Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y49: Focus Session: Oxide Tunnel Junctions, Metal-Ferroelectric Interfaces, A-site Ordering |
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Sponsoring Units: DMP Chair: Jorge Iniguez, ICMAB, Spain Room: Mile High Ballroom 1C |
Friday, March 7, 2014 8:00AM - 8:36AM |
Y49.00001: Enhancement of Tunneling Electroresistance in tunnel junctions using bilyer barriers with ferroelectric driven phase transition Invited Speaker: Qi Li Ferroelectric and Multiferroic tunnel junctions (magnetic tunnel junction with a ferroelectric barrier) have become one of the very promising approaches to new generation of multifunctional devices. A large tunneling electroresistance (TER) (the resistance on-off ratio) is very desirable for utilizing the device as a resistance switch or for signal processing. We have designed a bilayer tunneling barrier in which one layer is ferroelectric and the other layer is close to metal-insulator as well as ferromagnetic to antiferromagnetic phase transition with a goal to significantly change the barrier parameters and the interface state with the ferroelectric polarization reversal. The phase transition can occur when the ferroelectric polarization is reversed. In La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/BaTiO$_{3}$/La$_{0.5}$Ca$_{0.5}$MnO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ tunnel junctions\footnote{Y. W. Yin, et al, \textit{Nature Materials} 12, 397 (2013).} where the La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ is the phase transition layer, this has resulted an increase of TER from 30{\%} (without the La$_{0.5}$Ca$_{0.5}$MnO$_{3})$ to 10,000{\%} (with the inserted layer). The mechanisms of such large increase of TER come from two sources: one is the metal to insulator transition of the La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ which effectively change the barrier width for the two polarization states and hence the tunneling current; and the other is the polarization driven magnetic reconstruction of La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ from ferromagnetic to antiferromagnetic state. The antiferromagnetic phase in the barrier acted as a spin valve for spin polarized tunneling current to significantly reduce the tunneling current. The details of the sample structures, electrical characterization, and the magneto transport studies will be presented and the results will also be compared with the first principles calculation. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y49.00002: Enhanced Tunneling Electroresistance by Interfacial Phase Transitions in Ultrathin Oxide Heterojunctions Lu Jiang, Woo seok Choi, Hyoungjeen Jeen, Shuai Dong, Yunseok Kim, Takeshi Egami, Ho Nyung Lee, Sergei V. Kalinin, Elbio Dagotto The ferroelectric (FE) control of electronic transport is one of the emerging technologies. Many previous studies in FE tunnel junctions (FTJs) exploited solely the differences in the electrostatic potential across the FTJs that are induced by changes in the FE polarization direction. In this work, by using ultrathin PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$/(La,Sr)MnO$_{3}$ heterojunctions, we present that in practice the junction current ratio between the two polarization states can be further enhanced when correlated electron oxides are used as electrodes, and that FTJs with nanometer thin layers can effectively produce a considerably large electroresistance ratio at room temperature. To understand these surprising results, we employed an additional control parameter, which is related to the crossing of electronic and magnetic phase boundaries of the correlated electron oxide. Our study highlights that the strong coupling between degrees of freedom across heterointerfaces could yield versatile and novel applications in oxide electronics. *The work was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y49.00003: Electric control of tunnel magnetoresistance in oxide multiferroic tunnel junction J. Tornos, Liu Yaohua, G. Sanchez-Santolino, C. Munuera, S.G.E. te Velthuis, F. Mompean, M. Garcia-Hernandez, M. Varela, S.J. Pennycook, Z. Sefrioui, C. Leon, J. Santamaria Magnetic tunnel junctions with a ferroelectric barrier are systems amenable to control the spin dependent tunnel conductance by the electric field. We have investigated La0.7Sr0.3MnO3(LSMO)/BaTiO3(BTO)/LSMO tunnel junctions and, despite their symmetric structure, we have found very large tunnel electroresistance (TER) close to 1000{\%} at low temperatures. This is interpreted in terms of a variation of the effective barrier thickness due to a large modulation of electron charge at the BTO/LSMO interface that is induced by the switching of ferroelectric polarization in BTO. Moreover, for the orientation of ferroelectric polarization that leads to the larger conductance value, the bias and temperature dependence of the tunnel magnetoresistance (TMR) is consistent with a depolarization (spin filtering) of the tunneling current. This behavior might be related to the presence of an induced Ti magnetic moment in BTO interface, antiparallel to that of Mn in LSMO, as detected by XMCD measurements. Our results reveal the possibility to tune spin dependent transport by an electric field through the reversal of the ferroelectric polarization of the barrier. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y49.00004: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y49.00005: Effect of interface structure on Schottky barrier height in SrRuO$_{3}$/SrTiO$_{3}$ heterojunctions Vemulavada Sampath Kumar, Manish Niranjan Complex oxide heterostructures are highly promising for technological applications as they offer novel device concepts and functionalities. One of the fundamental parameters that influence the characteristics of the metal/oxide heterostructure is the Schottky barrier formed at the interface. The Schottky barrier height (SBH) is strongly influenced by the atomic structure of the interface and is of fundamental interest as an intrinsic property of the system. The SrRuO3/SrTiO3 (001) heterostructure is a prototypical system to study SBH at the oxide metal/dielectric interface. In recent years, the SRO has attracted a lot of attention as an electrode material for ultrathin ferroelectric films. Using \textit{ab-initio} calculations, we have studied the $p$-type SBH and its dependence on the interface structure in SRO/STO heterostructure. In addition, we have estimated the $p$-SBH using semi-empirical Metal-Induced-Gap-States (MIGS) model. In particular we have considered three types of interfaces: RuO2/SrO/TiO2, RuO2/BaO/TiO2 and MnO2/SrO/TiO2 the \textit{ab-initio} estimate of $p$-SBH comes out to be 1.27, 1.33 and 0.78 eV for respective interfaces. We find that semi-empirical MIGS model overestimate the p-SBH by $\sim$2 eV. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y49.00006: Ferroelectric control of spin injection across the ferromagnet/ferroelectric interface Xiaohui Liu, J.D. Burton, Evgeny Tsymbal Magnetoelectric coupling has become one of the most attractive fields in modern materials research due their promise to electrically control spintronics-based devices. Previous investigations have shown that at the ferromagnet/ferroelectric interface, magnetization could be tuned by the reversal of ferroelectric polarization. We had previously predicted that ferroelectric polarization reversal can control the nature of the resistive contact at the SrRuO3/n-BaTiO3 heterojunction interface, going from the Ohmic to Schottky regimes with reversal of ferroelectric polarization [1]. It is known, however, that SrRuO3 displays robust ferromagnetism below the Curie temperature of about 160K. In this work, using first-principles density functional calculations, we explore the effect of ferroelectric polarization of spin-polarized transmission across the SrRuO3/n-BaTiO3 interface. Our study reveals that the interface transmission is negatively spin-polarized, and that ferroelectric polarization reversal leads to a change in spin polarization from -65{\%} for the Ohmic contact to -98{\%} for the Schottky contact. This sizeable change in the spin polarization could provide an interesting non-volatile mechanism to electrically control spin injection into semiconductor-based spintronics devices.\\[4pt] [1] X. Liu, et al., Phys. Rev. B 88, 165139 (2013). [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y49.00007: Ab initio study of a symmetric SrRuO3/PbTiO3/SrRuO3 ferroelectric capacitor Simon Divilov, Judith Gabel, Matthew Dawber, Marivi Fernandez-Serra We performed a density functional study of the free standing capacitor (SrRuO$_3$)$_1$/(PbTiO$_3$)$_m$/(SrRuO$_3$)$_1$ using local density approximation and Hubbard U to study the effects of SrRuO$_3$ [100] surfaces on the bulk properties of PbTiO$_3$. In addition we analyze how the thickness, epitaxial strain and termination plane of PbTiO$_3$ modify its bulk behavior. We observe different rumpling patterns for both paraelectric (PE) and ferroelectric (FE) phases, based on the termination plane. For the FE phase, we observe oxygen octahedra tilting dominated by in phase and out of phase tilts around the [100] axis. In all our simulations the SrRuO$_3$ layers remain metallic, even those at the open surfaces.An analysis of Schottky barriers and coupling between magnetism and ferroelectricity will be presented. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y49.00008: Electronic transport properties of PbTiO$_{3}$/SrRuO$_{3}$ superlattices Hsiang-Chun Hsing, Sara Callori, Judith Gabel, Fen Guan, Marivi Fernandez Serra, Xu Du, Matthew Dawber First principles calculations on PbTiO$_{3}$/SrRuO$_{3}$ superlattices indicate that even when the SrRuO$_{3}$ layers in these structures are only a single unit cell thick they retain a metallic character. In the out of plane direction the resistivity of the structures are expected to depend on the thickness of the PbTiO$_{3}$ layers which act as ferroelectric tunneling barriers. We have successfully fabricated high quality specimens of these superlattices using off axis RF magnetron sputtering and here we report on their transport properties. In the out of plane direction, as well as showing ferroelectric polarization-field hysteresis loops, the samples reveal tunneling characteristics that confirm that the SrRuO$_{3}$ layers do indeed retain their metallicity in the experimental realization of these structures. In addition to studying the effect of changing the thickness of the PbTiO$_{3}$ layers in the superlattice we have examined the impact that the ferroelectric polarization and the compositionally broken inversion symmetry have on current-voltage characteristics. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y49.00009: Linear magnetoelectricity at room temperature in perovskite artificial superlattices Saurabh Ghosh, Hena Das, Craig J. Fennie The primary challenge in the field of multiferroics remains to identify materials that have a functional coupling between an electrical polarization and a magnetization, i.e., a magnetoelectric effect, at room temperature. Such materials may, for example, facilitate technologically important devices based on the electric field control of magnetism. Atomic scale heterostructures of transition metal ABO$_3$ perovskites are an ideal platform to realize designer properties and functionalities that don't exist in the bulk phase diagrams of the constituent materials. Here we take advantage of a recent direction in functional perovskites (where the combination of heterointerfaces with rotations/tilts of the BO$_6$ octahedra facilitate ferroelectric order) to create a new class of room temperature multiferroics in which ferroelectricity induces linear magnetoelectricity. We consider heterostructures of rare-earth orthoferrites of \textit{Pnma} perovskites, (LnFeO$_3$)$_1$/(Ln$^{\prime}$FeO$_3$)$_1$. Computed values of linear ME coefficients are found to be comparable to the prototype ME compound Cr$_2$O$_3$. Finally, we discuss the role of the Ln \textit{f}-states in the ME response. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y49.00010: Engineered spatial inversion symmetry breaking in an oxide hetero-structure built from isosymmetric room temperature magnetically ordered components John Claridge, Jonathan Alaria, Matthew Dyer, Matthew Rosseinsky, Pavel Borisov, Troy Manning, Serban Lepadatu, Markys Cain, Elena Mishina, Natalia Sherstyuck, N.A. Ilyin, Joke Hadermann, David Lederman The oxide heterostructure [(YFeO$_{3}$)$_{5}$(LaFeO$_{3}$)$_{5}$]$_{40}$,which is magnetically ordered and piezoelectric at room temperature, has been constructed from two weak ferromagnetic AFeO$_{3}$ perovskites with different A cations using RHEED-monitored pulsed laser deposition. The polarisation arises by combining ordering on the A site, imposed by the periodicity of the grown structure, with appropriate orientations of the octahedral tilting, according to simple symmetry-controlled rules. Magnetization and MOKE measurements show that the heterostructure's magnetic structure is similar to that of the individual components. Evidence of the polarity was obtained from second harmonic generation and piezoelectric force microscopy measurements. Modeling of the piezoresponse allows extraction of d$^{33}$ (approximately 10 pC/N) of the heterostructure, which is in agreement with DFT calculations. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y49.00011: Band gap engineering $via$ electrostatic chemical strain in cation ordered LaSrAlO$_4$ Prasanna V. Balachandran, James M. Rondinelli In this work, we employ density functional theory to examine a novel design route that employs A-site cation ordering to engineer the band gaps of (A,A$^\prime$)BO$_4$ Ruddlesden-Popper (RP) oxides. Using LaSrAlO$_4$ as a model material, we show that the band gap is highly sensitive to the A-site cation ordering ranging from 3-4.5 eV. When the [AlO$_2$]$^{-1}$ layers are interleaved between two chemically equivalent [LaO]$^{1+}$ or [SrO]$^{0+}$ layers, we obtain the smallest band gap with a reduction of $\sim$1 eV determined from the Heyd, Scuseria, and Ernzerhof (HSE) hybrid exchange-correlation functional. We relate the observed band gap reduction to the local bond distortions arising from electrostatic chemical strain induced changes to the O 2$p$ and La 5$d$ states in the valence and conduction bands, respectively. [Preview Abstract] |
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