Sergei V. Kalinin Oak Ridge National Laboratory
Anna N. Morozovska National Academy of Sciences of Ukraine
Nagarajan Valanoor University of New South Wales
William Brownell Baylor College of Medicine
JJ1: Nanoscale Electromechanics
Thursday PM, April 16, 2009
Room 3000 (Moscone West)
9:00 AM - **JJ1.1
Piezoforce Microscopy Studies of Domain Walls in Multiferroics.
Ramamoorthy Ramesh 1 Show Abstract
1 , UC Berkeley, Berkeley, California, United States
Over the past year, we have been exploring the properties of domain walls in the model multiferroic system, BiFeO3 using a combination of scanned probe techniques. We use epitaxial thin films and crystals as our model materials. In this talk, I will describe some very exciting results on three types of domain walls, namely the 180*, 109* and 71* domain walls. We find that the 180* and 109* domain walls exhibit electrical conduction at a level that is significantly higher than the bulk. Photoemission studies of the 109* domain walls indicate that these walls may exhibit a significant degree of uncompensated spins. We are presently carrying out detailed magnetotransport studies of such domain walls. I will present our understanding so far in my talk
9:30 AM - **JJ1.2
Phase-field Method of Ferroelectric Domain Structures and Evolution: Simulating Domain Nucleation and Switching under Piezoreponse Force Microscopy.
Samrat Choudhury 1 2 , Sergei Kalinin 3 , Long-Qing Chen 1 Show Abstract
1 , Penn State University, University Park, Pennsylvania, United States, 2 , University of Wisconsin, Madison, Wisconsin, United States, 3 , Oak Ridge National Lab, Oak Ridge, Tennessee, United States
This presentation will discuss the phase-field method of ferroelectric domain structures in ferroelectric thin films. In this method, a domain structure is described using a set of spatially inhomogeneous distributions of order parameters such as polarization and strains, and their temporal evolution toward equilibrium is obtained by solving the coupled time-dependent Ginzburg-Landau equations as well as the electrostatic and mechanical equilibrium equations. The focus will be on domain nucleation and evolution under a local switching condition, simulating domain nucleation under a piezoresponse force microscopy (PFM). A number of examples of phase-field simulations of domain nucleation at defect-free surfaces and in the vicinity of ferroelastic twin boundaries and localized defects will be presented. The results are compared to those obtained in actual PFM measurements.
10:00 AM - **JJ1.3
Probing Electro-mechanical Coupling in Biological Systems.
Brian Rodriguez 1 Show Abstract
1 Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Dublin Ireland
Biological systems are mechanosensitive and respond to physical and chemical environmental change, affecting biochemical reactions and providing a pathway for probing biological functionality (mechanotransduction). Similarly, coupling between electrical signals and mechanical motion in biological systems is essential to life and many biopolymers are piezoelectric (electromechanical coupling). Little is known, however, regarding the effect of electrical stimuli on the chemistry and functionality of biological systems, particularly at the nanoscale. Determining the functionality of electromechanics in biosystems necessitates first studying electromechanical coupling in biological systems, which requires the ability to apply bias in physiological environments and to measure small displacements at the cellular and molecular level of soft materials. Piezoresponse force microscopy (PFM) is widely used to probe electromechanical phenomena in ferroelectric materials and in recent years has been employed to study piezoelectric biomaterials. Examples of electromechanical imaging (in air) of cellular and biomolecular systems, including tooth dentin and red blood cells are presented. PFM has been extended to a liquid environment for model ferroelectric systems, allowing preliminary results of liquid electromechanical imaging of biosystems to be obtained. The challenges of electromechanical imaging of biological systems in physiological environments are discussed.
10:30 AM - JJ1.4
Lorentz Microscopy Investigation of Piezoelectric Induced Magnetic Domain Motion in Unclamped FeGa/BaTiO3 Thin Film Structures.
John Cumings 1 , Sung Hwan Lim 1 , Todd Brintlinger 1 , Yi Qi 1 , Lourdes Salamanca-Riba 1 , Ichiro Takeuchi 1 Show Abstract
1 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States
We have studied electromechanical coupling induced magnetic domain motion in unclamped FeGa/BaTiO3 thin film bilayer structures. Magnetostrictive FeGa layers were sputter-deposited on epitaxially grown BaTiO3 films on SrTiO3 substrates. Focused ion-beam milling was used to remove the substrate from underneath the BaTiO3 film, and electrodes were patterned in the metallic FeGa film to apply electric field across a patterned gap (1 micron). Lorentz microscopy was used to monitor the magnetic domains in FeGa, while electric field is applied to the piezoelectric BaTiO3. Lorentz microscopy allows direct and dynamic observation of magnetic domain motions. Reversible electric field induced magnetic domain motion was observed. This work is funded by NSF MRSEC (DMR 0520471), ONR-MURI N000140610530 and ARO W911NF-07-1-0410.
10:45 AM - JJ1.5
Nano-Electro-Mechanical Switching Devices For 3D Electronics
Anupama Kaul 1 , Paul von Allmen 1 , Krikor Megerian 1 , Richard Baron 1 , Julia Greer 2 Show Abstract
1 , Jet Propulsion Laboratory, Pasadena, California, United States, 2 Materials Science, California Institute of Technology, Pasadena, California, United States
Performance limitations of Si integrated circuits as a result of continued miniaturization has created an ever-increasing need for exploring new materials and architectures beyond solid-state transistors. Nano-electro-mechanical (NEM) switches may overcome these limitations, by reducing leakage currents and power dissipation during operation, while at the same time providing the added benefits of enhanced radiation tolerance, and high temperature resilience. We will describe our work on carbon nanotube-based 3D NEM switches that are being developed at JPL, primarily for extreme environment space electronics. The 3D nanoscale devices are formed using high throughput, manufacturable techniques, where the tubes are oriented perpendicular to the substrate. The electrical characterization of the devices will be presented, such as switching voltage and potential for nonvolatile operation, and results are compared to an electrostatic model developed using a custom Poisson finite element solver. Nanoscale mechanical characterization results will also be presented, where a custom nano-indentor is used to observe deflection characteristics of individual tubes in an SEM under compressive loading. The mechanical properties of the tubes obtained empirically are then used to determine the overall electro-mechanical response of the switches.
JJ4: Poster Session
Friday AM, April 17, 2009
Salon Level (Marriott)
9:00 PM - JJ4.1
Ferroelastic Domains and Electromechanical Interactions in Bilayered Ferroelectric Thin Films
Reza Mahjoub 1 , Anbu Varatharajan 1 , Nagarajan Valanoor 1 , Pamir Alpay 2 Show Abstract
1 , University of New South Wales, Sydney, New South Wales, Australia, 2 Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States
Ferroelastic domain fraction in a bilayer heteroepitaxial structure consisting of (001) tetragonal PbZrxTi1-xO3 and (001) rhombohedral PbZr1-xTixO3 on a thick (001) passive substrate is investigated theoretically, as a function of the lattice misfit between layers and the substrate, taking into account the self-strain in each layer and the indirect elastic interaction between the layers. In particular, we provide an theoretical analysis corresponding to a (001) PbZr0.2Ti0.8O3/(001PbZr0.8Ti0.2O3 bilayer structure as a function of the PbZr0.2Ti0.8O3 layer thickness on (001) LaAlO3, (001) SrTiO3, (001) and MgO. It is found that the self-strain coupling between the tetragonal and rhombohedral layers leads to an excess elastic energy in the tetragonal layer, resulting in a 2 to 3 times increase in the in-plane ferroelastic domain fraction in the tetragonal layer compared to a single-layer PbZr0.2Ti0.8O3 films of similar thickness. On application of bias, the domain fraction is shown to change and yield large piezoelectric coefficients. These results show new ways of engineering and manipulation of the ferroelastic domain structure of ferroelectric thin films.
9:00 PM - JJ4.3
Piezoresponse Force Microscopy Characterization of PTO Thin Films.
Alessio Morelli 1 , Sriram Venkatesan 1 , George Palasantzas 1 , Bart Kooi 1 , Jeff De Hosson 1 Show Abstract
1 Applied Physics, Zernike Institute for Advanced Materials, University of Groningen, Groningen, Groningen, Netherlands
Piezoresponse force microscopy (PFM) enables the characterization of ferroelectric films at the nanoscale, which it is required for further developments in ferroelectric random access memories (FeRAM) and transducers. In particular long polarization retention and low coercive voltage are key properties in FeRAM, whereas linearity and adequate magnitude of the piezoresponse signal are required for transducers. Although lead zircon titanate (PZT), has been studied extensively, there is a lack of data at the nanoscale about its precursor lead titanate (PTO). Therefore, we examined the functionality of PTO samples grown by pulsed laser deposition (PLD) on strontium ruthenate (SRO) electrodes, by means of PFM techniques in conjunction with transmission electron microscopy (TEM). PFM characterization revealed as grown upward polarization, inhomogeneous distribution of piezoelectric characteristics, imprint in hysteresis loops, nonlinearity in the piezoelectric deformation, and polarization reversal. TEM observations show the presence of defects at the electrode/film interface and confirm the absence of ‘a’ domains. The nonlinearity cannot be attributed to 90° domain walls movement as it was indicated in former studies, since this type of walls was absent in the studied films. The variations of piezoelectric properties measured with PFM are associated with the presence of defects near the interface as supported by TEM observations.
9:00 PM - JJ4.5
Piezoforce Microscopy Mapping of Rare-earth Doped BiFeO3 Composition Spreads.
Daisuke Kan 1 , V. Anbusathaiah 2 , Dwight Hunter 1 , V. Nagarajan 2 , Ichiro Takeuchi 1 Show Abstract
1 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States, 2 Materials Science, University of New South Wales, Sydney, New South Wales, Australia
We have mapped the piezoelectric properties of Bi1-x(RE)xFeO3 composition spreads using piezoforce microscopy. 200 nm thick epitaxial composition spread chips 6 mm long were fabricated by combinatorial pulsed laser deposition on SrRuO3 (50 nm) on SrTiO3 (100) substrates. Along the length of each spread chip, the composition continuously changes from BiFeO3 (BFO) to (RE)FeO3. Sm, Gd, and Dy were investigated as the A-site substituting rare-earth elements. 50 micron sized Pd capacitors were patterned across the chips, and Pt-Ir coated contact-mode tips were used to perform d33 hysteresis loop mapping across the spreads. In Bi1-xSmxFeO3, we have previously discovered a morphotropic phase boundary (MPB) at approximately x = 0.14. High-field d33 values increase from ~ 60 pm/V for undoped BFO to 110 pm/V for Bi0.86Sm0.14FeO3. As x increases beyond the MPB composition, the d33 hysteresis exhibits loops consistent with an antiferroelectric phase. Similar composition trend was observed for Gd and Dy doped composition spreads. This work is funded by NSF MRSEC (DMR 0520471), ONR-MURI N000140610530 and ARO W911NF-07-1-0410.
9:00 PM - JJ4.6
Nanoscale Studies of the Polarization/Morphology Correlation in Polymer Ferroelectric Films.
Pankaj Sharma 1 , D. Wu 2 , T. Reece 1 , S. Ducharme 1 , A. Gruverman 1 Show Abstract
1 Department of Physics and Astronomy, University of Nebraska Lincoln, Lincoln, Nebraska, United States, 2 Department of Physics, North Carolina State University, Raleigh, North Carolina, United States
In this study, Piezoresponse Force Microscopy (PFM) has been used to investigate correlation between polarization and morphology of the ferroelectric thin films of polyvinylidene fluoride trifluoroethylene (pVDF-TrFE 80:20). The films were deposited on highly doped Si substrate using the Langmuir-Blodgett technique. Films morphology and crystallinity have been controlled by depositing a varying number of molecular monolayers (ML) and subjecting films to annealing. The pVDF-TrFE films of 1 ML thickness exhibited isolated mesas approximately 200 nm in diameter, on average. The PFM images show that the mesas are in generally in a polydomain state which can be modified by applying a bias voltage to the PFM tip. The nominally 3 ML thick films exhibited either nano-mesa or complementary nanowell structures  with qualitatively different domain patterns. The PFM images have been analyzed using the auto-correlation function method showing that while the average size of domains in the nano-mesas is about 100 nm, the films with nanowell structure are characterized by much smaller domains of 20-50 nm in size. Annealing showed a profound effect on the morphology and orientation of 12 ML thick continuous films . Depending on annealing conditions, morphology of the 12 ML films varied from having irregular crystallites of 200-500 nm in size to rice like elongated crystallites approximately 200 nm long and 50 nm wide. These morphological changes are accompanied by a decrease in average domain size from ~300 nm to less than 100 nm. Results of the large scale and local polarization switching in 3 ML and 12 ML are also presented.
9:00 PM - JJ4.7
Single Multi-Walled Carbon Nanotube Biological Probe for Whole Cell Patch Clamp and Intracellular Stimulation and Recording of Neurons
Edward de Asis 1 2 , Sally Wood 1 , Joe Leung 3 , Cattien Nguyen 2 Show Abstract
1 Departments of Electrical Engineering and Bioengineering, Santa Clara University, San Francisco, California, United States, 2 , Eloret Corporation, Moffett Field, California, United States, 3 , NASA Ames Research Center, Moffett Field, California, United States
The glass micropipette electrode has enabled the study of neurons at the single cell and single channel level to provide fundamental understanding of cellular neurophysiology. It has been widely used to characterize the bioelectrical properties of neurons, elucidate ion channel kinetics, and investigate the effect of pharmacology on ion channel gating and neuronal electrical activity. Recent studies have examined the feasibility of using carbon nanostructure based electrodes as laboratory tools for studying neurons and neural networks in vitro and as implantable neuroprotheses for treating neuropathological disorders. However, these electrodes typically rely on bundles or arrays of many nanofilaments and are used for extracellular recording and stimulation, leading to limited spatial resolution, reduced stimulation efficiency, and suboptimal signal-to-noise ratio. We present a novel process for fabricating a high-aspect ratio needle-like biological MEMS device consisting of a single multi-wall nanotube electrode that can be potentially used to probe spatially confined areas of mammalian neurons such as the perisynaptic region which is inaccessible to glass microelectrodes. We demonstrate the ability of this probe to penetrate the cell membrane and perform single cell intracellular stimulation and recording potentially leading to increased stimulation efficiency and enhanced signal-to-noise ratio. Unlike glass micropipette electrodes, our carbon nanotube electrode eliminates the problem of washout of materials in the cell cytosol potentially facilitating the study of long-term potentiation and the cellular mechanisms of learning and memory. In this presentation, we will give details of the fabrication techniques as well as the electrochemical characterization of the nanoscale electrode.
Sergei V. Kalinin Oak Ridge National Laboratory
Anna N. Morozovska National Academy of Sciences of Ukraine
Nagarajan Valanoor University of New South Wales
William Brownell Baylor College of Medicine
Friday AM, April 17, 2009
Room 3000 (Moscone West)
9:00 AM - **JJ5.1
Electromechanically Coupled Magnetostrictive/piezoelectric Thin Film Multilayers for Device Applications.
Ichiro Takeuchi 1 Show Abstract
1 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States
In order to investigate the potentials of electromechanically coupled magnetic/piezoelectric devices, we are exploring various geometry magnetostrictive/piezoelectric multilayer structures. Sol-gel synthesis is used to deposit up to microns thick Pb(Zr,Ti)O3 films. Magnetron-sputtered individual layer Fe0.7Ga0.3 (FeGa) films show robust saturation magnetostriction lambda100 of approximately 150 ppm. FeGa/Pb(Zr,Ti)O3 all thin-film structures deposited on micromachined SiO2/Si cantilevers can be used to perform ac magnetic field detection with the ME coupling coefficient comparable to those of bulk laminate devices. To exploit the converse effect for possible electric-field tunable magnetic thin film devices, we have deposited FeGa films on PbZr0.3Ti0.7O3 (PZT(30/70))/PbZr0.7Ti0.3O3 (PZT(70/30)) tetragonal/rhombohedral bilayers on Pt/Ti/SiO2/Si wafers. The PZT(30/70)/PZT(70/30) bilayers show grain size as large as 1-3 micron. Previous piezoforce microscopy studies have shown that such bilayers exhibit presence of ferroelastic domains where the fraction of the local c/a domain ratio can be tuned by applied electric field. The FeGa top layer (300 nm) was patterned into 20 micron x 20 micron capacitor devices for applying electric field to the PZT(30/70) (70nm)/PZT(70/30) (70 nm) bilayers, and ferromagnetic resonance (FMR) measurements at 9.2 GHz were performed. Typically, a relatively sharp FMR signal observed from the FeGa pad before application of the electric field would get substantially broadened after applying +4 kV/cm. Angular dependent FMR indicates that magnetic anisotropy in FeGa is indeed affected by application of electric field. Other device configurations will also be discussed. This work was performed in collaboration with A. Luykx, D. Kan, P. Zhao, Z. Li, S. Lofland, F. Kartawidjaja, J. Wang, V. Nagarajan, V. Anbusathaiah, T. Brintlinger, J. Cumings, M Wuttig. This work is funded by NSF MRSEC (DMR 0520471), ONR-MURI N000140610530 and ARO W911NF-07-1-0410.
9:30 AM - **JJ5.2
Unconventional Phase Field Simulations of Domain Structures in Ferroelectrics and Multiferroics.
Jiangyu Li 1 , Yi-CHung Shu 2 , Liangjun Li 1 , Jui-Hen Yen 2 Show Abstract
1 , University of Washington, Seattle, Washington, United States, 2 Institute of Applied Mechanics, National Taiwan University, Taipei Taiwan
An unconventional phase field theory is developed to simulate the domain structures of ferroelectrics and multiferroics. It employs a set of characteristic functions motivated by energy-minimizing multi-rank laminated domain configurations as the internal variables, gives rising to explicit expression of the energy-wells that are implicit in conventional phase field approach. The adoption of characteristic functions also makes it easy to couple the multiple order parameters such as strain, polarization, and magnetization, facilitating the simulation of coupled domain structures in multiferroics. The theory we develop thus provides a powerful computational tool that complements piezoresponse force microscopic study of piezoelectric domain structures.The theory is applied to study three classes of ferroelectric and multiferroic materials, including ferroelectric rhombohedral crystals, ferromagnetic shape memory alloys, and multiferroic bismuth ferrite films. First of all, a sophisticated electromechanical self-accommodating domain pattern consisting of eight variants is simulated for ferroelectric rhombohedral crystal in the absence of external field. When the crystal is poled along a non-polar axis, an engineered domain configuration is also observed, and both simulations are in good agreement with those observed in experiments. Secondly, for ferromagnetic shape memory alloys, it is observed that magnetoelastic domains evolve through either variant rearrangement or magnetization rotation, resulting in large or small magnetic field-induced strain depending on the magnitude of applied compressive stress. This explains the small blocking stress observed in ferromagnetic shape memory alloys, and offers a method for its improvement. Finally, magnetoelectric domain and cross field switching in bismuth ferrite are simulated. We not only observe the coupled ferroelectric and antiferromagnetic domains, and demonstrate the electric control of antiferromagnetic ordering, both in consistency with experiments, but also revealed the switching of antiferromagnetic domains by mechanical stress that is yet to be explored in experiments.
10:00 AM - JJ5.3
Labile Ferroelastic Nanodomains in Bilayered Ferroelectric Thin Films.
Anbusathaiah Varatharajan 1 , Fransiska Kartawidjaja 2 , John Wang 2 , Nagarajan Valanoor 1 Show Abstract
1 School of Materials Science & Engineering, University of New South Wales, Sydney, New South Wales, Australia, 2 Department of Materials Science and Engineering, National University of Singapore, Singapore Singapore
The ability to engineer unique domain behavior in ferroic materials systems such as ferroelectrics is of immense practical as well as fundamental interest. In the special case of thin film ferroelectrics that have a large ferroelastic self-strain associated with their phase transformation, a key aspect is the interaction of this self-strain with the boundary conditions of the film. In this presentation we show that, by depositing a strongly tetragonal ferroelectric thin film on a soft ferroelastic buffer layer, we can harness these ferroelastic interactions to demonstrate a ferroelastic domain structure in the tetragonal film that is easily susceptible to external perturbation. High-resolution piezoresponse force microscopy images demonstrate gross movement (nm scale) of the ferroelastic domains under local bias. This movement creates enhanced electromechanical response (up to three times larger than constrained thin film values), which make the system attractive for sensor and nanoelectronic applications.
10:15 AM - JJ5.4
Piezoelectric Response in the Vicinity of Domain Walls in Ferroelectric Crystals with Engineered Domain Configuration
Alexei Bokov 1 , Haiyan Guo 1 , Zuo-Guang Ye 1 Show Abstract
1 Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada
The displacement of domain walls may contribute significantly to the piezoelectric response of ferroelectric materials, alongside with the intrinsic contribution of domains themselves. However, in the special case of so-called “engineered domain” configuration the walls are arranged in such a way that the electric field or stress cannot influence their positions and thereby the corresponding “extrinsic” piezoelectric contribution is absent. Surprisingly, the best piezoelectric performance is found right in crystals with engineered domain configuration (namely, in relaxor-based perovskite solid solutions), which makes them the materials of choice for a new generation of electromechanical transducers, sensors and actuators. To explain this fact it is tempting to assume that the regions near the walls possess enhanced piezoelectric properties even though the walls do not move. This assumption was supported by theoretical calculations . Measurement of local piezoelectric coefficients is needed for verifying this hypothesis, which cannot be done by conventional methods because of insufficient spatial resolution. In this work we use the technique of piezoresponse force microscopy (PFM) to solve the problem. The tetragonal (111) oriented relaxor-ferroelectric 0.63Pb(Mg1/3Nb2/3)O3-0.37PbTiO3 single crystal platelets were investigated with PFM and polarized optical microscopy. The engineered domain configuration should appear in this crystal after poling. The uncharged 90o domain walls were observed as expected for the poled crystals of tetragonal symmetry. The lateral PFM revealed the similar domain pattern as that observed by the polarized optical microscopy. The vertical piezoelectric response (which is determined by the d33 piezoelectric coefficient) showed an almost uniform distribution except within the distance of ~ 1 μm from the domain walls where it was appeared to be significantly reduced. This unexpected result indicates that the “giant” piezoelectric properties in relaxor-ferroelectric crystals can hardly be related to the domain walls contribution. This finding is essential for understanding the mechanisms of the giant piezoresponse in relaxor ferroelectric crystals and for designing and preparing new high-performance piezoelectric materials.1. Rao, W.-F. & Wang, Y. U. Appl. Phys. Lett. 90, 041915 (2007).
10:30 AM - JJ5.5
Nanoscale Studies of Ferroelectric Domain Wall Motion in Epitaxial BiFeO3/RuSrO3 Films
Yi-Chun Chen 1 , Ying-Hao Chu 2 , Wen-Chuan Hsieh 1 , Cheng-Hung Ko 1 , Hsiang-Hua Tai 1 Show Abstract
1 Department of Physics, National Cheng Kung University, Tainan Taiwan, 2 Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu Taiwan
Among the single-phase multiferroics, BiFeO3 (BFO) had attracted great scientific and technological interests due to its advantages of high remanent polarization and magnetoelectric applicability at room temperatures. Recently, it had been experimentally observed that the structures of antiferromagentic moments in BFO thin films are coupled with the ferroelectric domains. For the applications of BFO materials in novel devices with fast response and high-density data storage, the ferroelectric domain kinetics, such as the domain nucleation, growth, and relaxation, need to be investigated in details. In this study, we discussed the domain wall motion both qualitatively and quantitatively in epitaxial BFO films. The BFO films were prepared by pulsed laser deposition on the SrRuO3 (SRO) buffered SrTiO3 (STO) single crystal substrates. The domain dynamic behaviors were investigated by initially fabricating switched domains with voltage pulses, followed by performing in-plane and out-of-plane piezoresponse force microscope (PFM) imaging. BFO (111) epitaxial films were treated as the single-domain-like environment where 180-degree domain wall motion can be directly observed and analyzed by the creep motion equations. In contrast, the possible polarizations in BFO (100) were eight variants along the four cubic diagonals <111>, and so the domain wall motion involved ferroelectric (180-degree switching) and ferroelastic (71-degree and 109-degree switching) types. The ferroelectric domain walls in BFO (100) also followed the thermal activated motion while the ferroelastic walls were easily pinned by the initial domain walls in the as grown films. The domain wall motions during the relaxation process were also systematically investigated.
10:45 AM - JJ5.6
Impact of High Interface Density on Ferroelectric and Structural Properties of PbZr0.4Ti0.6O3 / PbZr0.2Ti0.8O3 multilayers.
Ludwig Feigl 1 , Yin Zhu 2 1 , Balaji Birajdar 1 , Brian Rodriguez 1 , Y. Kim 1 , Ionela Vrejoiu 1 , Marin Alexe 1 , Dietrich Hesse 1 Show Abstract
1 Exp. Dept. 2, Max Planck Institute of Microstructure Physics, Halle, Saxony-Anhalt, Germany, 2 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang China
Epitaxial strain and the interrelated formation of misfit and threading dislocations strongly influence the properties of ferroelectric thin films. It was recently shown that the introduction of an artificial interface by combining the two tetragonal compositions PbZr0.4Ti0.6O3 (PZT40/60) and PbZr0.2Ti0.8O3(PZT20/80) into a bilayer leads to a widespread distribution of the resulting bilayers’ properties. The domain pattern and dislocation state which are signs of relaxational processes vary with the thickness of the bilayer and the ratio of the layer thicknesses. This in turn notably changes the remanent polarization and the dielectric constant. These observations are mainly attributed to the relaxation which is triggered by the interface but the interface’s properties remain unclear.In order to elucidate these properties, in this work the density of interfaces is increased to gradually suppress the influence of the relaxation state. Layers with equal thickness consisting of PZT20/80 and PZT40/60 are grown by pulsed laser deposition onto a SrRuO3 bottom electrode on vicinal (100) SrTiO3 substrates. The bottom and the top layer have the same PZT composition to avoid interfering contributions resulting from an asymmetric structure. Multilayers with an interface density reaching from 0.05 nm-1 up to 0.63 nm-1 are fabricated.The remanent polarization and the dielectric constant are determined to evaluate the dependence of Pr×εr on the interface density. Pr×εr is chosen as a figure of merit to reduce the impact of the strain state which would in the case of tensile in-plane strain increase the dielectric constant and decrease the remanent polarization. To achieve a comprehensive structural characterization TEM, HRTEM, RSM and PFM investigations are performed.A maximum of Pr×εr is observed at an interface density of about 0.2 nm-1. For interface densities below 0.2 nm-1 (HR)TEM investigations reveal a changeover in the domain pattern. At a low interface density the a-domains are mainly constricted to the individual layers with only a few a-domains comprising several layers, whereas at higher interface densities all domains propagate through the whole film. RSM measurements show the occurrence of superlattice peaks due to this transition but no change of the average lattice constants is observed. For interface densities above 0.2 nm-1 the fraction of a-domains decreases as shown by PFM. Taking into account all of the investigations it can be judged that the increased interface density leads to a common strain state of all the layers, which avoids the formation of layers with inferior properties that in turn would be detrimental for the ferroelectric properties of the whole structure. If the interface density is increased above 0.2 nm-1, the high volume fraction of the distorted interface regions might lower the mobility of the domain walls which leads to a drop of Pr×εr.
11:30 AM - **JJ5.7
Nanoscale Imaging And Polarization Dynamics In Relaxor Ferroelectrics.
A. Kholkin 1 , I. Bdikin 1 , D. Kiselev 1 , B. Dkhil 2 Show Abstract
1 Dept. of Ceramics and Glass Engineering & CICECO, University of Aveiro, Aveiro Portugal, 2 CNRS-UMR 8580, Ecole Centrale Paris, Chatenay-Malabry France
Ferroelectric relaxors have been a puzzling problem in solid state physics for more than 30 years since the discovery of their unusual dielectric and electromechanical properties. Recent years witnessed a tremendous interest in relaxor materials due to the unique combination of useful functional properties and new possibilities offered by miniaturization and modern deposition and patterning techniques. Thes e include giant piezoelectric effect and electrostriction, outstanding electrooptic properties, and huge and diffuse-type dielectric constant. However, the central problem of relaxors – the origin and properties of so-called polar nanoscale regions (PNRs) that are responsible for their dielectric and piezoelectric phenomena remain unclear. Until now the properties of PNRs have been only tested via indirect scattering techniques such (e.g., neutron scattering) and TEM. New possibilities have been recently offered by the appearance of Piezoresponse Force Microscopy (PFM) where the polarization clusters can be probed via their internal piezoelectric response with the resolution approaching to several nm. In this presentation, we report our recent PFM results on Pb-based perovskite relaxors based on Pb1-yLay(Zr1-xTix)1-y/4O3 (PLZT), PbZn1/3Nb2/3O3 (PMN) and PbZn1/3Nb2/3O3 (PZN). These materials are classical objects of relaxors physics and their nanoscale investigations are extremely useful for the understanding of their properties. It will be shown that, on a nanoscale level, some of the referred materials behave as nanocomposites comprising polar islands with nonzero piezoelectric effect and polarization embedded in a nonpolar matrix where the apparent correlation length is smaller than the size of the tip. Clear correlation between the grain size and relaxor properties was observed in PLZT ceramics. This effect may explain, for example, broad dielectric curves, high-temperature hysteresis and shift of the transition temperatures typically observed in relaxors. It will be also shown that some solid solutions of relaxors with “normal” ferroelectrics often display a co-existence of micron-sized domains with nanodomains that may be considered as agglomerates of PNRs. These may contribute to the giant piezoelectric effect and thus important for applications. The effect of the bias-induced phase transitions in relaxors will be also presented. The nanoscale properties of relaxors will be discussed based on the existing theories taking into account PFM instrumentation and interface phenomena. The work is performed within the FCT project PTDC/FIS/81442/2006.
12:00 PM - **JJ5.8
Grain Boundaries-Ferroelectric Domains Interactions in Polycrystalline Ferroelectrics.
Sarah Leach 1 , R. Edwin Garcia 1 , Nagarajan Valanoor 2 Show Abstract
1 Materials Engineering, Purdue University, West Lafayette, Indiana, United States, 2 School of Materials Science, UNSW, Sydney, New South Wales, Australia
Ferroelectric Lead Zirconate Titanate (PZT) films display physical behavior that makes them an important candidate for random access memory applications. In such devices, ferroelectric domains are locally switched by the application of an electric field, thus fixating the state of a memory unit. Today's technological advancement, however, demands ever higher memory densities. Therefore, as the device size shrinks, the microstructural features become increasingly important and the spatial variation of the hysteretic behavior increases, making the memory unit potentially unreliable. The local crystallographic orientation and the local grain-grain interactions play an important role in determining the switching of domains. In particular, large spatial variations of the fields arise as a combined result of the stresses that develop due to the thermal expansion and lattice mismatch of the film-substrate system, the anisotropy of the properties of the involved materials, and the processing conditions.
12:30 PM - JJ5.9
Imaging Polarization of Ferroelectric Polymer Thin Films with Pyroelectric Scanning Microscopy.
Stephen Ducharme 1 , J. Travis Johnson 1 , Jihee Kim 1 , Stella Stephens 2 , Horatio Vasquez 2 Show Abstract
1 Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska, United States, 2 Department of Mechanical Engineering, University of Texas – Pan American, Edneberg, Texas, United States
The polarization pattern in a ferroelectric capacitor can be imaged by Pyroelectric Scanning Microscopy (PSM), because the pyroelectric response is proportional to polarization, the PSM image is, in effect, a polarization image as well. The PSM imaging system operates by scanning an amplitude-modulated laser beam across the surface of a ferroelectric capacitor while recording the pyroelectric current. We constructed an automated PSM system working with free-space optics. The main components of the system are the laser source, microscope objective, nano-positioning system, lock-in amplifier, and control computer. By using a blue 405-nm laser diode (405 nm) and high-numerical-aperture microscope objectives the optical resolution can reach 200 nm. Direct modulation of the laser diode at frequencies up to 5 MHz reduces the thermal diffusion length to the same range. Additional capabilities include the ability to image polarization states at any point on the hysteresis loop, and to obtain time-dependent images of polarization switching dynamics . The PSM system was used to study a ferroelectric capacitors consisting of Langmuir-Blodgett films of vinylidene fluoride – trifluoroethylene copolymer achieve an effective resolution of 500 nm or better. We thank Vladimir Fridkin for useful advice. This work was supported by the National Science Foundation through the Q-SPINS Materials Research Science and Engineering Research Center (DMR-0213808) and by the University of Nebraska Office of Graduate Studies.