Symposium Organizers
Guus Rijnders University of Twente
Rossitza Pentcheva University of Munich
Jak (Jacques) Chakhalian University of Arkansas
Ivan Bozovic Brookhaven National Laboratory
W2: Theory and Experiment II
Session Chairs
Harold Hwang
Rossitza Pentcheva
Tuesday PM, April 14, 2009
Room 3003 (Moscone West)
2:30 PM - **W2.1
Spectroscopy of Electronic Reconstructions at Oxide Interfaces.
Bernard Keimer 1
1 , Max Planck Institute for Solid State Research, Stuttgart Germany
Show AbstractWe will discuss recent experiments using a combination neutron reflectometry, x-ray spectroscopy, and spectral ellipsometry to probe charge transport as well as magnetic and orbital polarization at oxide interfaces. Of particular interest will be cuprate-manganate [1,2] and nickelate [3] interfaces. The latter experiments indicate an intrinsic metal-insulator transition at the interface driven by charge ordering.[1] J. Chakhalian et al., Nature Phys. 2, 244 (2006)[2] J. Chakhalian et al., Science 318, 1114 (2007)[3] A. Boris et al. (unpublished)
3:00 PM - **W2.2
Crystal-field vs Superexchange Effects in Strongly Correlated Oxides.
Eva Pavarini 1
1 IFF, Forschungzentrum Juelich, Juelich Germany
Show AbstractOrbital-ordering phenomena play a crucial role in determining the electronic and magnetic properties of many transition-metal oxide. The origin of these phenomena in real materials is, however, still controversial. While it is clear that Coulomb repulsion is a key ingredient, it remains uncertain whether it just enhances the effects of lattice distortions (and thus of the crystal-field) or it really drives orbital-order via a purely electronic Kugel-Khomskii superexchange mechanism. In this work, using the LDA+DMFT approach, we study the interplay between super-exchange and crystal-field effects and clarify the origin of orbital-order in some paradigm compounds: KCuF3, LaVO3 and YVO3. Consequences for the properties of oxides heterostructures are discussed.
3:30 PM - W2.3
Orbital Reconstruction at the SrTiO3/LaAlO3 Interface.
Marco Salluzzo 1 , Giacomo Ghiringhelli 2 , Julio Criginski Cezar 3 , Nicholas Brookes 3 , Valentina Bisogni 3 , Gabriella Maria De Luca 1 , Christoph Richter 4 , Thiel Stefan 4 , Jochen Mannhart 4 , Mark Huijben 5 , Alexander Brinkman 5 , Guus Rijnders 5
1 COHERENTIA, CNR-INFM, Napoli Italy, 2 Dipartimento di Fisica, Politecnico di Milano, Milano Italy, 3 European Synchrotron Radiation Facility, ESRF, Grenoble France, 4 Experimental Physics VI, Center for Electronic Correlations and Magnetism , Augsburg Germany, 5 Faculty of Science & Technology and MESA+ Institute for Nanotechnology, University of Twente, Enschede Netherlands
Show AbstractInterfaces between metal transition oxides exhibit unexpected physical properties that depend drastically on the mutual arrangement of each single atomic plane. Outstanding examples come from the discovery that a 2D conducting electron gas is realized at the TiO2/LaO n-type interface (IF) between LaTiO3/SrTiO3 [1], LaVO3/SrTiO3 [2], and LaAlO3/SrTiO3 compounds [3], i.e. by combining band and/or Mott insulators. In the case of the LaAlO3/SrTiO3 system many striking properties, including high carrier mobility, as well as low-T superconductivity [4] and signatures of magnetic scattering effects [5], have been found. These findings have raised strong interest in the material science community and a lively debate on the origin of the phenomena. A possible explanation is an ‘electronic reconstruction’ of the interface, realizing a transfer of electrons from the LaAlO3 band insulator to SrTiO3, thereby avoiding the “polarization catastrophe” associated with the alternating polar layers of the LaAlO3 film. The main criticism to this scenario is that similar properties could be obtained by supposing the realization of an oxygen defective SrTiO3 and/or an La1-xSrxTiO3, i.e. a purely extrinsic effect.
Here, by using X-ray Absorption Spectroscopy, i.e. an orbital and interface sensitive spectroscopy technique, we show that electronic reconstruction is in fact at the base for the realization of the electron gas at the LaAlO3/SrTiO3 n-type IF. In particular, we find that the 3d orbital degeneracy of the titanium states located at the interface is fully removed and that the 3dxy in-plane orbitals become the first available interface states when the system becomes conducting. The splitting is consistent with an ordering of the Ti 3dxy orbital belonging to the TiO6 octahedra at the interface, as theoretically proposed for the LaTiO3/SrTiO3 system [6].
References
[1] A. Ohtomo, D. A.Muller, J. L.Grazul & H.Y. Hwang, Nature 419, 378–380 (2002).
[2] Y. Hotta, T. Susaki, and H.Y. Hwang, Phys. Rev. Lett. 99, 236805 (2007).
[3] A. Ohtomo & H. Y. Hwang, Nature 427, 423–426 (2004).
[4] N. Reyren et al., Science 317, 1196 (2007).
[5] A. Brinkman et al., Nature Mater. 6, 493-496 (2007).
[6] Satoshi Okamoto & Andrew J. Millis, Nature 428, 630–633 (2004).
3:45 PM - W2.4
First-principles Study of Orbitally Degenerate Transition Metal Oxide Interfaces.
James Rondinelli 1 , Nicola Spaldin 1
1 Materials Department, UC Santa Barbara, Santa Barbara, California, United States
Show AbstractDue to the many competing interactions across oxide-oxide interfaces, bulk ground state behaviors can be manipulated with small degeneracy breaking perturbations to stabilize new magnetic configurations. In this work, we present results of first-principles density functional calculations comparing the LaNiO3/LaAlO3 and SrTiO3/SrFeO3 interfaces. We begin by examining the effect of strain and electron-electron correlation effects in stabilizing different electronic and magnetic ground states in the parent magnetic systems. Using this as a reference, we discuss the effect of heteroepitaxy on the orbital physics and the subsequent metal-insulator transitions that occur across the interface. We also examine the coupling/competition between electronic polarization and magnetism in the comparison of these systems, since SrTiO3 becomes ferroelectric under epitaxial strain, while LaAlO3 remains a non-polar band insulator. We conclude by commenting on which superlattices optimize the electron-lattice and spin-lattice couplings across the interface, and suggest material combinations for experimental investigation.
4:30 PM - **W2.5
Study of Model Hamiltonians for Oxide Interfaces.
Elbio Dagotto 1 2
1 Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee, United States, 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractUsing a variety of numerical techniques, mainly Monte Carlo simulations of double exchange models, several simple Hamiltonians for oxide interfaces have been studied. In particular, interfaces involving manganites with different hole doping concentrations, such as LaMnO3 (LMO) and SrMnO3, have been analyzed with focus on the distribution of charge, spin and orbital order, and comparison with recent experiments. Trilayers involving LSMO-LMO-LSMO, with LSMO being Sr-doped LMO, have also been studied with emphasis on the magnetoresistance and effects related with even or odd number of sites in the LMO barrier. In addition, interfaces between manganites and cuprates have been investigated, and predictions to obtain electron-doped Cu oxides via the transfer of charge from manganites were formulated. Finally, motivated by recent experiments, a study of the interface between LSMO and the multiferroic material BiFeO3 have also been carried out, unveiled interesting effects that will be reviewed in this presentation. -------------------------------------------------------------SUPPORT of this effort has been provided by NSF and DOE.-------------------------------------------------------------COLLABORATORS: This work has been carried out in collaboration with S. Dong, S. Yunoki, R. Yu, M. Daghofer, A. Moreo, J-M. Liu, J. Riera, I. Gonzalez, G. Alvarez, S. Kancharla, S. Okamoto, S. Picozzi, K. Yamauchi, R. Ramesh, M. Huijben, A. Fujimori, and others.-------------------------------------------------------------REFERENCES by our group, were more details can be found, are: (1) Shuai Dong, et al., cond-mat/0810.1441, to appear in PRB(RC); (2) S. Yunoki, et al., Phys. Rev. B 78 024405 (2008); (3) S. Yunoki, et al., Phys. Rev. B 76, 064532 (2007); (4) M. Huijben et al., in preparation.
5:00 PM - **W2.6
Effect of Stoichiometry on the Two-Dimensional Electron Gas at the LaAlO3/SrTiO3 Interface Grown by MBE.
Maitri Warusawithana 1 , C. Brooks 1 , D. Schlom 1 , S. Thiel 2 , J. Mannhart 2 , N. Reyren 3 , A. Caviglia 3 , S. Gariglio 3 , J. Triscone 3
1 Department of Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 2 Experimentalphysik VI, University of Augsburg, Augsburg Germany, 3 Department of Physics – Condensed Matter, University of Geneva, Geneva Switzerland
Show AbstractThe discovery of a quasi 2-dimensional electron gas (q2-DEG) at the interface between SrTiO3 and LaAlO3 has enabled a number of exciting developments. So far this q2-DEG has been observed only in films grown by pulsed-laser deposition, which raised a question as to whether this manifestation has a connection with defects that result from the dynamics of the growth scheme employed. We find that a q2-DEG can also be obtained using the more gentle growth technique, molecular-beam epitaxy, and that control of the stoichiometry of the LaAlO3 layer is key to its existence. Small changes in the composition of the LaAlO3 layer affect the conductivity at the heterointerface. With appropriate stoichiometry the electron gas transitions into a superconducting state below ~200 mK. These findings of composition control on conductivity and the ability to obtain this q2-DEG under the framework of molecular-beam epitaxy open up interesting possibilities. Future directions of fundamental and applied interest including strained interfaces between other oxide phases, modulation doping through artificially layered structures, and the realization of such a q2-DEG on silicon, will be discussed.
5:30 PM - **W2.7
Enhanced Ionic Transport At Complex Oxide Interfaces.
J. Garcia-Barriocanal 1 , A. Rivera-Calzada 1 , M. Varela 2 , Z. Sefrioui 1 , E. Iborra 3 , C. Leon 1 , S. Pennycook 2 , Jacobo Santamaria 1
1 GFMC, Universidad Complutense de Madrid, Madrid Spain, 2 Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, Tennessee, United States, 3 ETSIT, Universidad Politécnica de Madrid, Madrid 28040 Spain
Show AbstractInterfaces between complex oxides has become a very active research area due to the possibility of tailoring their electronic structure to display novel behaviors and functionalities. Recent advances in the growth of complex oxides have allowed the alternated growth of thin layers of different materials in heterostructures of high crystalline perfection and with very controlled interface structure. The symmetry breakdown occurring at the interfaces causes profound changes on the charge orbital and spin structure and new correlated states appear with unexpected spin and conducting properties. In this talk we describe that ionic transport may also be deeply modified at interfaces. Ionic transport is known to be strongly modified in nanomaterials, and thus, nanotechnology is expected to have a large impact on the next generation solid electrolytes of application in fuel cells or batteries within the emerging field of nanoionics. We will describe the strong enhancement of the conductivity occurring at the interfaces of superlattices made by alternating strontium titanate and yttria stabilized zirconia [1]. Lateral electrical conductivity measurements show an enhanced oxygen conductivity in ultra thin film heterostructures, which for 1 nm thick YSZ layers is found to be as high as 0.014 S/cm at 357 K (about eight orders of magnitude higher than that of bulk YSZ), with a substantial decrease of the activation energy for the dc ionic conductivity from 1.1 eV down to 0.64 eV. EELS analysis is consistent with a partial occupancy and high disorder in the interface oxygen plane between YSZ and STO layers, which would yield a large number of interfacial oxygen vacancies and simultaneously give rise to a decrease in the activation energy for oxygen migration. Our results demonstrate that the observed high oxygen conductivity in ultra thin layers is a genuine interface process, and that the design of suitable heterogeneous interfaces in epitaxial heterostructures might have important implications in the search of artificial nanostructures with high ionic conductivity for application in solid oxide fuel cells or other technological devices.Work at UCM and UPM supported by Spanish Ministry for Science and Innovation grants MAT2007 62162, and MAT2008 6517. Research at ORNL (MV and SJP) sponsored by the Division of Materials Sciences and Engineering of the US Department of Energy.[1] J. Garcia-Barriocanal, A. Rivera-Calzada, M. Varela, Z. Sefrioui, E. Iborra, C. Leon, S. J. Pennycook, J. Santamaria Science 321, 676 (2008)
Symposium Organizers
Guus Rijnders University of Twente
Rossitza Pentcheva University of Munich
Jak (Jacques) Chakhalian University of Arkansas
Ivan Bozovic Brookhaven National Laboratory
W5: Doping, Field Effect and Devices
Session Chairs
Wednesday AM, April 15, 2009
Room 3003 (Moscone West)
11:15 AM - **W5.1
A Novel Approach to Optimizing Field Effect Transistors
Thilo Kopp 1 , Martin Breitschaft 1 , Rainer Jany 1 , Arno Kampf 1 , Yang-Chung Liao 1 , Christoph Richter 1 , Stefan Thiel 1 , Cheng Cen 2 , Jeremy Levy 2 , Andrea Caviglia 3 , Stefano Gariglio 3 , Nicolas Reyren 3 , Jean-Marc Triscone 3 , Jochen Mannhart 1
1 Center for Electronic Correlations and Magnetism , University of Augsburg, Augsburg Germany, 2 Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 3 Département de Physique de la Matière Condensée, University of Geneva, Geneva Switzerland
Show AbstractAt interfaces between insulating oxides, robust electron gases have been generated that have unique electronic properties, such as a electric-field tunable metal-insulator transitions. Inspired by the properties of such electron systems we propose electronic devices that may resolve quantum mechanical limitations of standard, semiconductor electronics.
11:45 AM - **W5.2
Heterostructured Oxide Nanowires and Their Interface Properties
Tomoji Kawai 1 , Takeshi Yanagida 1
1 , Osaka University, Osaka Japan
Show AbstractAn interface of heterostructured oxides exhibits the novel functional properties including transport, magnetic, ferroelectric properties. Although such fascinating properties at heterointerfaces have been discovered in 2D heterostructured oxide thin films, heterostructured oxide 1D nanowires might be other potential candidates towards functional nano-oxide devices due to their dimensionality and huge specific surface area. Here we report our recent progress on the formation of heterostructured oxide nanowires and their properties [1-9]. The oxide core-shell heterostructured nanowires were fabricated by in-situ laser MBE technique. Various oxide heterostructured nanowires, using rock-salt-NiO and MgO, rutile-TiO2 and SnO2, and spinel-Fe3O4, were fabricated via the in-situ technique. In forming the oxide heterostructured nanowires, the lattice matching in 3D was found to be a crucial factor to fabricate the well-defined epitaxial heterointerface. The atomic inter-diffusion at the heterointerface was also found to play a crucial role on the transport and magnetic properties of fabricated heterostructured oxide nanowires. The transport properties of single heterostructured oxide nanowires including the resistive memory effects were investigated using nano-gap electrodes and C-AFM techniques. We will present and discuss the underlying issues on the heterostructured oxide nanowires toward the nano-oxide devices using fascinating oxide interfaces.References[1] Appl. Phys. Lett., 90, 233103 (2007), [2] Appl. Phys. Lett., 91, 061502 (2007), [3] J. Appl. Phys., 101, 124304 (2007), [4] J. Appl. Phys., 102, 016102 (2007), [5] J. Appl. Phys., 104, 016101 (2008), [6] Appl. Phys. Lett., 93, 153103 (2008), [7] J. Phys, Chem., (2008) in press, [8] J. Am. Chem. Soc., 130, 5378 (2008), [9] Appl. Phys. Lett., 92, 173119 (2008)
12:15 PM - W5.3
Doping, Optical Properties and Growth of a Transition Metal Oxide Based Semiconductor.
Yoshiharu Krockenberger 1 , Masashi Kawasaki 1 2 , Yoshinori Tokura 1 3
1 CMRG, RIKEN, Wako Japan, 2 WPI Advanced Institute for Materials Research, Tohoku University, Tohoku Japan, 3 Department of Applied Physics, University of Tokyo, Tokyo Japan
Show AbstractIn recent years, semiconductor based p-n junctions are used in a vast variety of applications. However, increasing demand demand for transition metal oxide semiconducting materials appeared due to huge amount of possibilities on combinations with materials showing various physical properties, e.g. ferromagetism, superconductivity, ferroelectricity, a.s.o. In our present investigation, a transition metal perovskite system was grown epitaxially on (001) SrTiO3 substrates utilizing pulsed laser deposition (PLD) technique. Since oxygen acts as a dopant, the films have been treated under various annealing procedures. The epitaxial relationship is confirmed by using four-circle x-ray diffraction. Transport property measurements as well as optical transmission- and reflection measurements have been carried out. Moreover, C-V characteristic measurements have been taken to determine the Fermi energy level respective to Nb doped SrTiO3 substrates.
12:30 PM - W5.4
Modulation Doping of a Mott Quantum Well by a Proximate Polar Discontinuity.
Takuya Higuchi 1 , Yasushi Hotta 1 , Tomofumi Susaki 1 , Atsushi Fujimori 2 , Harold Hwang 1 3
1 Dept. of Advanced Materials Science, Univ. of Tokyo, Kashiwa, Chiba, Japan, 2 Dept. of Physics, Univ. of Tokyo, Bunkyo-ku, Tokyo, Japan, 3 , Japan Science and Technology Agency, Kawaguchi, Saitama, Japan
Show AbstractWe present evidence for hole injection into LaAlO3/LaVO3/LaAlO3 quantum wells near a polar surface of LaAlO3 (001). Surfaces and interfaces of oxides have been of growing interest, partially because of the rich variety of bulk oxide functionalities, as well as their unique reconstruction mechanisms not found in conventional semiconductors. The observation of metallic interfaces between two perovskite insulators, LaAlO3 and SrTiO3 [A. Ohtomo and H. Y. Hwang, Nature 2004], has motivated many studies on the origin of this conductivity. Two scenarios have been proposed, one based on electronic reconstructions driven by the polar discontinuity at the interface and another based on growth induced oxygen vacancies. Discriminating between these two proposed scenarios has been controversial, in part because both mechanisms could give similar transport and spectroscopic signatures. However, if the hole-doping using the polar discontinuity is observed, it can be clearly distinguished from the role of the electron-donor oxygen vacancies. In order to test this possibility of hole-doping, we have studied the transport properties of the Mott insulator LaVO3 embedded in AlO2-terminated LaAlO3 in trilayer structures. The polar surface of the AlO2-terminated LaAlO3 requires a net half positive charge to reconcile the surface instability. In LaAlO3, because of the fixed valence of the constituent members, it is realized by atomic reconstructions through surface off-stoichiometry or lattice distortion. However, when we bury a material that can easily accommodate holes to form mixed valence states, an alternative electronic reconstruction can solve the instability: hole-doping to the embedded layer. LaVO3 is an attractive candidate since it can be readily hole-doped by chemical substitution, and because of its structural and thermodynamic compatibility with LaAlO3 for the growth of atomically precise thin film structures. As the surface is brought in proximity to the LaVO3 layer, an exponential drop in resistance and a decreasing positive Seebeck coefficient is observed below a characteristic coupling length of 10-15 unit cells. We attribute this behavior to a crossover from an atomic reconstruction of the AlO2-terminated LaAlO3 surface to an electronic reconstruction of the vanadium valence. These results suggest a general approach to tunable hole-doping in oxide thin film heterostructures.
Symposium Organizers
Guus Rijnders University of Twente
Rossitza Pentcheva University of Munich
Jak (Jacques) Chakhalian University of Arkansas
Ivan Bozovic Brookhaven National Laboratory
W7: Analysis II
Session Chairs
Thursday AM, April 16, 2009
Room 3003 (Moscone West)
9:00 AM - **W7.1
The Effect of Relaxation on the Electronic Interface Configuration in LaAlO3/SrTiO3 Superlattices.
Sara Bals 1 , Jo Verbeeck 1 , Dirk Lamoen 1 , Gustaaf Van Tendeloo 1 , Mark Huijben 2 , Guus Rijnders 2 , Alexander Brinkman 2 , Hans Hilgenkamp 2 , Dave Blank 2
1 Physics, EMAT - University of Antwerp, Antwerp Belgium, 2 , Faculty of Science and Technology and MESA+ Institute for Nanotechnology. University of Twente, Enschede Netherlands
Show AbstractThe electronic structure at the LaO-TiO2 and the SrO-AlO2 interfaces in multilayers of SrTiO3 and LaAlO3 have been investigated by electron energy loss spectroscopy in a scanning tranmission electron microscope. These results are combined with ab-initio calculations to interpret the experimental observations. Special attention is paid to the valence state of the Ti-ions near the LaO-TiO2 interface. In order to compensate for the dipole field created by the polar discontinuities present in these systems, structural relaxation is considered to be important. Therefore, in our calculations, atomic positions have been relaxed by minimizingthe forces on the atoms, which resultes in picometer shifts of the atom positions. The effect of this relaxation will be presented as a function of layer thickness by studying the bandgap, Ti valence and the projected density of states for the Ti d-band. Finally, we discuss the difference between systems consisting of only one type of interface and multilayers in which two types of interfaces occur.
9:30 AM - **W7.2
Novel Properties of Atomic Scale Complex Oxide Heterostructures
John Freeland 1
1 Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractStrongly correlated oxide heterostructures have been predicted to possess a wide variety of unique properties in systems that have yet to be fabricated. In the area of spin based electronics there have been predictions that single unit cell superlattices composed of interleaving single layers of LaVO3 and LaMnO3 could possess zero net magnetic moment yet be able to create a fully spin polarized current [1]. Another example is the atomic level layering of LaNiO3 and LaAlO3 to manipulate orbital ordering in Nickelates via strain and confinement[2]. However, disorder effects occurring during bulk synthesis make it hard to realize many of these structures by conventional routes. Unit-cell level digital synthesis of oxides however is now quite feasible and opens the possibility of pursuing these novel materials. X-ray probes offer a unique ability to extract bulk vs interface properties in an element-resolved manner as illustrated by our recent work on complex oxide heterostructures[3-5]. Polarization-dependent spectroscopies provide direct chemically-specific information about the valence, orbital occupancies, and magnetism. Here I will present our recent exploration of systems when we move from the case of isolated interfaces to those completely dominated by interface properties. These systems are layered perovskite structures of the form: LaMO3/LaM’O3 grown using pulsed laser deposition with RHEED control to achieve unit cell level control. This work ranges from creating magnetic materials out of systems for which the bulk are nominally antiferromagnetic (e.g. LaFeO3 and LaCrO3) as well as interleaving metallic systems and insulators to create novel materials (e.g. LaNiO3 and LaAlO3).Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.[1] W.E. Pickett, Phys. Rev. B 57, 10613 (1998).[2] J. Chaloupka and G. Khaliulin, Phys. Rev. Lett. 100, 16404 (2008).[3] J. Chakhalian, J.W. Freeland, et. al. Nature Physics 2, 244 (2006). [4] J.W. Freeland, et. al. Appl. Phys. Lett. 90, 242502 (2007).[5] J. Chakhalian, J.W. Freeland, et. al. Science 318, 1114 (2007).
10:00 AM - W7.3
Electronic Phenomena in Cuprate/manganite Interfaces.
Maria Varela 1 , Stephen Pennycook 1 2 , Weidong Luo 2 1 , Sokrates Pantelides 2 1 , Javier Garcia-Barriocanal 3 , Zouhair Sefrioui 3 , Carlos Leon 3 , Jacobo Santamaria 3
1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , Vanderbilt University, Nashville, Tennessee, United States, 3 , Universidad Complutense de Madrid, Madrid Spain
Show AbstractCuprate/manganite interfaces are of fundamental interest since they allow the interaction between competing phenomena such as high temperature superconductivity and ferromagnetism to be studied. In the aberration corrected scanning transmission electron microscope (STEM), we can investigate their structure simultaneously with their chemistry and electronic properties through atomic resolution electron energy loss spectroscopy (EELS). This talk will show some examples of the application of STEM-EELS to the study of new physical phenomena these interfaces. We will show how the band structure of high Tc superconductors can be mapped atomic plane by atomic plane since the lower and upper Hubbard bands in YBa2Cu3O7 (YBCO) give rise to distinctive features in the O K edge fine structure. These changes in spectral features can be tracked through EELS spectrum images, allowing the density of holes (superconducting carriers) in the material to be mapped. Such images together with Density-Functional Theory calculations show how the occupation of these bands can be affected by charge transfer processes in heterostructures where YBCO is combined with an electron donor, such as LaMnO3.This research was sponsored by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, U.S. Department of Energy.
10:15 AM - W7.4
Magnetic Anisotropy and Interfacial Coupling in Oxide Thin Films and Heterostructures Probed with Soft X-ray Dichroism.
Elke Arenholz 1 , Gerrit van der Laan 2
1 , ALS/LBNL, Berkeley, California, United States, 2 , Diamond Light Source, Didcot United Kingdom
Show AbstractThe combination of novel magnetic properties induced by reduced dimensionality and strong magnetic interactions across interfaces leads to intriguing new properties in magnetic hetero- and nanostructures not observed in the constituent materials in bulk form. It is the careful optimization of the characteristics of the individual layers as well as the magnetic coupling across the interface that allows us to control the magnetic properties and tailor them for devices, e.g., in information storage and processing technology.Soft x-ray magnetic spectroscopies can make unique contributions to improving our understanding of complex magnetic nanostructures since these techniques provide elemental, valence- and site-symmetry specific information with high sensitivity and tunable probing depth. X-ray magnetic circular dichroism (XMCD) is sensitive to (unidirectional) ferromagnetic order, while x-ray magnetic linear dichroism (XMLD) can also detect (uniaxial) antiferromagnetic order. A crystalline electric field with cubic symmetry induces only a weak angular dependence in XMCD spectra but can cause a very pronounced anisotropy in XMLD spectra. Furthermore, non-magnetic sites with a distorted local cubic symmetry can give rise to an x-ray linear dichroism (XLD). In this presentation, we discuss how to distinguish between the individual contributions to soft x-ray dichroism spectra in order to extract the wealth of information about magnetic thin films, interfaces and hetero- and nanostructures contained in the data.We measured the asphericity and the energy splitting of the 4f states in EuO thin films [1] – a material with fascinating properties and of technological importance for spintronics applications - using XMLD. Our measurements, which are confirmed by multiplet calculations, show that there is significant 4f anisotropy. This suggests that pinning of the 4f states by the local environment becomes feasible and can be tuned by external conditions, chemical doping, and strain for use in device applications.Revisiting previous XMLD studies of the Co/NiO(001) interface taking the impact of the crystal electric field on the XMLD into account for the first time, we show that NiO(001) exhibits a crystallographic and magnetic domain structure near the surface that is identical to that of the bulk. Upon Co deposition perpendicular coupling of Co and Ni moments is observed [2, 3] that persists even in the presence of uncompensated interface moments.Moreover, we discuss the impact of epitaxial strain on the magnetic properties and XMLD spectra of complex oxide thin films.The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. [1] G. van der Laan et al., Phys. Rev. Lett. 100, 067403 (2008).[2] E. Arenholz et al., Phys. Rev. Lett. 98, 197201 (2007).[3] E. Arenholz et al., Appl. Phys. Lett. 93, 162506 (2008).
10:30 AM - W7.5
Local Electronic Structure at Oxide-Oxide Interfaces Probed by Atomic Resolution Electron Energy Loss Spectroscopy.
Amish Shah 1 2 , Quentin Ramasse 3 , Steven May 4 , Jianguo Wen 2 , Xiaofang Zhai 2 5 , James Eckstein 2 5 , Anand Bhattacharya 4 6 , Jian Zuo 1 2
1 Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Fredrick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 National Center for Electron Microscopy, Lawrence-Berkeley National Laboratory, Berkeley, California, United States, 4 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 5 Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 6 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractUnderstanding interfacial electronic reconstruction requires fabrication of chemically abrupt and defect-free interfaces and experimental probes to characterize the interfacial atomic structure and charge distribution. Electron microscopy has the advantage to see atoms directly at interfaces and probe their electronic structure by electron energy loss spectroscopy (EELS). Here, we report an atomically resolved study of the electronic structure of 12 x 4 LaMnO3-SrMnO3 and 2 x 2 LaMnO3-SrTiO3 superlattices synthesized using molecular beam epitaxy. Using EELS, we correlated the interfacial electronic structure with the interfacial atomic structure observed by atomic resolution Z-contrast scanning transmission electron microscopy (STEM). The characterization was carried out using aberration corrected electron probes of 1 Å or smaller. In STEM-EELS, we measured the site-specific unoccupied states of oxygen and transition metals. In the LMO-SMO system, we will show experimental evidence of extra holes near the Fermi level and their dependence on the interfacial abruptness. In the LMO-STO system, we will present evidence of site-dependent electronic structure of oxygen and the Mn valence based on the L-edge ratios. The interfacial electronic structure is mapped using a step size of one spectrum per 1 Å or per 0.5 Å. The high spatial resolution allows a direct mapping of the distribution of holes. We find an asymmetric hole occupation directly related to the asymmetric atomic structure observed in STEM.
W9: Multiferroic and Magnetic Interactions
Session Chairs
Thursday PM, April 16, 2009
Room 3003 (Moscone West)
2:30 PM - **W9.1
Novel Oxide-based Spintronic Nanodevices.
Stuart Parkin 1
1 Magnetoelectronics, IBM Almaden Research Center, San Jose, California, United States
Show AbstractSpintronic nanodevices using ultra thin oxide layers have found important applications over the past decade as components of magnetic field sensor and memory elements. Our understanding of spin dependent tunneling from magnetic electrodes through ultra thin oxide tunnel barriers has made major progress in recent years. In particular, magnesium oxide tunnel barriers give rise to highly spin polarized current of 80-90%, enabling tunneling magnetoresistance values at room temperature of several hundred percent [1]. These values are, however, highly dependent on the detailed structure of the tunnel barrier and its interfaces with the magnetic electrodes. We discuss the role of interfaces in spin dependent tunneling via MgO and a number of other oxides including SrTiO3. We compare the properties of nanodevices incorporating oxide electrodes and barriers prepared by reactive magnetron sputtering, pulsed laser deposition and molecular beam epitaxy. Defects in the oxide layers and at their interfaces can significantly affect the magnetic and transport properties of these devices. We show that significant concentrations of nitrogen can be incorporated in thin films of MgO by thermal evaporation of Mg in the presence of atomic nitrogen and atomic oxygen. After activation of the N dopants by annealing (at ~500-900 C) in vacuum, the MgO layers exhibit magnetic moments. Finally, we discuss the voltage driven metal-insulator transition in VO2 nanodevices and how this may be incorporated with other spintronic devices to build a novel memory device with synaptic functionality.[1] S. S. P. Parkin et al., Nature Mater. 3, 862 (2004).
3:00 PM - **W9.2
Exchange Bias at La0.667Sr0.333MnO3/BiFeO3 Interfaces: a Theoretical Approach.
Silvia Picozzi 1
1 CASTI Regional Lab, CNR-INFM L'Aquila, L'Aquila Italy
Show AbstractExchange bias [1], which induces a unidirectional shift of the magnetic hysteresis loop at a ferromagnetic/antiferromagnetic interface, is an important effect for spintronic applications at the nanoscale. Within this framework, ferromagnetic/antiferromagnetic-ferroelectric heterostructures are considered as promising candidates due to their multifunctional behavior [2]. In this talk, we will focus on the theoretical analysis of exchange bias and magnetoelectric effects in La0.667Sr0.333MnO3 (LSMO) / BiFeO3 (BFO) heterojunctions. First, we performed ab-initio density functional simulations of structural, electronic and magnetic properties of the LSMO/BFO system. Using first-principles total energies mapped into a Heisenberg Hamiltonian, the superexchange interaction energies Jij were estimated and used in Monte Carlo simulations to obtain hysteresis loops. Our results show that the exchange bias shift is induced by the presence of antiferromagnetic domains in the BFO side, arising from the intermixing of Mn and Fe sites in proximity to the interface. A discussion of the main factors affecting the exchange-bias will be presented, along with a comparison between theoretical and experimental results.[1] W. P. Meiklejohn and C. P. Bean, Phys. Rev. 102, 1413 (1956).[2] H. Béa et al., Phys. Rev. Lett. 100, 017204 (2008); Y.H.Chu et al., Nature Mater. 7, 478 (2008)* Work done in collaboration with M. Huijben, K. Yamauchi, S. Dong, S. Yunoki, R. Yu, S. Liang, E. Dagotto, R. Ramesh
3:30 PM - W9.3
Electrically Controlled Magnetization in an Oxide Multiferroic/ferromagnet Heterostructure.
Mark Huijben 1 , Lane Martin 2 3 , Ying-Hao Chu 2 4 , Martin Couillard 5 , Hajo Molegraaf 1 , Pu Yu 2 , Guus Rijnders 1 , David Muller 5 , Kunihiko Yamauchi 6 , Shuai Dong 7 8 , Silvia Picozzi 6 , Elbio Dagotto 7 8 , Ramamoorthy Ramesh 2 3 , Dave Blank 1
1 Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, Enschede Netherlands, 2 Department of Physics & Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 3 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 4 Department of Materials Science and Engineering, National Chiao Tung University, HsinChu Taiwan, 5 School of Applied and Engineering Physics, Cornell University, Ithaca, New York, United States, 6 , CNR INFM, L’Aquila Italy, 7 Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee, United States, 8 Division of Materials Science & Technology, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe ability to control local ferromagnetism by an electric field is a very intriguing phenomenon, which has enormous potential for applications in magnetic data storage, spintronics and high-frequency magnetic devices. Multiferroic materials, such as BiFeO3, exhibiting simultaneously multiple order parameters such as magnetism and ferroelectricity, offer an exciting way of magnetoelectric coupling in which charges are controlled by applied magnetic fields or spins by applied electric fields. Single-component multiferroic thin films only display weak magnetoelectric coupling, therefore multiferroic/ferromagnet heterostructures are investigated, because larger coupling, through exchange bias, is anticipated.Exchange anisotropy or bias describes the phenomenon associated with the exchange interactions at the interface between an antiferromagnet and a ferromagnet. Strong exchange interactions in a static manner are found in heterostructures based on multiferroic antiferromagnetic materials, including YMnO3 and BiFeO3. Recently, dynamic switching of the local magnetism with an applied electric field has even been demonstrated at room temperature for BiFeO3 heterostructures and at low temperatures for YMnO3 heterostructures. These are exciting developments in the research on magnetoelectric coupling, but typically the investigated heterostructures consisted of a complex oxide multiferroic, BiFeO3 or YMnO3, and a conventional ferromagnet, such as Co0.9Fe0.1 or Ni0.81Fe0.19. The challenging next step of utilizing magnetoelectric coupling through exchange bias in an all oxide epitaxial multiferroic/ferromagnet heterostructure has not been realized yet. By using oxide perovskite materials, high quality epitaxial heterostructures can be produced with atomically sharp interfaces between the multiferroic and the ferromagnet, which enables the controlled, detailed study of the magnetic interface ordering and the subsequent electric field control of it.Here, we report the observation of such electric-field control of local magnetism through exchange bias coupling at the interface between the ferroelectric/anti-ferromagnet (multiferroic) BiFeO3 and the ferromagnet La0.7Sr0.3MnO3. We will provide a suggestion for the cause of this interface effect based on magnetic and structural analysis measurements (such as STEM-EELS) as well as first principles calculations of the coupling of the Mn and Fe interfacial spins.
3:45 PM - W9.4
Antiferromagnetic / Ferromagnetic Coupling in Perovskite Oxide Superlattices.
Fan Yang 1 , Yayoi Takamura 1 , Nihan Kemik 1 , Elke Arenholz 2 , Michael Biegalski 3 , Hans Christen 3
1 Department of Chemical Engineering and Materials Science, University of California, Davis, Davis, California, United States, 2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractPerovskite oxides possess intriguing and technologically important physical properties such as ferromagnetism, superconductivity, and ferroelectricity. In particular, interfacial effects in superlattice structures resulting from structural distortions as well as band and charge discontinuities, can introduce enhanced or completely unexpected properties compared to the individual constituent materials. In this work, the structural, magnetic, and electronic properties of an all perovskite oxide superlattice consisting of alternating layers of the antiferromagnetic insulator La0.7Sr0.3FeO3 (LSFO) and the ferromagnetic metal La0.7Sr0.3MnO3 (LSMO) were studied as a function of the superlattice period. With decreasing superlattice period, the ferromagnetic properties of the LSMO layers gradually approach the case of the LSMO/LSFO solid solution, i.e. decreasing saturation magnetization and Curie temperature as well as increasing resistivity are observed. By contrast, the orientation of the antiferromagnetic axis in all the superlattices differs from that of the solid solution and single LSFO layers. Moreover, the Néel temperature is nearly independent of the LSFO layer thickness. Anisotropic x-ray linear and circular dichroism was utilized to obtain information on the coupling between the LSFO and LSMO layers in the superlattices. For six unit cell thick sublayers, a strong magnetic LSFO/LSMO coupling exists. The orientation of the antiferromagnetic axis of the LSFO layer lies perpendicular to the magnetic axis of the LSMO layer and can be controlled by an applied magnetic field. Below a sublayer thickness of six unit cells, the LSMO layer does not exhibit ferromagnetic order down to the lowest measurement temperature of 10 K. For greater superlattice periods, it appears that the anisotropy of the LSMO and LSFO layers dominate over the exchange coupling and a weak parallel orientation of the Mn and Fe moments is observed. This superlattice period dependence of the relative orientation of the LSFO/LSMO easy axes provides a unique way to control the magnetic properties of perovskite oxide superlattices, which may have potential applications in next generation magnetic sensors and spintronic devices. The research at the ALS and the CNMS is funded by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy, and the work at U.C. Davis by the National Science Foundation Award DMR-0747896.
4:30 PM - **W9.5
Tuning the Electronic and Magnetic Properties of LaMnO3/SrMnO3 Interfaces by Epitaxial Strain and Layer Thickness
Birabar Nanda 1 , Sashi Satpathy 1
1 Physics and Astronomy, University of Missouri-Columbia, Columbia, Missouri, United States
Show AbstractFrom density-functional calculations and model studies, we discuss the effect of epitaxial strain and layer thickness on the electronic and magnetic properties in oxide interfaces. Taking the example of LaMnO3/SrMnO3 interface, we show how the epitaxial strain affects the orbital ordering and in turn the magnetic ordering in this superlattice. The basic results will be interpreted from a model Hamiltonian involving Mn-O-Mn exchange interactions [1]. Concrete density-functional results will be presented for a delta-doped superlattice: (SMO)/(LMO)1/(SMO) and a periodic superlattice: (LMO){2n}/(SMO)n to study the effect of layer composition. In the delta-doped superlattice, we predict the formation of a fully spin-polarized two dimensional electron gas [2]. Finally, in the periodic superlattice, the evolution of magnetism from a uniform ferromagnetism in short period superlattice (n = 1) to a magnetic structure with inner bulk antiferromagnetic phases and interfacial ferromagnetic phase in long period superlattices (large n) [3] will be discussed. These results will be compared with experiments where available.[1] B. R. K. Nanda and S. Satpathy, Phys. Rev. B. 78, 054427 (2008)[2] B. R. K. Nanda and S. Satpathy, Phys. Rev. Lett. 101, 127201 (2008)[3] B. R. K. Nanda and S. Satpathy, arXiv:0810.2126
5:00 PM - **W9.6
Correlated Electron Interfaces with Magnetic, Electronic and Structural Reconstructions.
Hiroyuki Yamada 1
1 Nanoelectronics Research Institute (NeRI), AIST, Tsukuba Japan
Show AbstractPerovskite oxide interfaces often exhibit fascinating magnetic and electronic reconstructions which cannot be expected from those of constituent materials or simple combination of them. We have been exploring interface-specific phases in correlated electron oxides, by focusing on artificial superlattices composed of perovskite manganites or ruthenates. As in the case of prototypical SrTiO3 (STO)-based interfaces [Ohtomo et al., Nature 427 (2004) 423, ibid 419 (2002) 378], the manganite interface is also affected by the chemical potential difference, as typically seen in the LaMnO3/SrMnO3 superlattice, or the valence mismatching, observed as the broken symmetry in the LaAlO3/(La,Sr)MnO3/STO superlattice [Yamada, Ogawa, Kawasaki, Tokura, APL81 (2002) 4793]. The characteristic point in manganites is that the electronic phases are accompanied with the orbital structures, leading to the various substrate-controlled interface states [Yamada, Lottermoser, Kawasaki, Tokura, APL89 (2006) 052506]. In pervskite ruthenates (SrRuO3), we have found that, the interface structure can be coupled with a crystal structure of the adjacent layer, such as orthorohombic distortion in DyScO3 or in-plane ferroelectricity in the strained STO [Schlom et al., Nature 430 (2004) 758]. This interfacial structural modulation in ruthenates, evaluated as orthorhombic distortion, is also sensitive to the epitaxial strain [Koster et al., APL93 (2008) 051909], providing another determinant parameter at the interface. Those controlled interfaces with ferromagnetism and electronic reconstructions have potential to induce novel phenomena such as magneto-electric (ME) effect [Kida, Yamada, Kawasaki, Tokura et al., PRL99 (2007) 197404].This work was partly conducted in Correlated Electron Research Center (CERC)-AIST, under collaboration of Spin Superstructure and Multiferroics Projects, ERATO-JST. This work was partially supported by the Sumitomo Foundation and the Mitsubishi Foundation.
5:30 PM - W9.7
Origin of Magnetism at the Interface Between Two Antiferromagnetic Oxides.
S. Hasan Sadat Nabi 1 , Rossitza Pentcheva 1
1 Dept. of earth and environmental science , Ludwig Maximilians University (LMU), Munich Germany
Show AbstractTo explain the strong remanent magnetisation in nanoscale exsolutions observed between the canted antiferromagnet haematite (α-Fe2O3) and the RT paramagnet ilmenite (FeTiO3) [1] we have performed density-functional theory calculations, where we have varied systematically the concentration, distribution and charge state of Ti (Fe) in a hematite (ilmenite) host. We find that the polar discontinuity at the interface is compensated through Ti4+ and a disproportionation in the Fe contact layer into Fe2+, Fe3+[2], providing first evidence from correlated band theory for the lamellar magnetism hypothesis proposed by Robinson et al. [1]. The calculated phase diagram reveals that layered configurations are more stable than solid solutions. Moreover, Ti-ions show a preference for incorporation in the same spin-sublattice which maximizes the magnetic moment of the system. This interface magnetism is associated with impurity levels in the band gap. Their position with respect to the Fermi level can be tuned by straining the system to the lateral constants of different substrates (e.g. Fe2O3, FeTiO3, Al2O3) leading in some cases to halfmetallic behaviour relevant for future applications in spintronics devices. Funding by the German Science Foundation (DFG PE883/4-1) and ESF (EuroMinSci-MICROMAGN) and a grant for computational time at the Leibniz Rechenzentrum are gratefully acknowledged.[1] P. Robinson et al., Nature 418, 517 (2002).[2] R. Pentcheva, H. Sadat Nabi, Phys. Rev. B, 77, 172405 (2008).
5:45 PM - W9.8
Control of Nanoscale Periodic Multifunctional Domain Walls in La-Substituted BiFeO3 Thin Films
Ying-Hao Chu 1 , Qing He 2 , Pu Yu 2 , Lane Martin 4 , Chan-Ho Yang 2 , Jan Seidel 2 4 , Padraic Shafer 3 , R. Ramesh 2 3 4
1 Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, Taiwan, 2 Department of Physics, University of California, Berkeley, Berkeley, California, United States, 4 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States
Show AbstractComplex oxide heterostructure, such as LaAlO3 (LAO) and SrTiO3 (STO) interface, have piqued the general interests since its unpredicated phenomena. The polarity discontinuity of the interface can lead to conductive, magnetic, and even superconducting. On the other hand, magnetoelectric coupling in multiferroic materials has attracted much attention because of the intriguing science or significant application potential underpinning this phenomenon. A ferroic material owns its specific order parameters (electric polarization in ferroelectrics, magnetization in ferromagnets, or spontaneous strain in ferroelastics), which will bring to at least two equivalent spontaneous directions. This leads to the appearance of domains with different polarities coexisting in the crystal, separated by domain walls, which can also considered as another kind of interface. Such domain walls will become more important as the dimensions of individual elements shrink well to the nano-regime. The domain walls in ferroic materials can show novel functionality, which can be different from ferroic material itself. BiFeO3 (BFO) is a room temperature multiferroic. Recently, such special interfaces (domain walls) in BFO have been shown interesting properties, such as conducting behavior and magnetic interaction. However, in those studies, the domain wall indentified by piezoforce microscope (PFM) can be a combination of several domain walls due to the tip resolution, which make it difficult to eliminate the influence between different types of domain walls. In this study, we demonstrate an approach to create a 1-dimensional nanoscale periodic domain walls in epitaxial La-substituted BiFeO3 films. We have used DyScO3 (110)O single-crystal substrate to provide an anisotropic strain to exclude two of the possible ferroelectric polarization variants. Furthermore, by careful control of electrostatic boundary condition, such as the thickness of SrRuO3 bottom electrode to induce the self-poling effects, we can choose to obtain either 109° or 71° one dimensional periodic domain walls. Such domain walls show conducting behaviors and magnetic response, which creates a pathway to new functional device.