Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C14: Focus Session: Magnetic Oxide Superlattices and Multiferroics |
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Sponsoring Units: DMP GMAG Chair: Susanne Stemmer, UC Santa Barbara Room: 316 |
Monday, March 18, 2013 2:30PM - 2:42PM |
C14.00001: First-principles study of spin-lattice and spin-phonon couplings in SrMnO$_3$/LaMnO$_3$ superlattice Yuanjun Zhou, Karin Rabe We have studied the influence of epitaxial strain on magnetic orderings and the couplings between the spin and optical phonons in SrMnO$_3$/LaMnO$_3$ superlattices using first principles. We first couple octahedral rotations with structural relaxations in ferromagnetic (FM), A-type antiferromagnetic (A-AFM) and C-type AFM (C-AFM) states, and obtain the sequence of magnetic phases with epitaxial strain. We also find that oxygen octahedral rotations lower the ground state energy but do not destroy the strain induced magnetic phase transitions. Next, the zone-center phonon modes in FM, A-AFM, and C-AFM states are computed using the frozen phonon method. A substantial increase of the coupling strength between the spin and the lowest polar mode is observed for tensile strains. From the analysis of the eigenvectors, the effect is inferred to be the consequence of the enhanced amplitudes of oxygen atoms in the phonon mode. Finally, spin-phonon coupling parameters are computed in a Heisenberg formulism. They reveal the changes in exchange couplings due to specific atomic displacements or phonon modes, as well as the inequality of the out-of-plane exchange couplings across LaO layers and across SrO layers, the latter being the result of the artificial structuring in the superlattice. [Preview Abstract] |
Monday, March 18, 2013 2:42PM - 2:54PM |
C14.00002: Charge transfer and magnetism in (LaNiO$_3$)$_n$/(LaMnO$_3$)$_2$ superlattices Jason Hoffman, I-Cheng Tung, Brittany Nelson-Cheeseman, Ming Liu, John Freeland, Anand Bhattacharya Interfaces in solids have been an enduring them in materials physics, where dimensionality and proximity effects cooperate to create interfacial states that are distinct from their bulk counterparts. In this work, we investigate the interfacial ferromagnetism induced in the the paramagnetic metal LaNiO$_3$ via proximity to the antiferrmagnetic insulator LaMnO$_3$. We fabricated a series of (LaNiO$_3$)$_n$/(LaMnO$_3$)$_2$ ($2 \leq n \leq 5$) digital superlattices on (001) SrTiO$_3$ substrates using ozone-assisted molecular beam epitaxy. The total superlattice thickness is maintained at $\sim$30 nm by varying the number of superlattice periods. X-ray absorption and x-ray magnetic circular dichroism measurements at the Mn and Ni $L$-edges confirm the presence of charge-transfer at the LaNiO$_3$/LaMnO$_3$ interface, with magnetism residing on both Mn and Ni sites. Magnetotransport measurements performed on superlattices with $n \leq 3$ show insulating behavior between 5 K and 300 K, while samples with $n$ = 4,5 are metallic. We observe an anomalous Hall effect in the sample with $n = 4$, that vanishes in the more metallic $n = 5$ sample. We discuss possible models for the electronic and magnetic behavior of LaNiO$_3$. [Preview Abstract] |
Monday, March 18, 2013 2:54PM - 3:06PM |
C14.00003: Designing ferromagnetism in early transition metal oxides in bulk and superlattice forms Hung Dang, Andrew Millis The circumstances under which early transition metal oxides could exhibit ferromagnetism are determined using density functional plus single-site dynamical mean field methods. Particular attention is paid to the consequences of the GdFeO$_3$ distortion and other octahedral rotations. Ferromagnetism is favored by the combination of intermediate carrier concentration (formal valence $\sim d^{1.5}$) and large tilt angle. The decrease of GdFeO$_3$ distortion amplitude with hole doping away from $d^2$ is shown to keep the bulk solid solution La$_{1-x}$Sr$_x$VO$_3$ outside of the ferromagnetic regime. In superlattices such as (LaVO$_3$)$_m$(SrVO$_3$)$_1$, carrier concentration and tilt angle may be decoupled, potentially enabling ferromagnetism as suggested by experiment[1].\\[4pt] [1] U. L\"uders, W. C. Sheets, A. David, W. Prellier, and R. Fr\'esard, Phys. Rev. B 80, 241102(R) (2009). [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:42PM |
C14.00004: Designing Magnetism in Oxide Superlattices Invited Speaker: Andrew Millis Dramatic improvements in pulsed laser deposition and oxide molecular beam epitaxy suggest that it may be possible to create ``designer'' materials with desired correlated electron properties. This talk presents the results of theoretical studies based on the density functional plus dynamical mean field approximation aimed at determining design rules for creating or optimizing magnetism in oxide superlattices. Among the topics covered will be the physics of long-period antiferromagnetic states in nickelate-based heterostructures and the relation of strain-induced octahedral rotations in creating ferromagnetic states in vanadate-based superlattices. We show in particular how appropriately designed superlattices may lead to structure-doping combinations which do not occur in bulk solid solutions but which can produce high Curie temperature ferromagnetism. Limitations of present theoretical capabilities and opportunities and needs for conceptual, methodological and algorithmic improvements will also be discussed. This work is based in part on collaborations with C. Marianetti, B. Lao and H-T Dang. [Preview Abstract] |
Monday, March 18, 2013 3:42PM - 3:54PM |
C14.00005: Tunable spin-density-wave order in nickelate heterostructures A. Frano, E. Schierle, M. Haverkort, Y. Lu, M. Wu, S. Blanco-Canosa, U. Nwankwo, A.V. Boris, P. Wochner, G. Cristiani, H.U. Habermeier, V. Hinkov, E. Benckiser, E. Weschke, B. Keimer Antiferromagnetic spin-density-wave (SDW) order in metals has been proposed as the basis for a new generation of spintronic devices. However, SDWs have been observed only in a few materials to-date, and it has proven difficult to systematically control their properties. Using resonant x-ray diffraction, we demonstrate SDW order in epitaxial thin films and superlattices based on metallic $\bf \it R$NiO$_3$ with $\bf \it R$ = La, Nd, Pr. The materials remain highly conductive in the SDW state, and the amplitude of concomitant charge order is dramatically reduced with respect to their bulk analogs. We also show that the SDW polarization is tunable through two independent control parameters -- epitaxial strain and dimensional confinement of the conduction electrons. Nickelate heterostructures are thus a powerful new model platform for SDW physics and antiferromagnetic spintronics. [Preview Abstract] |
Monday, March 18, 2013 3:54PM - 4:06PM |
C14.00006: Magnetic and orbital order in (RMnO3)n/(AMnO3)2n superlattices Shuai Dong, Qinfang Zhang, Elbio Dagotto The magnetic and orbital orders in (RMnO3)n/(AMnO3)2n (R: rare earths; A: alkaline earths, n=1 and 2) superlattices have been studied using both the double-exchange model and density functional theory calculations. For large bandwidth manganites, the A-type antiferromagnetic order is found to be robust when the superlattices are grown on a SrTiO3 substrate, as in recent experiments on (LaMnO3)n/(SrMnO3)2n. In addition, a C-type antiferromagnetic state is predicted for these superlattices when using substrates like LaAlO3 with smaller lattice constants. The physical mechanism for the stabilization of the A- and C- magnetic transitions is driven by the orbital splitting of the x2-y2 and 3z2-r2 orbitals, which is induced by the Q3 mode of Jahn-Teller distortions created by the strain induced by the substrates. If the superlattices were prepared employing narrow bandwidth manganites, several non-homogeneous magnetic profiles are predicted to exist, highlighting the importance of carrying out investigations in this mostly unexplored area of research. [1] S. Dong, Q.F. Zhang, S. Yunoki, J.-M. Liu, and E. Dagotto, Phys. Rev. B in press. (ArXiv: 1211.1943) [2] Q.F. Zhang, S. Dong, B.L. Wang, and S. Yunoki, Phys. Rev. B 86, 094403 (2012). [Preview Abstract] |
Monday, March 18, 2013 4:06PM - 4:18PM |
C14.00007: Half-Metallic Ferromagnetism in LaAlO$_3$/SrMnO$_3$ Nanosheet Superlattices Fang Hou, Tian-Yi Cai, Sheng Ju, Ming-Rong Shen Based on first-principle density-functional theory, we have revealed a robust half-metallic ferromagnetism in LaAlO$_{3}$/SrMnO$_{3}$ nanosheet supperlattices. Interface electronic reconstruction, where electrons transfer from the (LaO)$^{+}$ layer to the adjacent (MnO$_{2})^{0}$ layer, is found to lead to the partially occupied e$_{g}$ orbitals at the Mn sites and the half-metallic state in nn-type superlattice via the Zener double-exchange mechanism. On the other hand, holes transfer from (AlO$_{2})^{-}$ layer to (SrO)$^{0}$ layer and reside mainly at oxygen sites in SrMnO$_{3}$, leading to either the preserved G-type AFM ordering in pp-type superlattices or complex magnetic ordering in np-type superlattices. When these systems transist to ferromagnetic ordering by an external magnetic field, an obvious change of electronic states at the Fermi level is found, suggesting a large magnetoresistive effect therein. [Preview Abstract] |
Monday, March 18, 2013 4:18PM - 4:30PM |
C14.00008: Magnetoelectric coupling across the BiFeO3/manganite interface Di Yi, Jian Liu, Pu Yu, Weidong Luo, Suresha Jaganatha, Guneeta Bhalla, Gunnar Palsson, Elke Arenholz, satoshi Okamoto, Ramamoorthy Ramesh Artificially constructed heterointerfaces between strongly correlated systems provide researchers an extensive playground to investigate the novel physics and fascinating states. Recently it has been shown that an exotic magnetoelectric coupling exists at the ferromagnetic manganite La0.7Sr0.3MnO3 (LSMO) and the multiferroic BiFeO3 (BFO) interface, in which the magnetization, the coercive field and exchange bias of LSMO can be controlled by the ferroelectric polarization of BFO. First principle calculations illustrate that different charge screening of polarization lead to different coupling mechanism. To further explore the magnetoelectric coupling, we also investigate the heterostructure between BiFeO3 and half-doped manganite La0.5Ca0.5MnO3 (LCMO). Unlike LSMO which is a ferromagnetic metal, LCMO thin film exhibits a paramagnetic semiconducting behavior in the temperature range we studied, yet the magnetization of LCMO in an applied magnetic field enhanced by a factor of 2 by switching the ferroelectric polarization. X-ray absorption data reveals the different valence states of Mn, consistent with the charge screening model. [Preview Abstract] |
Monday, March 18, 2013 4:30PM - 4:42PM |
C14.00009: Engineering the magnonic and spintronic response of BiFeO$_{3}$ films by epitaxial strain Maximilien Cazayous, P. Rovilain, J. Juraszek, A.K. Zvezdin, L. Bellaiche, B. Dkhil, A. Barthelemy, M. Bibes Multiferroics display cross-coupling effects between ferroelectricity and magnetism. BiFeO$_{3}$ has many properties such as a cycloidal magnetic order in the bulk and conductive domain walls, most related to its ferroelectric order. However its antiferromagnetic properties have not been investigated deeply in thin films. Here we show how the strain engineering can be applied to modify its static and dynamic magnetic properties. We have used Mossbauer and Raman spectroscopies combined with Landau-Ginzburg theory and effective Hamiltonian calculations. We show that the cycloidal spin modulation that exists at low compressive strain is driven towards collinear antiferromagnetism at both tensile and compressive high strain. Morover, we find that the spin excitations are entirely modified with the suppression of the magnon modes as strain increases and that the strain modifies the average spin angle from in-plane to out-of-plane. Our results illustrate the power of strain engineering for designing functional materials on demand. [Preview Abstract] |
Monday, March 18, 2013 4:42PM - 4:54PM |
C14.00010: Order Parameter Interaction at Interfaces and Domain Walls in a BiFeO3 Thin Film Young-Min Kim, Mark Oxley, Anna Morozovska, Eugene Eliseev, Pu Yu, Ying-Hao Chu, Ramamoorthy Ramesh, Stephen Pennycook, Sergei Kalinin, Albina Borisevich Atomic scale studies of the different structural, electronic, and chemical order parameters at domain walls and interfaces are vital for optimization and design of the ferroelectric-based thin film devices. In this study, we use quantitative scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy to study atomic-scale phenomena in a multiferroic thin film heterostructures of BiFeO$_{3}$ (BFO) epitaxially grown on (La,Sr)MnO$_{3}$ (LSMO) layer on a SrTiO$_{3}$ (STO) substrate. We find that charged and uncharged domain walls, as well as interfaces to domain of different polarity, have distinct structural signatures. Charged domain walls are associated with local lattice expansion, suggesting segregation of oxygen vacancies; uncharged domain walls show increased Debye-Waller factors for Bi, suggesting structural frustration. At the LSMO/BFO interface, downward polarization direction is associated with change in local valence state of near-interface Mn cations and lattice expansion. [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C14.00011: Probing of spontaneous polarization screened by defect-induced free carriers in gallium ferrite thin films S.H. Oh, R.H. Shin, W. Jo, C. Lefevre, F. Roulland, A. Thomassn, C. Meny, N. Viart Gallium ferrite, GaFeO3 (GFO), is known as a potential multiferroic material with spontaneous polarization and magnetization [1,2]. However, it was difficult to measure the polarization reversal of GFO thin films because conduction electrons screen the polarization switching responsible for ferroelectricity. Therefore, controlling charge conduction of a multiferroic material is key issue. In this study, we investigated the carrier transport behavior and the charge conduction mechanism in epitaxial GFO thin films deposited on metallic oxide-coated single crystal SrTiO3 substrates by pulsed laser deposition. Macroscopic carrier transports result showed that the interface limited model was the dominant conduction mechanism of the large leakage current and the nature of carrier transport at interface between GFO thin films and substrates was demonstrated by band profiles. Local charge conduction of GFO thin films was studied by conducting atomic force microscope. The polarization switching behavior of GFO thin films was showed by polarization-electric field curve and the positive-up-negative-down method. [1] A. Roy et al., J. Phys.: Condens. Matter 23 (2011) 325902. [2] D. Stoeffler, J. Phys.: Condens. Matter 24 (2012) 185502. [Preview Abstract] |
Monday, March 18, 2013 5:06PM - 5:18PM |
C14.00012: Interfacial Magnetic Response of PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ /La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ Heterostructures San-Wen Chen, Hongyu Guo, Karine Dumesnil, Valeria Lauter, Cecilia Sanchez-Hanke, Edwin Fohtung, Moses Marsh, Oleg Shpyrko, Eric Fullerton, Sunil Sinha There is increasing interest in modifying magnetism by electric fields for both scientific and technological point of view. In a ferromagnetic/piezoelectric composite structure, it is reported that the electric field can alter both the Curie temperature and the magnetization in the ferromagnetic material by inducing charge accumulation or depletion at the interface.$^{\mathrm{1}}$ To understand the detailed changes in the magnetization profile at the interface, we performed both polarized neutron reflectivity and resonant soft X-ray reflectivity measurements on an epitaxially grown PbZr$_{\mathrm{0.2}}$Ti$_{\mathrm{0.8}}$O$_{\mathrm{3\thinspace }}$(PZT)/La$_{\mathrm{0.67}}$Sr$_{\mathrm{0.33}}$MnO$_{\mathrm{3}}$ (LSMO) bilayer. A clear magneto-electric effect was observed below the Curie temperature of LSMO; i.e., the magnetization in LSMO decreases when the electric field was applied. The magnetization depth profile as a function of applied electric field will be presented.\\ \\$^{\mathrm{1}}$H. J. A. Molegraaf et al. \textit{Adv. Materials} \textbf{21}, 3470 (2009).\\$^{2}$This work is supported by DOE/BES through grant number DW-SC0003678. [Preview Abstract] |
Monday, March 18, 2013 5:18PM - 5:30PM |
C14.00013: Investigation on valences and strains in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/PbZr$_{0.2}$Ti$_{0.8}$ O$_{3}$ Heterostructures Jinling Zhou, Disheng Chen, Andreas Scholl, Ying-Hao Chu, Mikel Holcomb Magnetoelectric (ME) coupled materials have electric and magnetic properties coexisting and coupled together, promising novel applications. Understanding the coupling mechanisms responsible for this behavior would allow a strategic approach to device design. Our group studies the interfacial properties of the known magnetoelectric system of ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and ferroelectric PbZr$_{0.2}$Ti$_{0.8}$ O$_{3}$ (PZT). Through photoemission electron microscopy imagining, ME coupling was confirmed at the interface. X-ray absorption spectroscopy of Mn and Ti was taken across wedged samples of varying ferroelectric and ferromagnetic thicknesses. X-ray microdiffraction was analyzed at different thickness to investigate the strain effect on ME coupling. The effect of thickness and strain on Mn and Ti valences suggest an ideal model for improving interfacial coupling in these systems. [Preview Abstract] |
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