Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q10: Focus Session: Multiferroics III and Dielectrics |
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Sponsoring Units: FIAP Chair: Delphine Lebuegle, University of Paris-Sud 11 Room: 304 |
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q10.00001: Low-energy Electron Microscopy (LEEM) Imaging and Diffraction of Ho$_{1-x}$Y$_{x}$MnO$_{3}$(0001) M.D. Ulrich, Relja Vasic, J.T. Sadowski, J.E. (Jack) Rowe, H.D. Zhou, C.R. Wiebe We have investigated the (0001) surfaces of several hexagonal perovskite alloys Ho$_{1-x}$Y$_{x}$MnO$_{3}$ (x=0-1), by low-energy electron microscopy (LEEM) using both mirror-mode imaging and diffraction (LEED) techniques. We find LEEM structured domains for our (0001) surfaces of single crystals grown by a traveling-solvent-floating zone technique which are likely due to work function variations on a scale of $\sim $100 nm to 5 $\mu $m. The domains are visible up from 500\r{ }C to $\sim $900\r{ }C, giving possible evidence for the expected 180\r{ } ferroelectric domain structure below and above the Curie temperature of $\sim $630 \r{ }C. The domain contrast in LEEM was lower upon cooling and we are exploring several mechanisms for this result. In addition, we observed LEED patterns from $\sim $24 to 1100\r{ }C, with electron energies in the range of 15eV to 50eV. Above 500\r{ }C, 1$\times $1 and 2$\times $2 patterns were obtained for all samples with the sharpest LEED contrast near T $\sim $1000\r{ }C. In some cases a 2$\times $4 surface reconstruction was also evident but with less consistent order. The 2-D lattice constant parameters in (0001) plane are consistent with previous literature values for pure YMnO$_{3}$ (see: Aberdam \textit{et al., Surface Science }\textbf{14, }pp.121-140 (1969)). [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q10.00002: Spin Correlation and Magnetically Induced Ferroelectricity in YMn$_2$O$_5$ J. Okamoto, D. J. Huang, W. B. Wu, S. L. Cheng, C. T. Chen, C. Y. Mou, K. S. Chao, S. W. Huang, S. Park, S-W. Cheong There is great interest in understanding the microscopic nature of the coupling between ferroelectricity and magnetic ordering in several multiferroic frustrated manganites $R$MnO$_3$ and $R$Mn$_2$O$_5$ ($R$= rare earth and Y). For $R$MnO$_3$, the multiferroicity can be understood in terms of the anti- symmetric spin interaction, whereas the underlying mechanism of multiferroicity in $R$Mn$_2$O$_5$ remains controversial, because of its structural complexity. We unraveled the temperature-dependent spin ordering of multiferroic YMn$_2$O$_5 $ by using resonant soft x-ray magnetic scattering at Mn $L_3$ edge. For temperatures below the onset temperature $T_C$ of ferroelectricity, the handedness of cycloidal spin spirals exists, but vanishes above $T_C$. The spin handedness perpendicular to the induced polarization reverses at the temperature where the polarization changes its sign. The temperature dependence of spin correlation along the propagating direction of spin spirals resembles the temperature behavior of polarization. These data imply that both symmetric and antisymmetric spin interactions involve in the magnetoelectric coupling in YMn$_2$O$_5$. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q10.00003: Magnetic field effects on charge and magnetic structures in a new multiferroic LuFe$_2$O$_4$ Jinsheng Wen, Guangyong Xu, Genda Gu, Steve Shapiro LuFe$_2$O$_4$ is a new multiferroic where the ferroelectric polarization originates from valence order of Fe$^{2+}$ and Fe$^{3+}$ ions instead of cation displacements. It evolves from two- to three-dimensional charge ordered state upon cooling, and the bulk polarization appears when the charges order three dimensionally. A ferrimagnetic order appears with further cooling. Bulk polarizations and the charge order are both enhanced when the magnetic order occurs, suggesting a strong coupling between the two orders in the system. We have performed neutron scattering measurements on both the charge and magnetic orders under external magnetic fields. We will discuss the field effects and their implications. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q10.00004: Reentrant ferroelectricity and the multiferroic phase diagram of Mn$_{1-x}$Fe$_{x}$WO$_{4}$ Rajit Chaudhury, Bernd Lorenz, Yaqi Wang, Yanyi Sun, Ching Chu Recently MnWO$_{4}$ has attracted attention because of its multiferroic properties. In MnWO$_{4}$ the Mn$^{2+}$ ions can be substituted by Fe$^{2+}$ since MnWO$_{4}$ and FeWO$_{4}$ are isomorphic. This opens the possibility to tune the magnetic orders by Fe-substitution for a better understanding of the microscopic interactions resulting in the multiferroic properties. We report the discovery of reentrant ferroelectricity in the phase diagram of multiferroic Mn$_{1-x}$Fe$_{x}$WO$_{4}$ single crystals. At zero magnetic field (H) the spin-spiral ferroelectric (FE) state is completely suppressed at Fe substitutions (x) exceeding 0.04. For x<0.04 a ferroelectric phase exists in a narrow temperature (T) range at zero magnetic field. This FE phase shows a reentrant behavior at lower T above a critical magnetic field H$_{c}$(x). The reentrant FE transition is explored by polarization, dielectric constant, and magnetization measurements. The complete multiferroic x-T-H phase diagram of Mn$_{1-x}$Fe$_{x}$WO$_{4}$ is derived. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q10.00005: X-ray resonant magnetic scattering study of multiferroic $R$MnO$_{3 }(R$~=~Dy, Ho, Er) compounds A.I. Goldman$^{1}$, S. Nandi$^{1}$, A. Kreyssig$^{1}$, L. Tan$^{1}$, J.W. Kim$^{1}$, J.Q. Yan$^{1}$, M.D. Vannette$^{1}$, J.C. Lang$^{3}$, D. Haskel$^{3}$, T.A. Lograsso$^{2}$, R.J. McQueeney$^{1}$ Element specific x-ray resonant magnetic scattering (XRMS) investigations were undertaken to determine the magnetic structure of multiferroic hexagonal RMnO$_{3}$ compounds. In the intermediate temperature phase (ITP) (8-68~K for the Dy$^{3+}$ and 4.5-40~K for Ho$^{3+})$ the moments are aligned and antiferromagnetically correlated in the \textbf{c }direction according to the same magnetic representation $\Gamma _{3}$. Below the ITP, the Dy$^{3+}$/Ho$^{3+}$ moments order differently and according to the magnetic representations $\Gamma _{2 }$/$\Gamma _{1}$. The temperature dependence of the observed intensity in the ITP can be modeled assuming the splitting of ground-state doublet/quasi-doublet crystal-field levels of Dy$^{3+}$/Ho$^{3+ }$by the exchange field of Mn$^{3+}$. No resonant signals could be found for Er$^{3+}$ from 2-80~K. Specific magnetic representations can be excluded for the magnetic order of Er$^{3+}$ moments but can not be uniquely determined within the sensitivity of XRMS. -- The support by U.S. DOE (DE-AC02-07CH11358 and DE-AC02-06 CH11357) is acknowledged. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q10.00006: Frequency dependent magneto-dielectric effect in bilayer manganite Pr(Sr$_{0.1}$Ca$_{0.9})_{2}$Mn$_{2}$O$_{7}$ Barnali Ghosh-Saha, D. Bhattacharya, S. Patnaik, A.K. Raychaudhuri, S. Arumugam We report novel frequency dependent magneto-dielectric effect and a strong dielectric anomaly near Neel temperature (T$_{N})$ in a single crystal of bilayer manganite Pr(Sr$_{0.1}$Ca$_{0.9})_{2}$Mn$_{2}$O$_{7}$ system. The magneto-dielectric effect measured in a field of 3T shows large frequency dependence and reaches a maximum ($\sim $25{\%}) near T$_{N}$ at a measurement frequency of 1 kHz. Change in frequency leads to a change in the sign of the effect. There is a sizeable dielectric relaxation process near T$_{N}$, which exhibits an activated behavior and strongly non-Debye nature at or below T$_{N}$ while becoming Debye like at higher temperature. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q10.00007: Flexoelectricity in nanostructures Invited Speaker: Crystalline piezoelectric dielectrics electrically polarize upon application of uniform mechanical strain. Inhomogeneous strain, however, locally breaks inversion symmetry and can potentially polarize even non-piezoelectric (centrosymmetric) dielectrics. Flexoelectricty--the coupling of strain gradient to polarization-- is expected to show a strong size-dependency due to the scaling of stain gradients with structural feature size. In this study, using a combination of atomistic and theoretical approaches, we elucidate the ``effective'' size-dependent piezoelectric and elastic behavior of inhomogeneously strained non-piezoelectric and piezoelectric nanostructures. We argue, through computational examples, the tantalizing possibility of creating ``apparently piezoelectric'' nano-composites without piezoelectric constituents and the emergence of size-dependent ``giant piezoelectricity'' in paradigmatical nanostructures. Finally, we propose that flexoelectricity is an important and essential contributor to the intrinsic dead-layer effect in high permittivity ferroelectric based nanocapacitors. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q10.00008: Evidence for the Potential Barrier Height Reduction in Metal-Oxide-Metal Tunnel Junctions due to the Interface Dependent Metal-Induced-Gap-States Lee Hosik, Hyuntae Jung, Yongmin Kim, Kyooho Jung, Hyunsik Im, Yuri Pashkin, O. Astafiev, J. S. Tsai, Yoshiyuki Miyamoto We have performed transport measurements on Al and Nb based metal-oxide-metal tunnel junctions with an AlO$_{x}$ tunnel barrier and found a strong dependence of the effective potential barrier height on the oxide-metal interface properties. Our estimations of the barrier height based on a phenomenological Simmons' model are consistent with the values obtained from the first-principle calculations. The calculations clearly show that the barrier height is strongly affected by the formation of metal induced gap states originating from the hybridization between metallic bands and Al$_{2}$O$_{3}$ conduction band. These findings are important for nanoelectronic devices containing tunnel junctions with a thin insulating layer. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q10.00009: Role of defects in the interfacial conductivity at interfaces in Pt/ SrTiO3/Pt heterostructures Naga Phani Aetukuri, Guenole Jan, Mahesh Samant, Kevin Roche, Stuart Parkin Oxides exhibit a wide variety of diverse phenomena including ferromagnetism, anti-ferromagnetism, ferroelectricity, superconductivity and multiferroicity. Furthermore, they display complex phase diagrams which result from the sensitivity of their properties to subtle changes in structure, doping, temperature and pressure. We will present the effect of interfaces and interface defects on the electrical switching properties of metal-insulator-metal (MIM) structures where the insulator is strontium titanate (STO) grown by pulsed laser deposition. The role of growth temperature and pressure and the effect of intentionally created oxygen vacancies on MIM device characteristics will be discussed. An attempt will be made to rationalize the role of intrinsic mechanisms and interface effects on the variability seen in device properties. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q10.00010: High-quality SrTiO$_{3}$ films using a hybrid MBE approach Bharat Jalan, Roman Engel-Herbert, Nick Wright, Susanne Stemmer A novel hybrid molecular beam epitaxy (MBE) approach for atomic-layer controlled growth of high-quality, epitaxial, stoichiometric SrTiO$_{3}$ films is presented. A solid source is used for Sr, an rf oxygen plasma source for oxygen and a metal-organic source for Ti. High-resolution x-ray diffraction revealed high quality single crystalline films with rocking curve full width half maxima similar to those of the substrates (i.e., 0.0095\r{ } for a LSAT substrate). RHEED showed persistent layer-by-layer growth ($>$ 180 oscillations), which has previously been observed only in a very few other systems (Si and GaAs). Surface reconstructions were observed during growth and related to the growth modes. Depending on growth conditions, step flow growth was also observed. The film surface RMS roughness was less than 0.2 nm. Excellent film stoichiometry was confirmed by homoepitaxy (prefect overlap of film and substrate reflections). Films were very insulating, consistent with oxygen stoichiometry. Oxygen gettering Ti contacts and vacuum anneals were used to produce n-type, oxygen deficient films to study carrier mobilities. We demonstrate the relative influence of hydrogen and oxygen vacancies on the electrical conductivity of SrTiO$_{3}$. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q10.00011: Conductance bistability in metal oxide junctions Zhongkui Tan, Vijay Patel, Esteban Monge, Shih-Sheng Chang, Shawn Pottorf, James Lukens, Konstantin Likharev We are exploring conductance bistability (memory) effects in junctions based on metal oxides, in the context of their possible applications in hybrid CMOS/nanoelectronic (e.g., CMOL [1]) circuits. So far, we have investigated CuO$_{x}$, NbO$_{x}$ and TiO$_{x}$ formed by thermal and plasma oxidation, with or without rapid thermal post-annealing (at 200 to 800$^{\circ}$C for 30 to 300 seconds). Conductance switching effects have been observed for all these materials. Particularly high endurance (over 1000 switching cycles) has been obtained for TiO$_{x}$ junctions plasma oxidized in 15mTorr oxygen and then post-annealed at 700$^{\circ}$C. However, the ON/OFF conductance ratio for these junctions is only about 5, and the sample-to-sample reproducibility is much lower than required for integrated circuit applications. Our plans are to extend our studies to a-Si junctions with one Ag electrode, and multilayer TiO$_{x}$ junctions, with the main goal to improve device reproducibility. The work is supported in part by AFOSR. \\[3pt] [1] K. K. Likharev, ``Hybrid CMOS/Nanoelectronic Circuits,'' accepted for publication in \textit{J. Nanoelectronics and Optoelectronics}, Nov. 2008. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q10.00012: Electrical characterization of epitaxial MgO/SiC capacitor structures Agham Posadas, Fred Walker, Charles Ahn, Trevor Goodrich, Zhuhua Cai, Kate Ziemer Epitaxial heterostructures of MgO (111) have been grown on hydrogen-cleaned 6H-SiC (0001) substrates and characterized using capacitance vs. voltage (C-V) and current vs. voltage (I- V) measurements. Low frequency capacitance measurements of MgO/SiC under strong accumulation conditions as a function of MgO layer thickness show that the epitaxial MgO has a dielectric constant of 10. The C-V measurements show modulation of the SiC from accumulation to depletion, consistent with the wafer conductivity type. The density of interface states was determined from ac conductance vs. frequency measurements, indicating a density of 6 x 10$^{11}$ eV$^{-1}$ cm$^{-2}$ at an energy level close to the conduction band edge, similar to what has been reported for native SiO$_{2}$ capacitor structures. In order to determine the dielectric breakdown field of the epitaxial MgO, leakage current as a function of gate voltage was measured for the capacitor structures. The median gate voltage at which the MgO breaks down was found to correspond to an electric field of 12 MV/cm, similar to that of bulk, single crystal MgO. These results show that epitaxial MgO has potential for use as a gate dielectric in SiC MOSFET applications. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q10.00013: Observation and coupling of domains in a spin spiral multiferroic Dennis Meier, Michael Maringer, Thomas Lottermoser, Manfred Fiebig, Petra Becker, Ladislav Bohaty The intrinsically strong cross coupling between magnetism and ferroelectricity in spin spiral multiferroics suggests these systems as prime candidates for novel multifunctional devices. Comprehension and controlling of the correlated antiferromagnetic (AFM) and ferroelectric (FE) domain structures by external fields is an indispensible prerequisite for future device design. However, very few is know about the domain topology and switching of AFM spin spirals and the magnetically induced FE domains. Here we present the spatial distribution of AFM and FE domains in MnWO$_4$, revealed by optical second harmonic generation. Electric fields are used to uniquely control the magnetic domain structure, while applied magnetic fields influence the poling behavior of the FE domains. [Preview Abstract] |
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