Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J13: Focus Session: Ferroelectric and Multiferroic Oxide Heterostructures |
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Sponsoring Units: DMP Chair: Jayakanth Ravichandran, University of Southern California Room: 007D |
Tuesday, March 3, 2015 2:30PM - 3:06PM |
J13.00001: Complex-oxide multilayers by design: a treasure trove of unusual ferroic functionalities Invited Speaker: Serge Nakhmanson While inheriting most of the traits of their parents, layered variations of ABO$_3$ perovskites allow for a number of additional channels for property manipulation and fine-tuning. Their remarkable flexibility toward structural and chemical modification can be exploited for the design of new and advanced functionalities not originally present in the parent ABO$_3$ compounds. With the help of first-principles-based computational techniques, we have predicted intriguing electroactive behavior in layered-perovskite compounds of the Ruddlesden-Popper (RP) type. Specifically, we showed that Goldstone-like states (collective, close to zero frequency excitations, requiring practically no consumption of energy) can be induced in a PbSr$_2$Ti$_2$O$_7$ RP superlattice, manifesting themselves as easy rotations of the in-plane polarization vector. Examination of a fictitious epitaxial Ba$_2$TiO$_4$ RP compound demonstrated that it exhibits an assortment of competing incommensurate distortions, including ones that promote in-plane polarization. In this presentation we highlight the unusual behavior of a RP Ba$_2$ZrO$_4$ structure, which has already been synthesized as a bulk ceramic. An investigation of the properties of a (fictitious) epitaxial thin-film variant of this material reveals that under compression it undergoes a transition into an incommensurate state, while under tension it shows hints of a Goldstone-like polar instability, which surprisingly occurs without the presence of a lone-pair active ions like Pb or Sn. In both cases, we observe anomalies in the planar static dielectric susceptibility of the system, with large dielectric response predicted for the phase displaying the Goldstone-like instability. [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:18PM |
J13.00002: Theoretical study of electronic transport properties in pillar-embedded multiferroic transition-metal oxides Yuan-Yen Tai, Jian-Xin Zhu Multiferroics show great potential in novel application to nanotechnologies based on well-established experimental techniques. Recently, vertically aligned nanocomposite (VAN) multiferroic thin films have demonstrated a significant amount of research interest owing to their promising results to give more delicate device, such as a larger interfacial area and intrinsic heteroepitaxy in this 3D structure. In order to understand the basic influence of the nano-pillar structure to the bulk multiferroics, we apply scaling theory to study the quasiparticle localization/delocalization effects of this novel nanostructure. Within an effective tight-binding model, we apply the transfer matrix method to calculate the wave function behavior throughout its transverse direction. We will show that how the critical behavior varies with various disordered nano-pillar patterns. We will also give a qualitative connection of our results to the transport experiments. [Preview Abstract] |
Tuesday, March 3, 2015 3:18PM - 3:30PM |
J13.00003: Electric Field and Structural Phase Transition Induced Magnetization Effects in BaTiO3-FeRh Heterostructures Probed using Polarized Neutron Reflectometry Steven Bennett, Thomas Ward, Michael Biegalski, Tony Wong, Zhiqi Liu, Haile Ambaye, Artur Glavic, Valeria Lauter The ability to change the magnetic state of a material with an electric field opens up a plethora of possible devices in spintronics and memory applications. A strong candidate material for such a control is FeRh, whose magneto-structural phase transition from antiferromagnetic (AFM) to ferromagnetic (FM) at T$\approx $350K, has shown to be controllably changed by an electric field when grown on ferroelectric BaTiO3 (BTO). It has been suggested that this shift is largely due to the -0.47{\%} in plane compressive strain induced by the piezoelectric BTO. Here we show a sharp repeatable change in magnetization as the system is heated/cooled through the tetragonal to orthorhombic (280-290K) and orthorhombic to rhombohedral (180-205K) crystalline phase transitions of BTO. To further characterize the effect polarized neutron reflectometry (PNR) was used to evolve the depth profile of magnetization in FeRh within the temperature vicinity of these transitions with and without the application of electric field. [Preview Abstract] |
Tuesday, March 3, 2015 3:30PM - 3:42PM |
J13.00004: Emergent phenomena in LaAlO3/GdAlO3 superlattices by breaking inversion symmetry Hai-Tian Zhang, Ryan Haislmaier, Jason Lapano, Venkat Gopalan, Roman Engel-Herbert Recently, tremendous interest has been focused on materials by design, where hybrid improper ferroelectricity was proposed from first principles predictions in perovskite superlattice structures (ABO3)n/(A'BO3)m , which is directly coupled to the oxygen octahedral rotations [1]. Such oxygen octahedron rotations are anticipated stable at high temperatures, thus being a potential materials candidate for high temperature piezoelectric sensor and transducer applications. In this talk we will present the homoepitaxial growth of LaAlO3 and GdAlO3 by hybrid molecular beam epitaxy, where a volatile aluminium alkoxide precursor and elemental La/Gd (flux 2 $\times$ 10$^{13}$ cm$^{-2}$s$^{-1}$) were co-supplied in the presence of an oxygen plasma (RF plasma power 250 W). Growth mode, film surface morphology and defect type for films grown under La/Gd-rich and Al-rich growth conditions will be discussed and directly related to structure and properties characterization of the LaAlO3 and GdAlO3. Changes in the octahedral tilts, probed by scanning transmission electron microscopy in the proximity of GdAlO3/LaAlO3 interface are discussed and compared to first principle predictions. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J13.00005: Ferroelectric tunnel junctions with electron doped SrTiO$_{3}$ elelctrodes Xiaohui Liu, J.D. Burton, Evgeny Tsymbal Increasing the tunneling electroresistance (TER) is crucial for the application of ferroelectric tunnel junctions (FTJs) for electronic devices.[1] Normal FTJs are composed of a thin ferroelectric layer sandwiched by two metallic electrodes, where TER results from the ferroelectric polarization dependent electric potential height in the ferroelectric barrier. Since the resistance depends exponentially not only on potential height but also on barrier width, TER is expected to be greatly enhanced by modulation of the polarization dependent barrier width when one of the electrodes is substituted by a semiconductor.[2] Recently, experiment reported sizable (10$^{4})$ TER in a FTJ where one electrode is n-type SrTiO$_{3}$, namely Pt/BaTiO$_{3}$/Nb:SrTiO$_{3}$.[3] To reveal the mechanism of this phenomenon, we perform theoretical studies on a representative system SrRuO$_{3}$/BaTiO$_{3}$/n-SrTiO$_{3}$ using on first-principles modeling. We analyze as a function of doping level the effect of ferroelectric polarization on the electronic structure near the interface, its influence on the barrier width and on the transport properties of such a system. [1] E. Y. Tsymbal and H. Kohlstedt, Science 313, 181 (2006) [2] M. Y. Zhuravlev, et al., Phys. Rev. Lett. 94, 246802 (2005) [3] Z. Wen, et al., Nat. Mater. 12, 617 (2013) [Preview Abstract] |
Tuesday, March 3, 2015 3:54PM - 4:06PM |
J13.00006: Role of interface termination in SrRuO3/PbTiO3/SrRuO3 capacitors under epitaxial strain Simon Divilov, Marivi Fernandez-Serra, Matthew Dawber We perform a DFT analysis of (SrRuO$_3$)$_1$/(PbTiO$_3$)$_m$/(SrRuO$_3$)$_1$ with varying PbTiO$_3$ using both LDA and LSDA+U. The goal of the study is to analyze the the effects of symmetrical termination along the [001] plane on the magnetic and electric properties of SrRuO$_3$ and PbTiO$_3$, respectively. We observe the electrical polarization of the thin film is highly sensitive to the termination plane, as compared to the bulk. In addition, the termination plane determines the oxygen octahedra tilting (OOT) pattern in the thin film, although the OOT are not coupled to the polarization. Despite having a single layer of SrRuO$_3$, we observe a magnetic polarization dependent on the termination. Our results show the importance of \textit{ab initio} calculations in the presence of a rich parameter space to aid experiments in synthesis of superlattice and thin film capacitors. [Preview Abstract] |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J13.00007: The road towards the ferroelectric-FET -- Carrier density modulation by ferroelectric switching in BaTiO$_{3}$/Ge Patrick Ponath, Kurt Fredrickson, Agham Posadas, Yuan Ren, Xiaoyu Wu, Rama Vasudevan, Baris Okatan, Stephen Jesse, Toshihiro Aoki, Martha McCartney, David Smith, Sergei Kalinin, Keji Lai, Alex Demkov Germanium, with its higher hole and electron mobility is a potential candidate to replace silicon as a channel material in a field effect transistor in the future. The ferroelectric high-k dielectric barium titanate (BTO) can be integrated on germanium (001) due to the small lattice mismatch between BTO and Ge and could therefore be a potential candidate for a ferroelectric memory. We report the epitaxial growth of BTO on a germanium (001) substrate with a thin STO buffer layer, which imposes compressive strain on BTO and causes it to be out of plane polarized. The BTO film crystallizes as-deposited which is monitored by RHEED. XRD measurements of the BTO film indicate an out-of-plane ferroelectric polarization which can be confirmed by piezoresponse force microscopy. Using microwave impedance microscopy we could show that we can effectively modulate the charge in germanium; this charge modulation constitutes the field effect, which is an important step towards the development of a ferroelectric-FET. [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J13.00008: Enhanced ferromagnetism in ferroelectric {\&} ultrathin ferromagnetic digital superlattices Hangwen Guo, Zhenyu Diao, Wang Zhen, Mohammad Saghayezhian, Lina Chen, Rongying Jin, Ward Plummer, Jiandi Zhang, Shuai Dong Electric control of magnetism has generated much activity due to both intellectual and technical interests. Epitaxial growth of transition-metal oxides with different ordering parameters offers unique platform to study magnetoelectric effect. In particular, quantum confinement of correlated electrons lead to interesting phenomena such as metal-insulator transition. In this work, we explore such effect in quantum confined regime by growing digital superlattices. We selected ferroelectric BaTiO$_{\mathrm{3\thinspace }}$(BTO) and La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3\thinspace }}$(LSMO) which exhibits ferromagnetic to paramagnetic transition at critical thickness. We explore how confined LSMO behaves when sandwiched by BTO. Superlattices are fabricated in formula [(BTO)$_{\mathrm{24}}$/(LSMO)$_{x}$]$_{y}$. BTO thickness is constant {\&} LSMO thickness are varied from 40u.c. to 2u.c. For all samples, total thickness of LSMO remains 40 u.c. Stable RHEED oscillations and sharp patterns ensure layer-by-layer growth and decent structural ordering. We find that LSMO has the ferromagnetic ground state down to 2 u.c. In addition, enhancement of ferromagnetism on ultrathin LSMO samples is observed compared to thicker films. Possible explanation involving interfacial magnetization will be discussed. [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J13.00009: Strain-mediated SrTiO$_{3}$/PbTiO$_{3}$ superlattice: The role of oxygen vacancy Menglei Li, Wenhui Duan Using first-principle calculations, we comprehensively study the oxygen vacancy effects on the ferroelectric(FE) and antiferrodistortive(AFD) properties of the [001]-oriented SrTiO$_{3}$/PbTiO$_{3}$ 1/1 superlattice at different epitaxial strains. Oxygen vacancies form most easily under intermediate strains while the oxygen on the PbO-plane is more difficult to lose than in other positions. Without vacancies, the superlattice is in the \emph{r}-phase where the polarization has both in-plane and out-of-plane components at intermediate strain. The oxygen vacancy can pin the polarization to the out-of-plane direction and remarkably reduce the polarization in the direction of Ti-V$_{\mathrm{O}}$-Ti chain by inducing local tail-to-tail polarization patterns. Furthermore, the oxygen vacancies suppress the octahedra rotations around the direction of Ti-V$_{\mathrm{O}}$-Ti chain while promote the rotations along the other two orthogonal directions. Therefore, the mediation of the FE and AFD properties in different directions in the superlattice can be achieved by the use of the anisotropic effect of oxygen vacancies. Our results provide a theoretical ground to the various coupling effects in ferroelectric-paraelectric superlattices. [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 4:54PM |
J13.00010: The metallic screening of the interfacial dipole field in multiferroic Bi6Fe2-xCoxTi3O18 thin films Xiaofang Zhai, Yu Yun, Chao Ma, Haoliang Huang, Dechao Meng, Jianlin Wang, Zhengping Fu, Ranran Peng, Yalin Lu The demand for superior room-temperature multiferroic materials pushes high-quality fabrication of novel Aurivillius-type complex oxides with a goal of revealing the intrinsic room temperature multiferroic properties. We have found a new route to fabricate single-crystalline quality multiferroic Bi6Fe2-xCoxTi3O18 thin-films utilizing a metallic screening effect. The films exhibit abrupt interfaces and greatly enhanced crystalline quality on conductive bottom layers while mixed types of interfaces on insulating bottom layers. The enhanced single-crystalline quality is explained by a metallic screening effect that compensates the diverging dipole field originating from the symmetry breaking at the interface. The films on the conductive bottom layers also exhibit an enhanced ferromagnetic spin coupling and a strong vertical piezoelectric polarization switching. This study demonstrates that screening the interface dipole can be crucial to fabricate high-quality thin films and then multiferroic devices. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J13.00011: Self-modulated growth of a super-large-period multiferroic BiFeCoTiO33 thin-film Yalin Lu, Dechao Meng, Xiaofang Zhai, Chao Ma, Haoliang Huang, Yu Yun, Yan Huang, Zhengping Fu, Ranran Peng, Xiangyu Mao, Xiaobing Chen The epitaxial growth of super-large-period Aurivillius thin films has been explored in order to study their period-modulated multiferroic property. BiFeCoTiO33 thin films with a pseudo-period of ten have been grown on SrTiO3 single crystal substrates using pulsed laser deposition. The films are found to be coherently strained to the substrates and atomically smooth. The X-ray diffraction indicates an average layer period of 10. While the high resolution scanning transmission electron microscopy reveals a self-modulated nanostructure in which the period changes as the thickness increases. The modulation to the period is tentatively explained by the volatile bismuth stacking difference between hetero-epitaxial growth and homo-epitaxial growth. Nonetheless, the self-modulated large period films exhibit intrinsic ferromagnetic and ferroelectric properties at room temperature. [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J13.00012: Reversible Magnetoelectric Control of Exchange Coupling in Monodomain BiFeO3 Heterostructures Julian Irwin, W. Saenrang, M.S. Rzchowski, K.J. Reierson, J.E. Podkaminer, S.B. Baek, C.B. Eom, J.W. Freeland, B.A. Davidson We investigate reversible switching of the ferromagnetic properties of a monodomain thin film of the multiferroic BiFeO$_3$ (BFO) [2] and a Co overlayer. For different electric polarization directions of the BFO film we observe changes in the Co layer magnetic anisotropy and coercive field as determined from in-plane and out-of-plane anisotropic magnetoresistance (AMR) and Magneto-Optic Kerr Effect (MOKE) measurements between 30K and 300K. The dependence of these results on BFO layer thickness is also investigated due to the thickness dependence of BFO's cycloidal magnetic ordering. X-ray linear dichromism (XLD) measurements of the BFO layer indicate a reversible change in the BFO magnetic ordering as a result of polarization switching, verifying the presence of a magnetoelectric effect in the BFO film and suggesting a magnetic coupling between the BFO and Co layers. Weak ferromagnetism and changing surface magnetic anisotropy energy are both explored as possible mechanisms for the observed coupling.\footnote{S.H. Baek et al., ``Ferroelastic Switching for Nanoscale Nonvolatile Magnetoelectric Devices'' Nature Materials, 9, 309, (2010)} [Preview Abstract] |
Tuesday, March 3, 2015 5:18PM - 5:30PM |
J13.00013: Spin-orbit engineering in perovskite heterostructures Byounghak Lee, Guru Khalsa There has been a steadily increasing interest in spin-orbit effects in systems with broken inversion symmetry. These effects may have technological applicability due to recent success in inducing dynamics and switching across heavy metal/ferromagnet interfaces through spin-orbit torque. In addition, broken inversion symmetry and large spin-orbit interactions can lead to novel magnetic and superconducting properties. Little effort has focused on developing a materials platform for studying these effects systematically. The versatility of perovskites along with recent advances in their epitaxial growth may provide such a playground. In this talk we discuss our theoretical efforts to engineer spin-orbit effects in materials systems based on perovskites. We show that Ruddlesden-Popper perovskites provide control of both spin-orbit strength and atomic scale broken inversion symmetry, providing new avenues for customizable materials. Using \textit{ab initio} tools, we predict the spin-splitting in SrTiO$_{3}$ and KTaO$_{3}$ based Ruddlesden-Popper perovskites and contrast results with bare surfaces and interfaces. [Preview Abstract] |
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