electron microscopy (PEEM) by femtosecond laser threshold excitation under UHV conditions [1]. For well-prepared BaTiO3(001), three ferroelectric domain types are clearly discriminable due to work function differences. At room temperature, the surface domains resemble the known ferroelectric domain structure of the bulk. Upon heating above the Curie point of 400 K, the specific surface domain pattern remains up to 500 K [2]. Ab-initio calculations explain this observation by a remaining tetragonal distortion of the topmost unit cells stabilized by a surface relaxation. The (001) surface of the single-phase multiferroic BiFeO3 which is ferroelectric and antiferromagnetic, shows clear ferroelectric work function contrast in PEEM. Additionally, the multiferroic domains show significant linear dichroism. The observation of a varying dichroism for different ferroelectric domains can be explained based on the coupled ferroelectric-antiferromagnetic order in BiFeO3. It demonstrates multiferroic imaging of different domain types within a single, lab-based experiment. [1]Hoefer et al., IBM J. Res. Dev. 55, 4:1 (2011) [2]Hoefer et al., PRL 108, 087602 (2012) [Preview Abstract] |
Monday, March 18, 2013 3:18PM - 3:30PM |
C21.00005: Bloch-type domain walls in rhombohedral BaTiO$_3$ Maryam Taherinejad, David Vanderbilt, Pavel Marton, Vilgelmina Stepkova, Jiri Hlinka Ferroelectric domain walls (FDWs) are usually considered to be of Ising type, but there have been suggestions in recent years that Bloch-type FDWs, in which the polarization rotates in the plane of the FDW, are also possible. The mechanically compatible and electrically neutral FDWs in rhombohedral BaTiO$_3$ are of 71$^\circ$, 109$^\circ$, and 180$^\circ$ type. We have investigated these FDWs based both on first-principles calculations and on a Ginzburg-Landau-Devonshire (GLD) model.\footnote{ P. Marton, I. Rychetsky, and J. Hlinka, Phys. Rev. B \textbf{81}, 144125 (2010).} The results from both approaches confirm the Ising nature of the 71$^\circ$ FDW and the Bloch nature of the 180$^\circ$ FDW, and predict both Ising-type and Bloch-type FDWs are possible for the 109$^\circ$ case. Considering the relatively small rhombohedral strain in BaTiO$_3$, the competition between the energies of Bloch and Ising FDWs can be discussed in terms of a picture in which a Bloch wall is regarded as being composed of a pair of smaller-angle Ising ones. A reduction by 40\% in the parameters describing the gradient term in the GLD model brings it into better agreement with the first-principles results for detailed properties such as the energies and widths of the FDWs. [Preview Abstract] |
Monday, March 18, 2013 3:30PM - 3:42PM |
C21.00006: Domain wall roughness and creep behavior in nanoscale crystalline ferroelectric oxide and polymer films Zhiyong Xiao, Shashi Poddar, Stephen Ducharme, Xia Hong We have studied the static and dynamic properties of domain wall (DW) in nanoscale crystalline ferroelectric oxide Pb(Zr,Ti)O$_{3} $ (PZT) and poly(vinylidene-fluoride-trifluorethylene) (PVDF-TrFE) films of 20 to 40 nm thick using piezo-response force microscopy. DW roughness exponent $\zeta $ is extracted from the correlation function of DW displacement. At room temperature, $\zeta $ of PVDF-TrFE is 0.4 to 0.48, much higher than those obtained on the PZT films (0.2-0.3). Combined with the dynamic studies of the DW creep behavior, this yields an effective dimensionality of 1.5 for PVDF-TrFE films, in sharp contrast to $d_{eff} \sim $ 2.5 observed in PZT films. We have also thermally quenched the DWs after heating them at high temperatures. Thermal quench causes significant change in the DW configuration in PZT films with $\zeta $ increasing to $\sim $ 0.5 after the films are heated close to the Curie temperature $T_{C} $. On the other hand, the DWs in PVDF-TrFE films exhibit very weak temperature dependence. We attribute this distinctly different behavior to the strong anisotropy between in-plane and out-of-plane interaction in PVDF-TrFE, which is absent in PZT. [Preview Abstract] |
Monday, March 18, 2013 3:42PM - 3:54PM |
C21.00007: Equilateral triangular ferroelectric closure domains in (111)-oriented epitaxial Pb(Zr,Ti)O$_3$ thin films Sang Mo Yang, Y.J. Shin, T.W. Noh, Y. Ehara, H. Funakubo, J.-G. Yoon, J.F. Scott Over 60 years ago, Charles Kittel predicted that intriguing quadrant flux-closure domains could spontaneously form in small ferromagnetic platelets [1]. Such quadrant flux-closure domains are considered as a precursor for the true vortex states, having attracted particular interest for storing memory devices. Although the vortex states are now ubiquitous and rather well understood in ferromagnets, even the flux-closure domains as their precursors are much less established in ferroelectrics (FEs) [2]. Here we report the generation of novel equilateral triangular FE closure domains. We generated such intriguing states by using the high crystalline anisotropy energy of a (111)-oriented epitaxial PbZr$_{0.35}$Ti$_{0.65}$O$_{3}$ thin film. Vector piezoresponse force microscopy analysis showed clearly the equilateral triangular closure states, consisting of three stripe domain bundles with three 120 degree orientation differences. Interestingly, the observed two closure states had the different rotation direction around the core, i.e., clockwise and anti-clockwise.\\[4pt] [1] C. Kittel, Rev. Mod. Phys. 21, 541 (1949);\\[0pt] [2] G. Catalan et al., Rev. Mod. Phys. 84, 119 (2012) [Preview Abstract] |
Monday, March 18, 2013 3:54PM - 4:30PM |
C21.00008: Probing the Atomic Structure and Dynamics of Ferroelectric Domain Walls during Electrical Switching in Real Time Invited Speaker: Xiaoqing Pan The ferroelectric switching occurs through the nucleation and growth of favorably oriented domains and is mediated by defects and interfaces. Dislocations, for example, are known to destroy ferroelectric order; neighboring grains and interfaces subject the ferroelectric to localized strain, electric fields, or the screening of electric fields. Thus, it is critical to understand how the ferroelectric domain forms, grows, and interacts with structural defects. This talk presents the nanoscale ferroelectric switching of BiFeO$_{3}$ and PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ thin films under an applied electric field using in situ transmission electron microscopy (TEM). We follow the kinetics and dynamics of ferroelectric switching in real time and at sub-angstrom spatial resolution. We observed localized nucleation events at the electrode interface, domain wall pinning on point defects, the formation of ferroelectric domains localized to the ferroelectric/electrode interface, and domain wall pinning by dislocations. Through a quantitative analysis of aberration-corrected TEM images we found that there is a strong structural coupling between ferroelectric film and substrate, resulting a polarization reduction in the ferroelectric layer and inducing a polar displacement in substrate. [Preview Abstract] |
Monday, March 18, 2013 4:30PM - 4:42PM |
C21.00009: Two mechanisms of resistive memories in complex oxide thin films Kui-Juan Jin, Can Wang, Zhongtang Xu Current-voltage hysteresis and switchable rectifying characteristics have been observed in epitaxial multiferroic BiFeO$_{3}$ thin films. [1, 2] It has been clearly demonstrated that ferroelectricity and conductivity coexist in a single phase. The forward direction of the rectifying current can be reversed repeatedly with polarization switching, indicating a switchable diode effect and large ferroelectric resistive switching phenomenon. LaMnO$_{3}$ (LMO) films are deposited on SrTiO$_{3}$:Nb (0.8 wt{\%}) substrates under various oxygen pressures for obtaining various concentrations of oxygen vacancies in the LMO films. An aberration-corrected annular-bright-field scanning transmission electron microscopy with atomic resolution and sensitivity for light elements is used, which clearly shows that the number of oxygen vacancies increases with the decrease of oxygen pressures during fabrication. Correspondingly, the resistive switching property becomes more pronounced with more oxygen vacancies contained in LMO films. *E-mail: kjjin@iphy.ac.cn [Preview Abstract] |
Monday, March 18, 2013 4:42PM - 4:54PM |
C21.00010: Shock wave mechanism for bipolar resistive switching S. Tang, V. Dobrosavljevi\'c, M. Rozenberg Many recently discovered systems displaying resistive switching phenomena have been widely studied as potential basis of future electronic memory devices. The hysteresis cycles observed in several such transition-metal oxide devices show a universal feature related to an abrupt onset of resistance switching. Here, we present an analytic analysis of a recently proposed phenomenological model\footnote{M. J. Rozenberg {\em et al.}, Phys. Rev. B {\bf 81}, 115101 (2010).}, via first principle derivation of an appropriate non-linear diffusion equation describing the rapid oxygen vacancy migration under strong time-dependent external electric fields. The non-linearity effect, which reflects the vacancy concentration dependence of the local resistivity, can be related to the modified Burger's equation describing shock waves. We show that the sudden resistance drop observed in the numerical solution of the model occurs exactly when the vacancy shock wave front reaches the interface between the highly resistive Schottky barrier and the bulk. We argue that the magnitude of the relevant nonlinear term is maximal for materials in the close-vicinity of the metal-insulator transition; this insight may facilitate the the optimization of device performance. [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C21.00011: Using Noise to Study Switching Dynamics of Oxide Memristors A.M. Bratkovsky, Wei Yi, G. Medeiros-Ribeiro, R.S. Williams, S. Savel'ev Oxide memristors present attractive opportunities in the areas of nonvolatile memory, random access storage, novel electronic circuits, and new cognitive computing paradigms. The progress in those areas requires detailed understanding of the origin of memristive (resistance switching) behavior, state evolution, and noise. We have found that in TaOx memristors, there is a boundary between semiconducting and metallic conductivity that is characterized by quantized conductance states, demonstrating the formation of an atomic-scale point contact within the oxide. We have measured the noise spectra of a wide range of conductance states, and observed a variety of conductance-dependent behaviors including a transition from 1/f$^{2}$ (semiconducting regime) to 1/f (flicker noise in the ``metallic'' regime) frequency (f) dependence and a peak in the noise amplitude at the conductance quantum G$_{\mathrm{Q}}=$2e$^{2}$/h. We have modeled the point contact using stochastic molecular dynamics and can understand the observed behavior in terms of thermally-activated atomic-scale fluctuations that make and break the contact in the non-conducting matrix. The data provides important input for circuit designs and other applications of memristors. [Preview Abstract] |
Monday, March 18, 2013 5:06PM - 5:18PM |
C21.00012: The rules of the resistive switching operation parameters based on Ta/Ta$_{2}$O$_{5}$ RRAM device Haitao Li, Curt Richter, Oleg Kirillov, Hui Yuan, Hao Zhu, Dimitris Ioannou, Qiliang Li The resistive switching (RS) of the TaO$_{x}$ based RRAM has been widely studied due to its excellent endurance and thermal stability. The RS mechanism is generally understood as the formation and dissolution of nanometer-size conductive filament (CF) formed in set and reset process, respectively. However the exact process of dielectric break down remains unknown. In this work we studied the RS of the Ta/Ta$_{2}$O$_{5}$ based RRAM devices from the dependences of operation parameters V$_{set}$, I$_{CC}$, V$_{reset}$, and I$_{reset}$ on device resistance. From statistical analysis of variation in the threshold parameters, we found that the set process is mainly determined by the voltage stress on the device, instead of current. The first forming process is different from the following set process. The forming voltage exponentially depends on the pristine resistance. The forming process gives a smallest low resistance (R$_{LRS})$ for each device. As a result change in compliance current (I$_{CC})$ has no obvious effects on this low resistance state. [Preview Abstract] |
Monday, March 18, 2013 5:18PM - 5:30PM |
C21.00013: Thermophoresis (Soret Effect) in Memristor Calculations Harold Hjalmarson, Patrick Mickel, Gad Haase, Andrew Lohn, Matthew Marinella, Michael McLain, Andrew Pineda Switching in memristive devices involves the formation of conductive filaments following the application of a voltage pulse that causes heating. The temperature gradient may cause migration of atoms and vacancies through thermophoresis, also called the Soret effect or thermal diffusion. Thus thermophoresis may contribute to the switching mechanism. In this talk, the inclusion of thermophoresis in continuum calculations of electrical transport will be discussed in terms of nonequilibrium thermodynamics. These calculations include the effects of Joule heating, chemical species migration, ionizing radiation and chemical reactions. The merits of various ways to include thermophoresis will be discussed. Some illustrative results will also be discussed. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
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