Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L19: Emergent Magnetic and Electronic States in Oxide HeterostructuresFocus
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Sponsoring Units: GMAG DMP Chair: Gervasi Herranz, ICMAB-CSIC Room: 318 |
Wednesday, March 16, 2016 11:15AM - 11:51AM |
L19.00001: Engineering a spin-orbital magnetic insulator by tailoring iridate-based superlattices Invited Speaker: Jobu Matsuno In 5$d$ Ir oxides with an interplay of spin-orbit coupling and electron correlations, we have tailored a spin-orbital magnetic insulator out of a semimetal SrIrO$_{\mathrm{3}}$ by tuning the structure through superlattices [(SrIrO$_{\mathrm{3}})_{m}$, SrTiO$_{\mathrm{3}}$] ($m=$ 1, 2, 3, 4, and $\infty )$ grown on SrTiO$_{\mathrm{3}}$(001) substrates. We observed the systematic decrease of the magnetic ordering temperature and the resistivity as a function of $m$. The transition from the semimetal to the insulator is found to be closely linked to the appearance of magnetism at $m\simeq $ 3. Long range magnetic ordering was realized even in the $m=$ 1 single layer superlattice, implying that the design and realization of novel electronic phases is feasible at the level of a single atomic layer in complex Ir oxides. We also report the fabrication of (111)-oriented superlattice structures with alternating 2$m$-layers ($m=$ 1, 2, and 3) of Ca$_{\mathrm{0.5}}$Sr$_{\mathrm{0.5}}$IrO$_{\mathrm{3}}$ perovskite and two layers of SrTiO$_{\mathrm{3}}$ perovskite on SrTiO$_{\mathrm{3}}$(111) substrates. In the case of $m=$ 1 bilayer films, the Ir sublattice is a buckled honeycomb, where a topological state may be anticipated. The ground states of the superlattice films were found to be magnetic insulators, which may suggest the importance of electron correlations in Ir perovskites in addition to the much discussed topological effects. [Preview Abstract] |
Wednesday, March 16, 2016 11:51AM - 12:03PM |
L19.00002: Novel magnetic and electronic states in manganite-iridate heterostructures John Nichols, Shinbuhm Lee, Jon Petrie, Tricia Meyer, Xiang Gao, Erjia Guo, John Freeland, Di Yi, Jian Liu, Daniel Haskel, Thomas Zac Ward, Gyula Eres, Valeria Lauter, Michael R. Fitzsimmons, Ho Nyung Lee Strong correlation between spin, charge, lattice, and orbital order parameters has proven to give rise to exotic physical phenomena, while epitaxial design of materials with strong interfacial coupling is an efficient technique to tune such parameters. Although there have been numerous studies of interfaces between 3$d$-3$d$ and 4$d$-3$d$ compounds, only few studies reported work on 3$d$ and 5$d$ materials and there has been no report on strong interfacial coupling in such systems. We have synthesized high quality [(AMnO$_{3})_{m}$/(SrIrO$_{3})_{n}$]$_{z}$ (A $=$ Sr, La) heterostructures by pulsed laser epitaxy on SrTiO$_{3}$ (001) substrates and have observed interesting novel magnetic and electronic ground states, which are highly sensitive to the degree of dimensional confinement in the heterostructures. Based on studies with x-ray diffraction, SQUID, \textit{dc}-transport, x-ray circular dichroism, and polarized neutron reflectometry measurements, we will report intriguing magnetic and transport properties that provide the first evidence of strong interfacial coupling between 5$d$ and 3$d$ materials. [Preview Abstract] |
Wednesday, March 16, 2016 12:03PM - 12:15PM |
L19.00003: Strain control of magnetic structure in Sr$_3$Ir$_2$O$_7$ Choong H. Kim We have studied from first principles the structural, electronic, and magnetic properties of the layered-perovskite iridates Sr$_3$Ir$_2$O$_7$ as a function of epitaxial strain. In Sr$_3$Ir$_2$O$_7$, bilayer iridates, an easy $c$-axis collinear antiferromagnetic structure have been reported, a significant constrast to single layer Sr$_2$IrO$_4$ with in-plane canted moments. This behavior is understood by competition among intra- and interlayer bond-directional pseudodipolar interactions. From our first-principles calculations, we show that these two energy scales are controllable via strain to drive spin-flop transition. [Preview Abstract] |
Wednesday, March 16, 2016 12:15PM - 12:27PM |
L19.00004: Realization of a Ferroelectric-Domain-Wall Tunnel Junction Jacobo Santamaria, G. Sanchez-Santolino, J. Tornos, D. Hernandez-Martin, J. I. Beltran, M. Cabero, A. Perez-Muñoz, Z. Sefrioui, C. Leon, M. Varela, C. Munuera, F. Mompean, M. Garcia-Hernandez, M. C. Muñoz, S. J. Pennycook Incorporating ferroelectric domain walls as an active part of electronic devices holds the promise of interesting new functionalities. Here we form a ferroelectric BaTiO3 tunnel barrier just 4.4-nanometer thick, with ferromagnetic La0.7Sr0.3MnO3 electrodes, containing a head-to-head domain wall within its thickness. A confined electron gas is formed at the domain wall, stabilized by oxygen vacancies, which controls the tunneling transport of the magnetic tunnel junction. Resonant tunneling assisted by the discrete levels of the ferroelectric quantum well gives rise to strong quantum oscillations of the tunneling conductance. Our engineered, highly constrained, domain wall provides a major step forward towards the new concept ``The Wall is the Device'', exploiting the electronic properties of domain walls for ferroelectric tunnel barriers with new functionalities. Work at UCM supported by MINECO MAT2014-52405-C02-01 and ERC Starting Investigator Grant {\#}239739 STEMOX. MCM acknowledges financial support from MICINN through grant MAT2012-38045-C04-04. [Preview Abstract] |
Wednesday, March 16, 2016 12:27PM - 12:39PM |
L19.00005: Tailoring magneto-electro-resistance in La0.7Sr0.3MnO3/BaTiO3multiferroic tunnel junctions Mariona Cabero, A.M. Perez-Muñoz, D. Hernandez-Martin, Z. Sefrioui, M. Varela, C. Leon, J. Santamaria, S. Valencia, R. Abrudan, S. J. Pennycook Controlling and manipulating the electronic states of oxide interfaces using external stimuli has become a major direction towards oxide-based electronics. Here, we present a study of the transport properties of multiferroic La0.7Sr0.3MnO3/BaTiO3 (LSMO/BTO) ferromagnetic/ferroelectric heterostructures. Multiferroic tunnel junctions (MTJ's) have been obtained introducing an ultrathin La/Sr cuprate (LSCO) layer between the ferroelectric barrier and the top ferromagnetic electrode. The LSCO introduces an asymmetry in the screening of polarization charges at both interfaces, which yields electroresistance values in excess of 105 {\%} and triggers an inversion of the sign of the tunneling magnetoresistance controlled by ferroelectric switching. We will discuss these results in the light of the generation and transport of oxygen vacancies. Work at UCM supported by MINECO MAT2014-52405-C02-01 and ERC Starting Investigator Grant {\#}239739 STEMOX. [Preview Abstract] |
Wednesday, March 16, 2016 12:39PM - 12:51PM |
L19.00006: DFT+$U$ study of electronic structure and Curie temperature of $A_{2}B$ReO$_6$ ($A$=Sr, Ca and $B$=Cr, Fe) Alex Lee, Chris Marianetti Re-based double perovskites (DPs) have attracted much attention due to their high Curie temperature ($T_C$) and colossal magneto resistance with large potential for spintronic applications. Here we investigate the electronic and magnetic properties of the Re-based DPs $A_2B$ReO$_6$ ($A$=Sr, Ca and $B$=Cr, Fe) using density functional theory + $U$ (DFT+$U$) calculations. While monoclinic Ca$_2$CrReO$_6$ and Ca$_2$FeReO$_6$ (monoclinic) are insulating within GGA+$U$, tetragonal Sr$_2$CrReO$_6$ ($a^{0}a^{0}c^{0}$) and Sr$_2$FeReO$_6$ ($a^{0}a^{0}c^{-}$) remain metallic. We show that both on-site interaction $U$ and octahedral tilting are critical to obtain the insulating phases. The $a^{0}a^{0}c^{-}$-phase of Sr$_{2}$CrReO$_{6}$ is most stable and insulating with nonzero $U$, suggesting that the high quality Sr$_{2}$CrReO$_{6}$ film on STO substrate can be a semiconductor as reported in recent experiments. We explain that the insulator-to-metal transition (MIT) of Ca$_2$FeReO$_6$ at 140K is predominantly due to a structural phase transition which drives the insulating state. Curie temperatures of Re-based DPs are calculated using the classical Monte Carlo simulations based on the Heisenberg model. [Preview Abstract] |
Wednesday, March 16, 2016 12:51PM - 1:03PM |
L19.00007: Magnetism and Nanoscale Structural and Compositional Irregularities in MBE-grown La$_{\mathrm{2}}$MnNiO$_{\mathrm{6}}$~on SrTiO$_{\mathrm{3}}$(001) Scott Chambers, Yingge Du, Timothy Droubay, Peter Sushko, Steven Spurgeon, Arun Devaraj, Mark Bowden, V Shutthanandan, Torgny Gustafsson Double perovskites (A$_{\mathrm{2}}$BB'O$_{\mathrm{6}})$ are a fascinating class of oxides with considerable potential for applications requiring ferromagnetic and semiconducting properties. We have investigated MBE-grown La$_{\mathrm{2}}$MnNiO$_{\mathrm{6}}$ and have found that despite the fact that Mn and Ni are present as 4$+$ ($d^{\mathrm{3\thinspace }}$: $t_{2g}^{\mathrm{3}}e_{g}^{\mathrm{0}})$ and 2$+$ ($d^{\mathrm{8\thinspace }}$: $t_{2g}^{\mathrm{6}}e_{g}^{\mathrm{2}})$ respectively, and exhibit suitable XMCD signatures, the volume-averaged moment per formula unit is considerably less than 5 Bohr magnetons. Our electron energy loss spectroscopy (STEM-EELS) and atom probe tomography (APT) results to date reveal that there is considerable disorder in the B-site sublattice for as-deposited films, despite excellent volume-averaged stoichiometry. While air annealing results in substantial ordering, the moment remains low due to the nucleation of NiO inclusions with needle-like shapes revealed only by APT. First principles modeling suggests that even though the double perovskite is quite stable if nucleated in excess O, the presence of O vacancies facilitates structural disorder. In this talk, we will present our latest results on this fascinating material. [Preview Abstract] |
Wednesday, March 16, 2016 1:03PM - 1:15PM |
L19.00008: Magnetism, Chemical Ordering, and Defects in Epitaxial Double Perovskite La$_{\mathrm{2}}$MnNiO$_{\mathrm{6}}$ Thin Films. Tim Droubay, Steven Spurgeon, yingge Du, Arun Devaraj, Steve Heald, Peter Sushko, Torgny Gustaffson, David Keavney, Scott Chambers Oxide double perovskites (A$_{\mathrm{2}}$BB'O$_{\mathrm{6}})$ exhibit an interesting variety of electronic and magnetic properties such as half-metallicity and high temperature ferromagnetism holding promise for potential technological applications. We have investigated La$_{\mathrm{2}}$MnNiO$_{\mathrm{6}}$/SrTiO$_{\mathrm{3}}$ grown using molecular beam epitaxy and have found different proportions of two ferromagnetic phase transitions (\textasciitilde 130K and \textasciitilde 290K) and various saturation magnetization values ( \textless 4.6 $\mu _{\mathrm{B}}$/f.u.) dependent upon post-growth annealing. Contrary to previous reports, neither the increase in the saturation magnetization nor the Curie temperature(s) after annealing can be attributed to changing Mn and Ni valence. Instead, using aberration-corrected transmission electron microscopy and atom probe tomography, we observe large-scale chemical ordering as a result of annealing. We also find the coexistence of NiO-derived extended defects that appear to prevent the magnetic moment from reaching the maximum possible value (5 $\mu_{\mathrm{B}}$/f.u.). We will describe these results in light of first principles calculations which suggest that local deviations from the ideal stoichiometry facilitate the formation of the NiO phase and structural disorder in the double perovskite phase. [Preview Abstract] |
Wednesday, March 16, 2016 1:15PM - 1:27PM |
L19.00009: Half-metallic ferromagnetism on surfaces of insulating and antiferromagnetic LaFeO$_{\mathrm{3}}$ thin films Rohan Mishra, Young-Min Kim, Qian He, Seong-Keun Kim, Seohyoung Chang, Anand Bhattacharya, Sokrates T. Pantelides, Albina Borisevich The surfaces of perovskite transition metal oxides having correlated electrons show novel electronic and magnetic phenomena. In this work, we combine scanning transmission electron microscopy imaging and electron energy loss spectroscopy (EELS) with density functional theory (DFT) calculations to study the surface of (LaFeO$_{\mathrm{3}})_{m}$/(SrFeO$_{\mathrm{3}})_{n}$ heterostructure thin films. Using EELS, we observe a reduction in the oxidation state of Fe on moving from the bulk to the surface over a length of \textasciitilde 5 unit cells. Simultaneously acquired STEM images allow us to map the associated changes in their structure, such as cation displacements and changes in oxygen polyhedral tilts. DFT calculations coupled with the STEM results show that by reducing the surface layer of a LaFeO$_{\mathrm{3}}$ film such that the surface is terminated with FeO$_{\mathrm{4}}$ tetrahedra instead of the FeO$_{\mathrm{6}}$ octahedra as present in the bulk, it is possible to stabilize an exotic phase where the surface layer displays a half-metallic ferromagnetic behavior, while the bulk remains antiferromagnetic and insulating, similar to the class of topological insulators. The calculations also predict that the magnetism and conductivity at the surface can be controlled by the partial pressure of oxygen. [Preview Abstract] |
Wednesday, March 16, 2016 1:27PM - 1:39PM |
L19.00010: X-ray Magnetic circular dichroism study of hexagonal YbFeO$_{\mathrm{3}}$ thin films Xiao Wang, Kishan Sinha, Xiaoshan Xu, Yaohua Liu, David Keavney, X.M. Cheng Multiferroic materials exhibit multiple ferroic orders simultaneously and thus have potential applications in information technology, sensing, and actuation. Hexagonal YbFeO$_{\mathrm{3}}$ is a promising candidate for a multiferroic material with room temperature ferromagnetism because of the expected enhanced Fe moment and higher transition temperature due to the exchange interaction between magnetic Yb and Fe ions. Here we report an x-ray magnetic circular dichroism (XMCD) study of (0001) Hexagonal YbFO$_{\mathrm{3}}$ thin films deposited on (111) yttria-stabilized zirconia substrates via pulsed laser deposition. XMCD spectra for the Fe L$_{\mathrm{2,3}}$ edges and Yb M$_{\mathrm{5}}$ edge were measured with the magnetic field applied parallel to the x-ray propagation direction and 20 degree away from the film normal at beamline 4ID-C of the APS at ANL. Field dependence of the XMCD spectra show that Fe and Yb each has a ferromagnetic ordering at around 6.7 K but with opposite orientations in between. The saturation magnetic moment for Fe is determined by the sum rules to be 0.07 $\mu_{\mathrm{B\thinspace }}$/ Fe cation at around 6.7 K, about 4 times larger than that in Hexagonal LuFeO$_{\mathrm{3}}$. [Preview Abstract] |
Wednesday, March 16, 2016 1:39PM - 1:51PM |
L19.00011: MAGNETO-OPTIC ENHANCEMENT IN NANO-SCALE IRON GARNET FILMS Ashim Chakravarty, Miguel Levy This work addresses dimensionality-induced magneto-optic effects in liquid-phase-epitaxy magnetic garnet thin films. It is found that the Faraday rotation (FR) per unit length evinces a marked and steady enhancement as the film thickness is reduced approximately below 100 nm in Bi$_{\mathrm{0.8}}$Gd$_{\mathrm{0.2}}$Lu$_{\mathrm{2}}$Fe$_{\mathrm{5}}$O$_{\mathrm{12}}$, although it remains constant in the micron- and most of the sub-micron-regime. The reported specific FR change in such reduced dimensions is due to size-dependent modifications in diamagnetic transition processes in the garnet film. These processes correspond to the electronic transitions from the singlet $^{\mathrm{6}}$S ground state to spin-orbit split excited states of the Fe$^{\mathrm{3+}}$ ions in the garnet. A measurable reduction in the corresponding ferrimagnetic resonance linewidths is found, thus pointing to an increase in electronic relaxation times and longer lived excitations at reduced thicknesses than in the bulk. These changes together with a shift in vibrational frequency of the Bi-O bonds in the garnet at reduced thicknesses result in magneto-optical enhancement in ultra-thin garnet films. [Preview Abstract] |
Wednesday, March 16, 2016 1:51PM - 2:03PM |
L19.00012: Enhancement of Magnetization in Y$_{\mathrm{3}}$Fe$_{\mathrm{5}}$O$_{\mathrm{12}}$ Epitaxial Thin Films. Jack T. Brangham, James C. Gallagher, Angela S. Yang, Shane P. White, Rohan Adur, Willam T. Ruane, Bryan D. Esser, Michael R. Page, P. Chris Hammel, David W. McComb, Fengyuan Yang The ability to generate pure spin currents has applications in telecommunications, radar, and spin-based logic. Y$_{\mathrm{3}}$Fe$_{\mathrm{5}}$O$_{\mathrm{12}}$ (YIG) is one of the best materials for dynamic generation of spin currents due to its low damping, narrow ferromagnetic resonance (FMR) linewidth, and insulating behavior. We grow stoichiometric, high quality, epitaxial YIG thin films with thicknesses ranging from 4 to 250 nm on Gd$_{\mathrm{3}}$Ga$_{\mathrm{5}}$O$_{\mathrm{12}}$ by off-axis magnetron sputtering and characterize the YIG films by various techniques. The temperature dependence of the saturation magnetization was independently measured by in-plane vibrating sample magnetometry, out-of-plane magnetic shape anisotropy, and angular-dependent FMR absorption from 10 K to the Curie temperature of 530 K. The room temperature saturation magnetization was also measured with frequency dependent FMR. All measurements show a magnetization enhancement of 15{\%} or greater when compared to reported magnetization values of bulk YIG crystals. We speculate this is due to suppression of the long wavelength magnons due to the finite size of the films. [Preview Abstract] |
Wednesday, March 16, 2016 2:03PM - 2:15PM |
L19.00013: Strain induced structural, electronic, and magnetic properties of SrFeO$_2$ and BaFeO$_2$ Weidong Luo, Xiaole Zhang The structural, electronic and magnetic properties of SrFeO$_2$ and BaFeO$_2$ under tensile strains are studied using first-principles density-functional theory calculations. Strain-induced Jahn-Teller-like behaviors involving the cooperative displacements of oxygen atoms are predicted in both compounds. Lattice dynamical properties are also investigated and the strain-induced imaginary phonon modes are consistent with the Jahn-Teller-like distortion. The usual Jahn-Teller instability of degenerate energy levels does not contribute to the interesting phenomena. Besides the structural and electronic properties, a transition of magnetic orderings from G-type anti-ferromagnetic phase to C-type anti-ferromagnetic phase is predicted in both compounds, which originates from the combined effects of the lattice-orbital coupling and the spin-orbital coupling due to exchange interaction between orthogonal Fe 3$d$ orbitals.\\ We acknowledge funding support from the National Natural Science Foundation of China. [Preview Abstract] |
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