Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B17: Focus Session: Phase Transitions and Domains in Ferroelectric Nanostructures I |
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Sponsoring Units: FIAP Chair: Alex Bratkovsky, Hewlett-Packard Room: Baltimore Convention Center 313 |
Monday, March 13, 2006 11:15AM - 11:27AM |
B17.00001: Ferroelectric phase transitions in BaTiO$_{3}$/SrTiO$_{3}$ superlattices studied by ultraviolet Raman spectroscopy Dmitri A. Tenne, X.X. Xi, A. Soukiassian, W. Tian, Y.L. Li, L.Q. Chen, D.G. Schlom, A. Bruchhausen, A. Fainstein, X.Q. Pan, A. Cantarero, R.S. Katiyar Ferroelectric (BaTiO$_{3})_{m}$/(SrTiO$_{3})_{n}$ superlattices (SLs) grown by molecular beam epitaxy on SrTiO$_{3}$ substrates have been investigated by ultraviolet (UV) Raman spectroscopy. Using the UV excitation allowed us to overcome the problem of overwhelming substrate contributions in Raman spectra and made possible the observation of phonons in SLs having the ferroelectric BaTiO$_{3}$ layers as thin as 2 unit cells. The ferroelectric-paraelectric phase transitions have been observed. Depending on the thickness of the BaTiO$_{3}$ layers and strain, the phase transition temperature varies by hundreds of degrees from $\sim $140 K to 630 K, which is over 200 degrees higher than in bulk BaTiO$_{3}$. Below $T_{c}$, the SLs likely remain in the single (tetragonal) ferroelectric phase down to 7 K, i.e. the low-temperature phases characteristic for bulk BaTiO$_{3}$, are suppressed by strain. The experimental data are in good agreement with the results of the thermodynamic calculations of polarization in SLs as a function of temperature. This work was supported by DOE, NSF, and ONR. [Preview Abstract] |
Monday, March 13, 2006 11:27AM - 11:39AM |
B17.00002: Interlayer Coupling and Dielectric Anomaly in Ferroelectric Bilayers and Multilayer Heterostructures S. Pamir Alpay, Shan Zhong, Alexander L. Roytburd, Joseph V. Mantese Ferroelectric multilayers and superlattices have gained interest for dynamic random access memory (DRAM) applications and as active elements in tunable microwave devices in the telecommunications industry. There have been a number of experimental studies that show that these materials have many peculiar properties that cannot be described by a simple series connection of the individual layers that make up the heterostructures. A thermodynamic analysis is presented to demonstrate that ferroelectric multilayers interact through internal elastic, electrical, and electromechanical fields and the ``strength'' of the coupling can be quantitatively described using Landau theory of phase transformations, theory of elasticity, and principles of electrostatics. The thermodynamic modeling indicates that the electrostatic coupling between the layers leads to the suppression of ferroelectricity at a critical paraelectric layer thickness for ferroelectric-paraelectric bilayers. This bilayer is expected to have a gigantic dielectric response similar to the dielectric anomaly near Curie-Weiss temperature in homogeneous ferroelectrics at this critical thickness. [Preview Abstract] |
Monday, March 13, 2006 11:39AM - 12:15PM |
B17.00003: Phase Transitions and Domain Structures in Nanoferroelectrics. Invited Speaker: A review of the Landau-type theory of size effects in ferroelectric phase transitions will be presented. An aspect of this theory, a question about the ``critical thickness'' of ferroelectric thin films will be the main emphasis. This question can be reduced to that of the size dependence of temperature of ferroelectric phase transition by taking into account two possibilities for such a transition: formation of (i) single- or (ii) multi-domain ferroelectric state. In a defect-free sample, two factors would define which of these possibilities is realized: the depolarizing field and the specific features of the sample surface reflected in the boundary conditions for the Landau-type equations in addition to the conventional electrodynamics boundary conditions. The possibility of the transition into the single domain state strongly depends on a character of electrodes and the additional boundary conditions, while it is much less important for the multi-domain case. In realistic conditions, the transition would proceed into the multi-domain state, especially in near cubic ferroelectrics, e.g. films of cubic perovskites with an elastic mismatch between the film and a substrate. Importantly, the shift of a transition temperature with respect to a bulk is relatively small in this case. The message is that, while studying the question about the ``critical thickness'', multi-domain states rather than single domain ones should be considered first of all, contrary to the approach in some recent papers where only monodomain state was studied.. In particular, there is no definite indication of ultimate ``critical thickness'' for a multi domain ferroelectric state in nearly cubic samples. Along with ultra thin films the ferroelectric nanopowders are also intensively studied now. Here the size effects are more complicated because of long-range interaction between the particles. The problems which the theory faces here are briefly commented upon. It is worth mentioning that several important results in the theory of the size effects have been obtained long ago but, unfortunately, seem not to be well known by the ferroelectrics community. They will be exposed together with more recent results obtained in collaboration with A.Bratkovsky at Hewlett-Packard Laboratories, Palo Alto. [Preview Abstract] |
Monday, March 13, 2006 12:15PM - 12:27PM |
B17.00004: Ferrolectric nanodots and nanowires under different electrical and mechanical boundary conditions Inna Ponomareva, Ivan Naumov, Laurent Bellaiche Intense effort has been recently made in synthesizing, characterizing and/or simulating ferroelectric nanostructures (FENs), because of their technological and fundamental promise. Among the different possible classes of FENs, thin films are, by far, the ones that have been the most investigated. However what is crucially missing nowadays is to gain a deep knowledge of 0D-like and 1D-like FEN and understand how their properties depend on mechanical and electrical boundary conditions. We report results on ferroelectric nanodots and infinite wires of $Pb(Zr_{0.4}Ti_{0.6})O_3$ alloy under different boundary conditions investigated via Monte-Carlo simulations using an atomistic first-principle-based effective Hamiltonian[1]. It was found that these nanosystems all exhibit a spontaneous polarization that points along a non-periodic direction, for situations close to short circuit electrical boundary conditions and independently of the epitaxial strain. On the other hand, unusual dipole patterns arise in these systems when they experience a large-enough depolarizing field. The dependency of these patterns on the nanostructure's dimensionality is revealed and explained. [1] L. Bellaiche {\it et al}, Phys. Rev. Lett. {\bf 8}, 5427 (2000). [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 12:39PM |
B17.00005: Self-assembled Nanoscale Domain Structures in Ferroelectrics: Formation and Evolution Vladimir Shur The formation and propagation of self-assembled nanodomain structures have been experimentally studied in lithium niobate single crystals. It has been shown that the ''discrete switching'' through appearance of the quasi-regular patterns consisting of individual nanodomains is a result of decay of highly non-equilibrium domain state. We have demonstrated that the necessary and sufficient condition for such abnormal domain behavior is ineffective screening of depolarization fields, which is characterized by the ratio between bulk screening and switching rates. We have systematically studied this effect in three different experimental situations: (1) ''super-fast'' switching in external electric field, (2) spontaneous backswitching, (3) intensive pulse irradiation by UV laser. The obtained nanoscale structures were classified and explained within unified approach. The main laws of formation of oriented short arrays and growth of strictly oriented ''super-long'' nanoscale domain ''rays'' accompanied by discrete turning and branching have been revealed. The geometry of the domain patterns obtained by computer simulation demonstrates one to one coincidence with experimental images. [Preview Abstract] |
Monday, March 13, 2006 12:39PM - 12:51PM |
B17.00006: Potential distribution and domain structure of metal-ferroelectric-semiconductor-metal heterostructures Rene Meyer, Paul McIntyre Recently, we proposed a novel resistive non-volatile memory concept based on the ferroelectric effect. The resistance switching originates from the unscreened polarization charge at the ferroelectric/semiconductor interface, which affects the distribution of the inner electric potential. A depletion or enrichment of mobile charge carriers leads to a reduced or increased conductivity of the near-interface region. The depolarizing field, however, which is inherently present in the ferroelectric in the case of imperfect screening, causes the formation of 180 deg. domains. The resulting alternation of positive and negative polarization charges at the ferroelectric/semiconductor interface could deteriorate the performance of the proposed resistance switch. In this contribution, the domain pattern is studied numerically for a metal-ferroelectric-semiconductor-metal structure. A 2-dimensional finite differences method is used to calculate the inner electric field, the potential distribution and the electrostatic energy under short circuit conditions and for external electric fields. Based on empirical data, the domain size is estimated as a function of the screening efficiency of the electrodes and the applied field. Results of the 2-dimensional model are compared to a 1-dimensional approach, where a voltage dependence of the macroscopic polarization is approximated by an effective polarization. [Preview Abstract] |
Monday, March 13, 2006 12:51PM - 1:03PM |
B17.00007: Fabrication and Electrical Measurements of CoFe$_{2}$O$_{4}$ Nanopillars in a BiFeO$_{3}$ matrix Scott Rutherford, Rasmi Das, Xianglin Ke, Dmitry Ruzmetov, Dong-Min Kim, Seung Hyub Baek, Mark Rzchowski, Chang-Beom Eom Coupling between ferromagnetic and ferroelectric ordering has recently stimulated many scientific and technological interests. This ``coupling'', would provide an additional degree of freedom in the design of micro and nano-electronic devices such as actuators, transducers, or memories. Unfortunately, the clamping effect of the substrate negates any such magnetoelectric coupling through elastic interactions which evident in a multilayer structures. Therefore our focus is directed towads the design of a novel vertically aligned oxide nano-structures, which will allow us to switch the magnetic domains by applying the electric field and vice versa. These nano-structures will also be used as model system to understand the physics of order parameter coupling in ferrroelctric and ferromagnetic systems. We have fabricated ferromagnetic nanopillar arrays of CoFe$_{2}$O$_{4}$ (CFO), surrounded by a ferroelectric BiFeO$_{3}$ (BFO) and BaTiO$_{3}$ matrix. 90$^{0}$ off-axis sputtering is used to deposit SrRuO$_{3}$ (SRO), followed by CFO on single surface TiO$_{2}$-terminated SrTiO$_{3}$ (001) substrates. SRO provides a good lattice match and electrode capabilities for the subsequent deposition of CFO. E-beam patterning defines pillar dimensions and spacing, while ion milling etches down to the SRO layer. The pillar dimensions range between 100 nm and 500 nm in diameter and are spaced 0.5 to 1 $\mu $m apart. Atomic force microscopy and scanning electron microscopy measurements confirm the structure of the pillars following the pattering and etching steps. The BFO ferroelectric matrix is then deposited by on-axis sputtering. Fabrication of these pillars along with piezo force micrcroscopy and magnetic force microscopy was used to understand the microstructure and domain switching. The detailed scanning probe measurements of domain switching in these novel oxide nanostructures will be discussed. [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B17.00008: Structure and electronic properties of GeTe nanoparticles from ab initio calculations. Gregory Hammad, Philippe Ghosez, Jean-Yves Raty We use density functional theory to study the structure of small Ge-Te nanoparticles close to stoichiometry with various geometries. We particularly investigate the geometrical distortion of the rocksalt structure that gives rise to the ferroelectric effect in bulk GeTe. We observe that similar distortions appear in nanoparticles down to very small sizes (Ge15Te16). The electronic structure and the HOMO-LUMO states localization are shown to be in most cases related to the surface termination. Using linear response calculation (ABINIT code) we compute effective charges and compare these to the bulk value. We finally discuss the effect of the various factors (aspect ratio, facets, surface termination, stoichiometry) on the possibility that small (a few nm in size) Ge-Te nanoparticles could be ferroelectric. [Preview Abstract] |
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