Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A20: Focus Session: Complex Oxide Thin Films Surfaces and Interfaces I: Superlattice Fabrication and Properties |
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Sponsoring Units: DMP GMAG Chair: John Mitchell, Argonne National Laboratory Room: Baltimore Convention Center 317 |
Monday, March 13, 2006 8:00AM - 8:36AM |
A20.00001: An atomic-scale view of the structure and electronic properties of manganite thin films and superlattices. Invited Speaker: Epitaxial manganite ultrathin films and heterostructures constitute an ideal system to study the nature and effects of inhomogeneity and phase competition in these materials. The presence of interfaces and epitaxial strain may cause new phenomena to arise, and a comprehensive study of the relations between structural, chemical and electronic properties at the atomic scale is needed. The combination of scanning transmission electron microscopy and energy loss spectroscopy represents a very powerful tool to analyze locally such relations. Unlike diffraction techniques, STEM-EELS allows clear separation of electronic valence from structural relaxations. Several examples will be presented, including the study of charge ordering in Bi$_{0.37}$Ca$_{0.63}$MnO$_{3}$. We find atomically-resolved striping of the Mn L$_{2,3}$ spectra that correlate with two distinct formal charge states (+3/+4). Theoretical results obtained by first-principles density-functional calculations reproduce the structural relaxation that leads to the striping with minimal charge transfer. In contrast, charge transfer may occur across YBa$_{2}$Cu$_{3}$O$_{7-x}$/La$_{0.67}$Ca$_{0.33}$MnO$_{3}$interfaces, where extensive charge transfer processes drastically affect the ferromagnetic/superconducting properties. Epitaxial strain can also play a determining role in ultrathin LCMO films, inducing a nanoscale modulation in the Mn+3/Mn+4 ratio which evidences the existence of nanoclusters with different electronic densities. [Preview Abstract] |
Monday, March 13, 2006 8:36AM - 8:48AM |
A20.00002: Doping without Disorder: Digital Synthesis of Manganite Superlattices Anand Bhattacharya, Xiaofang Zhai, Maitri Warusawithana, Jerald Kavich, John Freeland, Jim Eckstein, Sam Bader We have synthesized digital superlattices of (LaMnO$_{3})_{m}$(SrMnO$_{3})_{n }$ using ozone-assisted oxide MBE. This technique of \textit{digital synthesis} allows the realization of superlattices with the same overall stoichiometry as random alloys of composition La $_{m/m+n}$ Sr $_{n/m+n}$MnO$_{3}$, without introducing the random $A$-site disorder associated with bulk synthesis. In these digital superlattices, the `doping' or charge transfer occurs at well ordered coherent interfaces, whose electronic and magnetic properties may be studied by local probes, scattering techniques, and transport and magnetization studies. We shall present our results on a series of samples synthesized for various values of $m$ and $n$, exploring both metallic and insulating phases of the nominal phase diagram for the bulk counterpart. The results will be discussed in the context of the interfacial states that arise in structures obtained with digital synthesis. [Preview Abstract] |
Monday, March 13, 2006 8:48AM - 9:00AM |
A20.00003: Electronic Interfacial Effects in Epitaxial Heterostructures based on LaMnO$_{3}$. Hans M. Christen, M. Varela, H.N. Lee, D.H. Kim, M.F. Chisholm, C. Cantoni, L. Petit, T.C. Schulthess, D.H. Lowndes Studies of chemically abrupt interfaces provide an ideal platform to study the effects of discontinuities and asymmetries of the electronic configuration on the transport and magnetic properties of complex oxides. In addition, the behavior of complex materials near interfaces plays the most crucial role not only in devices and nanostructures but also in complex structures in the form of composites and superlattices, including artificial multiferroics. Interfaces in the ABO$_{3}$ perovskite system are particularly attractive because structurally similar oxides with fundamentally different physical properties can be integrated epitaxially. To explore the electronic effects at interfaces and to probe the physical properties that result from local electronic changes, we have synthesized structures containing LaMnO$_{3}$ and insulating perovskites using pulsed laser deposition. The local electron energy loss spectroscopy (EELS) capability of a scanning transmission electron microscope (STEM) is used to probe the electronic configuration in the LaMnO$_{3}$ films as a function of the distance from the interfaces. The results are compared to macroscopic measurements and theoretical predictions. Research sponsored by the U.S. Department of Energy under contract DE-AC05-00OR22725 with the Oak Ridge National Laboratory, managed by UT-Battelle, LLC. [Preview Abstract] |
Monday, March 13, 2006 9:00AM - 9:12AM |
A20.00004: Phase Separation in Ferromagnetic/Charge-ordered Superlattices T. Zac Ward, Jianxing Ma, Dane Gillaspie, Hong-Ying Zhai, E. Ward Plummer, Jian Shen The coexistence of ferromagnetic metal (FFM) and charge-ordered insulator (COI) observed in manganites, such as La$_{5/8-y}$Pr$_{y}$Ca$_{3/8}$MnO$_{3}$, has been an intensive focus in the study of transition metal oxides (TMO). Theories related to structural nanoscale disorder of charge carrier dopants and self-organization caused by an intrinsic elastic energy landscape have been proposed. To understand the subtle balance and competition between the different electronic phases, we have grown La$_{5/8}$Ca$_{3/8}$MnO$_{3}$/Pr$_{5/8}$Ca$_{3/8}$MnO$_{3}$ superlattices with different stacking periods in order to control the chemical ordering of La/Pr ions and thus the elastic energy landscape. The magnetic and transport properties of FM/CO superlattices as well as La/Pr disordered La$_{5/16}$Pr$_{5/16}$Ca$_{3/8}$MnO$_{3}$ will be presented and the driving mechanism of FM/CO phase separation will be discussed. Research sponsored by the U. S. Department of Energy under contract DE-AC05-00OR22725 with the Oak Ridge National Laboratory, managed by UT-Battelle, LLC [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:24AM |
A20.00005: New metallic interface state in oxide artificial superlattices investigated by optical spectroscopy Sung Seok A. Seo, Woo Seok Choi, Kyungwan Kim, Ho Nyung Lee, Li Yu, Christian Bernhard, Tae Won Noh Interfaces between the artificial structures of oxides have been attracting a lot of attention because of their novel physical properties, which are usually not obtained in single-phase bulk materials. As a model system to understand the interfaces between the Mott insulators and the band insulators, high quality artificial superlattices of SrTiO$_{3}$ / LaTiO$_{3}$ were epitaxially grown by pulsed laser deposition equipped with reflection high energy electron diffraction. Mid infrared-visible optical transmittance and reflectance spectra were measured to show highly conducting interface, providing clear evidences for electronic reconstruction at the interface. Moreover, temperature-dependent infrared ellipsometry results showed that the interface state was different from conventional metal, indicating a new two dimensional metallic state. [Preview Abstract] |
Monday, March 13, 2006 9:24AM - 9:36AM |
A20.00006: Understanding the atomic structure of epitaxial SrTiO$_{3}$-GaAs (001) hetero-interfaces. Robert Klie, Yimei Zhu, Yong Liang, Eric Altman, Weronika Walkosz, Juan-Carlos Idrobo, Serdar Ogut The combination of high-resolution Z-contrast imaging, electron energy-loss spectroscopy (EELS), and density functional theory (DFT) calculations was used to study the interfacial structure of ultra-thin SrTiO$_{3}$ films on GaAs(001). Z-contrast imaging suggests an atomically sharp hetero-interface with SrTiO$_{3}$ [110] in registry with As-terminated GaAs [100] with no visible reconstruction of either surface. Our initial DFT-calculations reveal that such a stoichiometric interface would have states in the band gap, while X-ray photoelectron spectroscopy (XPS) shows a clean band gap and a valence band offset more than twice of the calculated offset. Therefore, we will use atomic-column resolved EELS and further DFT-calculations to explore the role of oxygen vacancies at the interface in compensating for the As dangling bonds and search for the low-energy semi-conducting SrTiO$_{3}$/GaAs-system. [Preview Abstract] |
Monday, March 13, 2006 9:36AM - 9:48AM |
A20.00007: Magnetic properties of La$_{2/3}$Sr$_{1/3}$MnO$_{3}$/Pr$_{2/3}$Ca$_{1/3}$MnO$_{3}$ superlattices Dario Niebieskikwiat, Luis Hueso, Myron Salamon, Neil Mathur We present a magnetization study of ferromagnetic/antiferromagnetic (FM/AFM) manganite superlattices, grown by pulsed laser deposition on SrTiO$_{3}$ substrates. The FM layers are 15-nm-thick La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) sheets and the AFM layers were made of Pr$_{2/3}$Ca$_{1/3}$MnO$_{3}$ (PCMO), with variable thickness t$_{A}$ between 0 and 7.6 nm. Although all our multilayers exhibit a PM-FM transition of the LSMO layers at T$_{C}\sim $340K, only for t$_{A}$=0 do we observe a FM moment M$_{0}$ close to the expected saturation for the 1/3 doping. As soon as the AFM layers are added (t$_{A}$= 0.8nm) M$_{0}$ decreases, related to the introduction of the FM/AFM interfaces. The lack of exchange bias would indicate that the reduction of the FM moment is due to the appearance of a magnetically dead layer in the LSMO close to the interface with the AFM volume. Upon a further increase of t$_{A}$, the FM moment increases again and develops a peak at t$_{A}\sim $3.5 nm. We explain this behavior in terms of the accommodation of nanometric FM droplets in the PCMO layers. [Preview Abstract] |
Monday, March 13, 2006 9:48AM - 10:00AM |
A20.00008: Strong interaction of ultra thin cuprate superconductor and ferromagnetic manganites Xiaofang Zhai, James Eckstein We have made bi-layer films with thick manganite layers on top of thin layers of the cuprate superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{7+\delta }$ (BSCCO). The samples were grown using ozone assisted atomic layer by layer molecular beam epitaxy. For samples in which the manganite is the ferromagnet La$_{2/3}$Sr$_{1/3}$MnO$_{3}$(LSMO), the normal and superconducting properties of the BSCCO can be strongly affected by the LSMO when the BSCCO layer is sufficiently thin. In particular, samples with ten molecular layers of BSCCO were largely unaffected by the LSMO. On the other hand, when the number of BSCCO molecular layers was reduced to 5 and then to 4, the normal state above 80K became much more resistive and the superconducting Tc was reduced. However, a sample with four molecular layers of BSCCO capped with insulating and presumably antiferromagnetic SrMnO3 showed no effect of the manganite of BSCCO transport. This indicates that the reduced conductivity of the 4-layer sample capped with LSMO is due to the interaction of mobile carriers from the two systems at the interface. This interaction extends about 4 unit cells into the BSCCO. [Preview Abstract] |
Monday, March 13, 2006 10:00AM - 10:12AM |
A20.00009: Oscillatory behavior of the magnetoresistance of F/S/F trilayers Norbert M. Nemes, M. Garcia-Hernandez, V. Pe\~na, Z. Sefrioui, J. Garcia-Barriocanal, C. Visani, D. Arias, C. Leon, J. Santamaria, S.G.E. te Velthuis, A. Hoffmann We have measured magnetoresistance of trilayers made of highly spin polarized La$_{0.7}$Ca$_{0.3}$MnO$_3$\ ferromagnets and high-T$_c$\ superconducting YBa$_2$Cu$_3$O$_{7-\delta}$\ with the current in plane (CIP) geometry. Large magnetoresistance peaks occur for antiferromagnetic (AF) alignment of the manganite layers, much resembling the GMR in metallic superlattices, and probably resulting of enhanced interface scattering in the AF configuration. We show RKKY-type oscillations in the intensity of the giant magnetoresistance (GMR) peak as a function of the thickness of the superconductor, perhaps originating in oscillatory magnetic coupling. The magnetoresistance vanishes for thickness of the superconductor in excess of 30 nm suggesting that this length could be an upper limit for the spin diffusion length. [Preview Abstract] |
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