Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session P41: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides III |
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Sponsoring Units: DMP Chair: Nicola Spaldin, University of California, Santa Barbara Room: Baltimore Convention Center 344 |
Wednesday, March 15, 2006 11:15AM - 11:51AM |
P41.00001: Luminescence and Raman based real time imaging of ferroelectric domain walls Invited Speaker: Novel methods for real-time imaging of ferroelectric domain walls open up new possibilities for advancing physical understanding of domain wall structure, properties, and defect interactions. Instead of destructive chemical etching and subsequent optical or electron microscopy, domain walls are imaged nondestructively in real-time by photoluminescence microscopy using dilute doping by rare earth ions as “designer defects” whose luminescence is affected by the domain walls. Uisng a combination of high spatial and spectral site-selectvity in laser confocal and near field opptical microscopy, domain structure changes on a 100 nm length scale can be observed with a temporal resolution of 5ms, as demonstrated in LiNbO$_3$ and LiTaO$_3$. Imaging using Raman spectroscopy (that does not require rare earth doping) will also be described. These new imaging methods reveal that domain wall widths and structures are velocity-dependent, and they provide active, real time feedback needed for precise laser-writing of ferroelectric domain patterns. [Preview Abstract] |
Wednesday, March 15, 2006 11:51AM - 12:03PM |
P41.00002: Study of Ferroelectric Domains in a Phase Separated Multiferroic Mixture by Variable Temperature Electrostatic Force Microscopy Alex de Lozanne, Tien-Ming Chuang, Weida Wu, Casey Israel, Chenglin Zhang, Sang-Wook Cheong We present a variable temperature Electrostatic Force Microscopy (VTEFM) study on a mixed multiferroic crystal. The sample was synthesized by the floating zone method. It was cut and polished with the surface normal to the growth direction. The chemical phase separation is clearly seen by polarized optical microscopy. The transition temperature is about 25K and 900K for the two different phases. The VTEFM images taken at 77 K reveal the ferroelectric domains, with typical sizes in the order of micrometers. [Preview Abstract] |
Wednesday, March 15, 2006 12:03PM - 12:15PM |
P41.00003: A new fundamental limit of ferroelectric devices and its domain dynamics in ultrathin ferroelectric BaTiO$_{3}$ films J.Y. Jo, D.J. Kim, Y.S. Kim, T.W. Noh, T.K. Song, J.-G. Yoon Phenomena in ultrathin ferroelectric (FE) films, such as the critical thickness and the domain structures, have attracted much interest for a few years. We fabricated fully-strained SrRuO$_{3}$/BaTiO$_{3} $/SrRuO$_{3}$ capacitors, whose BaTiO$_{3}$ layer thicknesses were between 5 and 30 nm, using the laser molecular beam epitaxy. We found that rapid decay of net polarization occurs due to large depolarization field [1]. Using the Monte-Carlo simulations, this decay can be explained by the domain formation dynamics, governed by the domain nucleation process. We found a universal relation between the decay exponent and nucleation energy barrier, regardless of film thickness and temperature. This universal relation will provide a fundamental thickness limit for practical FE devices, set by net polarization decay. [1] D. J. Kim et al. Phys. Rev. Lett, in press. [Preview Abstract] |
Wednesday, March 15, 2006 12:15PM - 12:27PM |
P41.00004: Raman Studies of Ferrolectric Domain Walls in Lithium Tantalate and Niobate Pavel Capek, Greg Stone, Volkmar Dierolf, Chad Althouse, Venkat Gopalan The local structure of ferroelectric domain walls and its dependence on intrinsic defects and dopants is of great interest both from a basic science and a application point of view. For instance, in the ferroelectrics LiNbO$_3$ and LiTaO$_3$ that are widely used in nonlinear and electro-optical devices, the stability, shape, switching fields and smallest achievable domain size are determined by the defect concentration. Using confocal Raman spectroscopy we investigated the perturbation of the phonon modes across a domain wall as a function of sample stoichiometry (i.e.: the number of intrinsic defects. For all samples, we find that in the spectral vicinity of the E(TO$_8$) and E(TO$_9$) the Raman intensity is enhanced in the domain wall region. In order to elucidate the origin of this enhancement, we investigate the directional dispersion of the observed change and perform measurements under variation of pump and probe light polarization and sample orientation. On the basis of these results, we will discuss structural models of the domain wall. [Preview Abstract] |
Wednesday, March 15, 2006 12:27PM - 12:39PM |
P41.00005: Time-Resolved Observations of Soft Phonon Modes in Strained BaTiO$_{3}$/Si Heterostructures Cheng Cen, Jeremy Levy, Venugopalan Vaithyanathan, James Lettieri, Wei Tian, Darrell Schlom Ferroelectric thin films such as BaTiO$_{3}$, grown on Si(100) substrates, have enormous potential for applications ranging from non-volatile random access memories to electro-optic gates for quantum information processing architectures. Optical techniques provide powerful means for obtaining time-resolved information about the ferroelectric soft mode in these materials. Using a two-color pump-probe arrangement, we observe THz-frequency soft modes in strained BaTiO$_{3}$/Si heterostructures grown by oxide-molecular beam epitaxy. [Preview Abstract] |
Wednesday, March 15, 2006 12:39PM - 12:51PM |
P41.00006: Distribution function of random electric fields in disordered ferroelectrics thin films Vladimir Stephanovich, Elena Kirichenko We present the calculation of first moment $E_0$ and variance $\Delta E$ of distribution function of random fields in a ferroelectric of finite size. This defines completely the distribution function in gaussian limit. Specific calculations have been performed for the case of slab-shaped ferroelectric thin film. We have shown that $E_0$ and $\Delta E$ can be expressed through the integrals from first and second degree of Green's function of such confined geometry ferroelectric in $k$ - space. To obtain the Green's function, we solve the differential equation minimizing Landau free energy of a ferroelectric with respect to the boundary conditions on its surfaces. We show, that the distribution function of random fields in the finite-size ferroelectric differs from that of the unbounded bulk material. For example, both $E_0$ and $\Delta E$ depends on film thickness $L$. Knowledge of this distribution function permits to calculate the observable physical properties of ferroelectric thin films made from ferroelectric relaxors. Our method of calculation of $E_0(L)$ and $\Delta E(L)$ can be easily generalized for ferroelectric of arbitrary shape. [Preview Abstract] |
Wednesday, March 15, 2006 12:51PM - 1:03PM |
P41.00007: Interface Induced Ferroelectric Phase Transformation in SrTiO3. Dawn Bonnell, Rui Shao, Gerd Duscher, Matthew Chisolm The transport properties across bicrystal interfaces in SrTiO3 are quantified with 4-pt probe, Hall measurements, scanning impedance microscopy and scanning tunneling microscopy. The properties are related to the structure determined by transmission electron microscopy, energy loss spectroscopy and first principles calculations. An anomaly in the temperature dependence of the transport properties arises from the charge trapped at the interface, which induces dipole ordering adjacent to the boundary. This represents the first observation of interface induced ferroelectricity in SrTiO3. [Preview Abstract] |
Wednesday, March 15, 2006 1:03PM - 1:15PM |
P41.00008: Nanoscale structural dynamics of ferroelectric thin films Alexey Grigoryev, Dal-Hyun Do, Dong-Min Kim, Chang-Beom Eom, Paul Evans, Eric Dufresne, Bernhard Adams The emerging capability to visualize dynamical phenomena at small scales in both distance and time simultaneously has important implications in understanding ferroelectric materials. We have used time-resolved synchrotron x-ray microdiffraction to probe polarization switching and piezoelectric response at the sub-nanosecond time scale and the sub-micrometer spatial scale in lead zirconium titanate thin films. Based on time resolved maps of the polarization and piezoelectric distortion, the polarization switching domain wall velocity can be measured directly. The magnitude of this velocity and its scaling with electric fields suggest that significant improvements in switching speed can be made in optimized thin film structures. [Preview Abstract] |
Wednesday, March 15, 2006 1:15PM - 1:27PM |
P41.00009: External and internal magnetic-field effects on ferroelectricity in orthorhombic rare-earth manganites H. Kuwahara, K. Noda, M. Akaki We report the dielectric and magnetic properties of the perovskite (Eu,Y)MnO$_3$ crystal {\it without} the presence of the $4f$ magnetic moments of the rare earth ions. The subject compound, (Eu,Y)MnO$_3$, was controlled the average ionic radius of the $A$ site so that it was the same as that of TbMnO$_3$ in which the intriguing magnetoelectric effect has been recently discovered. The (Eu,Y)MnO$_3$ crystal was found to have two distinct ferroelectric phases with polarization along the $a$ ($P_a$, $T$$\le$23K) and $c$ ($P_c$, 23K$\le$$T$$\le$25K) axes in the orthorhombic $Pbnm$ setting in a zero magnetic field. In addition, we have demonstrated a magnetic-field-induced switching between these ferroelectric phases: $P_a$ changed to $P_c$ by the application of magnetic fields parallel to the $a$ axis ($H_a$). In analogy to the case of $P_c$ in TbMnO$_3$, this result is possibly interpreted as follows. In the case of (Eu,Y)MnO$_3$, Mn $3d$ spins rotate in the $ab$ plane and $P_a$ would emerge in a zero field. In the $H_a$, the field will force the spins to rotate in the $bc$ plane, in which $P_c$ would be stabilized. Magnetization measurements supported this interpretation: We confirmed the change of the spin rotation axis of the helix from the $c$ axis to the $a$ axis induced by application of the $H_a$ because there is no $4f$ moments acting as an internal magnetic field and interacting with the $3d$ spins. Results obtained with other rare-earth manganites such as (Gd,Tb)MnO$_3$ and (Eu,Ho)MnO$_3$ will be presented. [Preview Abstract] |
Wednesday, March 15, 2006 1:27PM - 1:39PM |
P41.00010: Dielectric and vibrational properties of crystalline and amorphous high-k lanthanum aluminate Pietro Delugas, Vincenzo Fiorentini, Alessio Filippetti High-k oxides are the focus of intense research for their applications in MOS and FLASH devices. A material currently in focus is LaAlO$_3$, with a dielectric constant of ~23-24 in the crystal phase, and similar values of around 20-22 in the amorphous phase (although values as low as 15 have also been reported). We have studied LaAlO$_3$ in both phases [1] to identify possible reasons for this apparent conservation of the dielectric properties upon amorphization. Amorphous samples were generated by melt-and-quench using a combination of pair potentials and ab initio dynamics. The linear response density-functional perturbation-theory approach was used to study dynamical response and phonons. We indeed find a large dielectric constant (~24) in the amorphous: the rationale is that the expected reduction of the anomalous effective charges is compensated by the appearance of new low-frequency (weakly) IR-active modes, whose character is a mixture of La translations (IR in the crystal) and Al-O octahedra rotations (Raman in the crystal). A similar behavior is expected in any rare earth aluminate exhibiting a similar perovskite-related structure (e.g. scandates). \\ 1) P. Delugas, V. Fiorentini, and A. Filippetti, Phys. Rev. B 71, 134302 (2005), and to be published. [Preview Abstract] |
Wednesday, March 15, 2006 1:39PM - 1:51PM |
P41.00011: Composition dependence of the diffuse scattering in relaxor (1-$x$)Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$-$x$PbTiO$_{3}$ ($0\leq x\leq0.40$) M. Matsuura, K. Hirota, P. M. Gehring, Zuo-Guang Ye, W. Chen, G. Shirane We have studied composition dependence of diffuse scattering in the relaxor system (1-$x$)Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$-$x$PbTiO$_{3}$ (PMN-$x$PT) with $x=0$, 10, 20, 30, and 40\% by neutron diffraction. The addition of ferroelectric PbTiO$_{3}$ (PT) modifies the ``butterfly" and ``ellipsoidal" diffuse scattering patterns observed in pure PMN ($x=0$), which are associated with the presence of randomly oriented, polar nanoregions (PNR). The spatial correlation length $\xi$ derived from the width of the diffuse scattering increases from 12.6~\AA\, for PMN ($x=0$) to 350~\AA\, for PMN-20\%PT, corresponding to an enlargement of the PNR. The integrated diffuse scattering intensity, which is proportional to $\chi''$, grows and reaches a maximum at $x = 20$\%. Beyond $x =$30\%\,PT, a concentration very close to the morphotropic phase boundary (MPB), no diffuse scattering is observed below $T_{C}$, and well-defined critical behavior is observed. By contrast, the diffuse scattering for $x \leq 20$\% persists to low temperatures, where the system retains an average cubic structure ($T_{C}=0$). We can simulate the wave vector dependence of the diffuse scattering by assuming that it arises from the condensation of a soft transverse-optic (TO) phonon. [Preview Abstract] |
Wednesday, March 15, 2006 1:51PM - 2:03PM |
P41.00012: Multiresonance, multifrequency spectroscopy of rare-earth and transition ions in ferroelectrics. Galina Malovichko, Valentin Grachev The usual scheme of many methods for material investigation includes an emitter of electromagnetic waves and detectors for the wave registration. Typical output of one-frequency methods is an image with a space resolution of about the used wavelength. The methods are very successful for the study of lattice structures, their transformations at phase transitions, domains etc. Another approach uses sweeping of the frequency or another external parameter: electric or magnetic field, pressure etc. Typical result of a measurement is a spectrum or a dependence of measured characteristic on the sweeping parameter. The spectra do not contain direct evidence about the space structure of a lattice or defects. However, they contain very important information about the energetic characteristics of interactions of lattice ions, intrinsic and extrinsic defects. We present results of multifrequency research of defects in oxide crystals involving optical spectroscopy, microwave and radiofrequency spectroscopy: electron paramagnetic resonance, and electron nuclear double resonance. [Preview Abstract] |
Wednesday, March 15, 2006 2:03PM - 2:15PM |
P41.00013: Femtosecond Spectroscopy of LuMnO$_{3}$ Shitao Lou, Frank M. Zimmermann, Robert A. Bartynski, Namjun Hur, Sang-Wook Cheong Hexagonal LuMnO$_{3}$ manganite is a ferroelectric and strongly frustrated antiferromagnetic crystal. Strong coupling between lattice, electronic, and magnetic degrees of freedom makes it a promising electronic material. We have used femtosecond pump-probe spectroscopy to study the interaction of electron excitations with lattice vibrations in real time. Optical excitation of a Mn $d_{(x^2-y^2),(xy)} \to d_{(3z^2-r^2)} $transition served as the primary excitation step. With both pump and probe beam polarization perpendicular to the c axis, the probe reflectivity shows a sharp drop due to saturation of the transition, recovering on a timescale of 1 ps. We also observed displacive excitation of a coherent optical phonon vibration at 3.6 THz, which is assigned to an A1 symmetry mode involving Lu ion motion along the c axis. This mode was excited in longitudinal (LO) and transverse mode (TO) geometries. While the LO-TO frequency splitting is small ($<$0.1 THz), a remarkable phase reversal of the reflectivity curve was observed. This is attributed to a large linear electro-optic effect (Pockels effect), induced by the THz electric field associated with the LO mode. [Preview Abstract] |
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