Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session C19: Epitaxial Engineering of Magnetic Oxide Thin FilmsFocus
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Sponsoring Units: GMAG DMP Chair: Mark Huijben, University of Twente Room: 318 |
Monday, March 14, 2016 2:30PM - 3:06PM |
C19.00001: Epitaxial Engineering of Domain Walls and Distortions in Ferrite Heterostructures. Invited Speaker: Julia Mundy The defining feature of ferroics is the ability of an external stimulus---electric field, magnetic field, or stress---to move domain walls. These topological defects and their motion enables many useful attributes, e.g., memories that can be reversibly written between stable states as well as enhanced conductivity, permittivity, permeability, and piezoelectricity. Although methods are known to drastically increase their density, the placement of domain walls with atomic precision has until now evaded control. Here we engineer the location of domain walls with monolayer precision and exploit this ability to create a novel multiferroic in which ferroelectricity enhances magnetism at all relevant length scales. Starting with hexagonal LuFeO$_{\mathrm{3}}$, a geometric ferroelectric with the greatest known planar rumpling, we introduce individual extra monolayers of FeO during growth to construct formula-unit-thick syntactic layers of ferrimagnetic LuFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ within the LuFeO$_{\mathrm{3}}$ matrix, i.e., (LuFeO$_{\mathrm{3}})_{m}$/(LuFe$_{\mathrm{2}}$O$_{\mathrm{4}})_{\mathrm{1}}$ superlattices. The severe rumpling imposed by the neighboring LuFeO$_{\mathrm{3}}$ drives the ferrimagnetic LuFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ into a simultaneously ferroelectric state and reduces the LuFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ spin frustration. This increases the magnetic transition temperature significantly---to 281 K for the (LuFeO$_{\mathrm{3}})_{\mathrm{9}}$/(LuFe$_{\mathrm{2}}$O$_{\mathrm{4}})_{\mathrm{1}}$ superlattice. Moreover, LuFeO$_{\mathrm{3}}$ can form charged ferroelectric domain walls, which we align to the LuFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ bilayers with monolayer precision. Charge transfers to these domain walls to alleviate the otherwise electrostatically unstable polarization arrangement, further boosting the magnetic moment. Our results demonstrate the utility of combining ferroics at the atomic-layer level with attention to domain walls, geometric frustration and polarization doping to create multiferroics by design. [Preview Abstract] |
Monday, March 14, 2016 3:06PM - 3:18PM |
C19.00002: Understanding the Interplay of Polar, Magnetic, and Electronic Order in Ferroic (LuFeO$_{3})_{m}$/LuFe$_{2}$O$_{4}$ Superlattices Alejandro Rebola, Hena Das, Craig Fennie Multiferroics are not only important from a technological point of view but also because of the rich and complex physics that results from the interplay between spin, charge and structural distortions. Hexagonal LuFeO$_{3}$ has recently been understood theoretical and experimentally, and shown to be an improper structural ferroelectric directly analogous to the hexagonal manganites. LuFe$_{2}$O$_{4}$ is structurally homologous to LuFeO$_{3}$ --both are characterized by a FeO$_{5}$ bipyramidal crystal field- but unlike the latter it exhibits a much larger magnetic moment and it is still a matter of debate whether it is ferroelectric. The double Fe-layer in LuFe$_{2}$O$_{4}$ is thought to be charge ordered and highly frustrated, resulting in possible polar, non-polar or anti-polar charge arrangements. Here we first investigate the relation between different charge and magnetic orders and structural distortions in bulk LuFe$_{2}$O$_{4}$ by DFT and Monte Carlo calculations. Then we concentrate on a system that combines both mechanisms -a structural improper ferroelectric and a charge frustrated polar structure- such as the (LuFeO$_{3})_{m}$/LuFe$_{2}$O$_{4\, \, }$superlattices. [Preview Abstract] |
Monday, March 14, 2016 3:18PM - 3:30PM |
C19.00003: Strain-mediated control of orbital ordering planes in heteroepitaxial lanthanum manganite thin films Yong-Jin Kim, Jin Hong Lee, Tae Yeong Koo, Chan-Ho Yang Strain engineering which controls the misfit strain of heteroepitaxial thin films leads to distinctive physical properties in contrast to the intrinsic properties of unstrained bulk materials Perovskite LaMnO$_{\mathrm{3}}$ (LMO) has attracted considerable attention due to strong coupling among the lattice, charge, spin and orbital degrees of freedom. Bulk LMO is known to be an A-type antiferromagnetic (T$_{N}$\textasciitilde 140 K) Mott insulator, and its orbital ordering plane is established due to cooperative Jahn-Teller distortion below \textasciitilde 750 K. Previous studies have focused on the orbital ordering planes of the bulk LMO but not researched on correlation between orbital planes and misfit stain. To figure out the strain dependence of orbital ordering planes, we have grown LMO thin films on four different substrates, $i.e.$, DyScO$_{\mathrm{3}}$(110), GaScO$_{\mathrm{3}}$(110), SrTiO$_{\mathrm{3}}$(001), and LSAT(001), using the pulsed laser deposition technique. The films have been characterized by atomic force microscopy and x-ray diffraction. We have performed resonant x-ray scattering to identify orbital ordering plane on each film. We have found that orbital ordering planes can be modulated depending on the misfit strain. [Preview Abstract] |
Monday, March 14, 2016 3:30PM - 3:42PM |
C19.00004: Ferroelectric-ferromagnetic coupling in hexagonal YMnO$_{3}$ film. Shaobo Cheng, Menglei Li, Shiqing Deng, Shanyong Bao, Peizhe Tang, Wenhui Duan, Jing Ma, Cewen Nan, Jing Zhu Simultaneously achieving ferroelectricity and ferromagnetism in a single phase material is an important research topic in recent decades. Here, we demonstrate that with the modulation of oxygen vacancies, the ferroelectric-ferromagnetic coupling can be realized in the typical hexagonal manganite: YMnO$_{3}$. The first-principal calculations are used to reveal the importance of oxygen vacancies on the alterations of magnetic behaviors for YMnO$_{3}$. In order to obtain net magnetic moments, the on-top oxygen vacancies of MnO$_{5}$ clusters should be created, thus the initial 2D spin frustration structure of Mn ions will be broken. By growing YMnO$_{3}$ film on Al$_{2}$O$_{3}$ substrate, large in-plane compressive strain is induced, thus we can experimentally realize the on-top oxygen vacancies. With the help of SQUID and spherical aberration corrected TEM, the magnetic moments are experimentally measured and the correlations between the crystal structures and magnetic properties can be clearly understood. Our findings may pave a way for future applications of single phase multiferroic materials. [Preview Abstract] |
Monday, March 14, 2016 3:42PM - 3:54PM |
C19.00005: An oxygen-deficiency modulated multiferroic: Cobalt-substituted perovskite Juan Manuel Florez In this work, we use density functional theory to model recently demonstrated room temperature ferromagnetism and ferroelectricity in polycrystalline and single crystal Cobalt-substituted SrTiO$_3$ thin films (SrTi$_{0.70}$Co$_{0.30}$O$_{3-d}$), deposited at different oxygen pressures to change their oxygen vacancy concentration. The modeling indicates an origin for both magnetism and electric polarization in the interactions between oxygen vacancies and the B-site cations. The magnetization saturation increases with the oxygen deficiency as a result of valence spin states changes, which depend on whether the oxygen octahedral of the respective local B-site cations are complete or not. On the other hand, a finite electric polarization appears as a result of a non-centrosymmetric distribution of different resulting local charges and such a polarization increases when the oxygen vacancies increase. Increasing of both order parameters, magnetic and ferroelectric, are analyzed respect to all possible Co-sites and O-vacancies distributions, showing that these results suggest a class of multiferroic materials with properties controlled by their oxygen stoichiometry. Agreement and discrepancies between experiments and modeling are discussed. [Preview Abstract] |
Monday, March 14, 2016 3:54PM - 4:06PM |
C19.00006: Transport and Raman signatures of electron-doped SmNiO$_{3}$ thin films Koushik Ramadoss, Nirajan Mandal, You Zhou, Yong Chen, Shriram Ramanathan We report low temperature transport and Raman spectroscopy measurements of electron-doped SmNiO$_{3\, }$(SNO) thin films. It has been shown that pristine SNO films can be doped with electrons using hydrogen. Our transport measurements indicate a Coulomb interaction dominated variable range hopping (VRH) for electron-doped samples whereas the pristine films show a Mott type VRH mechanism at low temperatures. The electron-doped samples display a strong localization which can be correlated with the high spin state of Ni$^{2+}$ ions. The spatial Raman map shows a remarkable shift of about 167 cm$^{-1}$ with electron doping thus serving as a spectroscopic tool to investigate hydrogen in our films. \textbf{References} \begin{enumerate} \item J. Shi, Y. Zhou and S. Ramanathan, Nat. Commun \textbf{5}, 4860 (2014) \item Jikun Chen \textit{et al}., Appl. Phys. Lett. \textbf{107}, 031905 (2015) \end{enumerate} [Preview Abstract] |
Monday, March 14, 2016 4:06PM - 4:18PM |
C19.00007: Temperature dependent near field infrared microscopy of La$_{\mathrm{0.67}}$Sr$_{\mathrm{0.33}}$MnO$_{\mathrm{3}}$ thin films Peng Xu, TJ Huffman, MM Qazilbash, Inhae Kwak, Amlan Biswas La$_{\mathrm{0.67}}$Sr$_{\mathrm{0.33}}$MnO$_{\mathrm{3}}$ thin films are studied with apertureless, scattering-type near field microscopy at mid-infrared wavelength and varied temperatures. Spatial resolution of about 20 nm is achieved with our technique. The temperature-dependent resistivity shows a continuous second order phase transition between insulating and metallic phases. At most temperatures, near-field infrared microscopy reveals local persistent phase separation that is independent of temperature. It is possible that the local persistent phase separation is induced by strain inhomogeneity in the thin films. Remarkably, we also observe global time-dependent changes in the infrared near-field signal upon repeated scanning of the same microscopic area at a fixed temperature. This observation is consistent with time-dependent, fluctuating conductivity in the vicinity of a second order phase transition. [Preview Abstract] |
Monday, March 14, 2016 4:18PM - 4:30PM |
C19.00008: Step-induced magnetic phase separation in La$_{\mathrm{0.67}}$Sr$_{\mathrm{0.33}}$MnO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ (100) thin films. In Hae Kwak, Amlan Biswas We investigated thickness dependent magnetic anisotropy in La$_{\mathrm{0.67}}$Sr$_{\mathrm{0.33}}$MnO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ (100) (LSMO/STO) thin films using a combination of magnetic force microscopy (MFM) and magnetization measurements. Atomically smooth thin films of LSMO were grown on STO using pulsed laser deposition. The thin films showed step flow growth with unit cell step heights. MFM images of a 40 unit cell (u.c.)-thick film showed out-of-plane magnetic domain structure indicating bulk-like rhombohedral crystalline anisotropy. As the film thickness was decreased to 20 u.c., the MFM images showed signatures of step-induced uniaxial anisotropy. Hence, the magnetic domain structure shows that tensile strain from lattice mismatch weakens the rhombohedral crystalline anisotropy in LSMO. Magnetization vs. field, $M(H)$ measurements for the 20 u.c. thick LSMO film revealed a clear in-plane uniaxial anisotropy with the direction along the steps being the easy axis and the coercive fields along the steps were consistently smaller than across the steps for a broad temperature range. Our combination of bulk and local magnetic measurements suggest that the microscopic origin of magnetic anisotropy is step-induced phase separation in the thinner films which are under higher tensile strain. [Preview Abstract] |
Monday, March 14, 2016 4:30PM - 4:42PM |
C19.00009: Electronic and magnetic properties of quadruple manganite Ca$_{1-x}$Sr$_{x}$Mn$_{7}$O$_{12}$ films Amanda Huon, Steven May We investigate the functional properties of epitaxial Ca$_{1-x}$Sr$_{x}$Mn$_{7}$O$_{12}$ films to better understand the underlying physical phenomenon in this perovskite system. We utilize oxide molecular beam epitaxy to fabricate Ca$_{1-x}$Sr$_{x}$Mn$_{7}$O$_{12}$ thin films. The epitaxial films were achieved through a two-step oxygen/ozone post-growth anneal. In parent x$=$0 films, we find bulk-like electronic and magnetic properties including an abrupt increase in resistivity at 425 K due to a nominal charge ordering transition and a net magnetization below 43 K likely arising from helical magnetic order. Finally, we will present on how tuning the Sr concentration alters the electronic and magnetic properties, providing a means to control the phase transition temperatures. The results highlight the scientific opportunities in heterostructures based on quadruple manganites. [Preview Abstract] |
Monday, March 14, 2016 4:42PM - 4:54PM |
C19.00010: Observation of a huge polaron gyrotropic response near room temperature in manganite thin films. Gervasi Herranz, Blai Casals, Rafael Cichelero, David Pesquera, Mariano Campoy, Florencio Sanchez, Josep Fontcuberta, Pablo Garcia Fernandez, Javier Junquera Magnetic materials induce rotation and ellipticity in the polarization of light. This phenomenon is exploited, e.g., to control the flux of light along optical fibers. In the pursuit for increased magneto-optic responses, strategies so far have been based on photonic or plasmonic effects. Here we uncover a novel physical mechanism by which the gyrotropic activity is hugely enhanced around the Curie temperature in optimally doped ferromagnetic manganites. This phenomenon is observed only for a narrow range of wavelengths and temperatures and is strongly dependent on the angle of incidence and polarization. We understand such an outstanding response as the result of the interplay between Jahn-Teller distortions and spin-orbit coupling in narrow-band manganites. The showcased material is La$_{\mathrm{2/3}}$Ca$_{\mathrm{1/3}}$MnO$_{\mathrm{3}}$, for which the extraordinary gyrotropic response is seen near room temperature. This raises the possibility of optimizing the stoichiometric composition to drive the effect to higher temperatures. The observed phenomenon gives an added functionality --unseen previously in any manganite or other magnetic oxides-- and puts a new perspective on the use of these materials for optical data storage and retrieval. [Preview Abstract] |
Monday, March 14, 2016 4:54PM - 5:06PM |
C19.00011: The Manipulation of Electronic Phase Separation in Manganites. Lifeng Yin The Electronic Phase Separation (EPS) is a common phenomenon in strongly correlated systems where two or more electronic phases coexist owing to a delicate balance of competition between these phases. A model system is (La$_{\mathrm{5/8-y}}$Pr$_{\mathrm{y}})$Ca$_{\mathrm{3/8}}$MnO$_{\mathrm{3\thinspace }}$(LPCMO), a colossal magnetoresistance (CMR) manganite that is known for its large-scale EPS. Since the transport and magnetic properties depend sensitively on EPS, it is crucial to manipulate the EPS domains, especially for the applications of CMR manganites in multifunctional electronic and spintronic devices. Through the broken symmetry induced edge states, we can control the nucleation and growth of ferromagnetic metallic domains, thus the spatial distribution of EPS domains in turn. When the Pr doping is chemical ordered, we found the size of EPS domains will be one order of magnitude smaller. Furthermore, the EPS phenomena can be fully eliminated by the spatial confinement when the sample size is smaller than 500nm. These findings could help to understand the origin of large-scale EPS in LPCMO. [Preview Abstract] |
Monday, March 14, 2016 5:06PM - 5:18PM |
C19.00012: Enhanced magnetization in ultrathin manganite layers via structural ``delta-doping'' of octahedral rotations Eun Ju Moon, Brian J. Kirby, Steven J. May The design of rotations and distortions of the corner-shared $B$O$_{6}$ octahedra has emerged as an exciting platform to control electronic or magnetic behavior in \textit{AB}O$_{3}$ perovskite heterostructures. Recent work has shown that purely structural effects can be used to spatially confined magnetism in oxide heterostructures and point to the design of rotational gradients as routes to realize novel electronic or ferroic states in oxide superlattices [Nat. Comm. \textbf{5}, 5710 (2014)]. Here, we demonstrate a structural ``delta doping'' approach for controlling magnetism in ultrathin layers within isovalent manganite superlattices. Polarized neutron reflectivity and temperature dependent magnetization measurements are used to correlate enhanced magnetization with local regions of suppressed octahedral rotations in the heterostructures. [Preview Abstract] |
Monday, March 14, 2016 5:18PM - 5:30PM |
C19.00013: Ultrafast structural dynamics of LaVO3 thin films grown by hybrid molecular beam epitaxy matthew brahlek, Jason Lapano, Vladimir Stoica, Lei Zhang, Hai-tian Zhang, Hirofumi Akamatsu, craig eaton, Venkatraman Gopalan, John Freeland, Haidan Wen, Roman Engel-Herbert LaVO$_{3}$, with a partially full d-shell is expected to be metallic, but due to electron-electron interactions a gap emerges and the ground state is a Mott insulator. Such effects are a strong function of the bonding geometry, and particularly the V-O-V bond angle. Controlling these structural effects on the ultrafast time scale can lead to control over the underlying electronic ground state. Here we report the ultrafast structural dynamics of 25 and 50 nm thick LaVO$_{3}$ thin films grown by the hybrid molecular beam epitaxy technique on SrTiO$_{3}$ when excited across the bandgap by 800 nm light. Using time-resolved x-ray diffraction on the 100 ps time scale at Sector 7 of the Advanced Photon Source, we directly measured the structural changes with atomic accuracy by monitoring integer Bragg diffraction peaks and find a large out-of-plane strain of 0.18{\%} upon optical excitation; the recovery time is \textasciitilde 1 ns for the 25 nm film and \textasciitilde 2 ns for the 50 nm film, consistent with the thermal transport from the film to the substrate. Further, we will discuss the response of the oxygen octahedral rotation patterns indicated by changes of the half-order diffraction peaks. Understanding such ultrafast structural deformation is important for optimizing optical excitations to create new metastable phases starting from a Mott insulator. This work was supported by the Department of Energy under Grant DE-SC0012375, and DE-AC02-06CH11357. [Preview Abstract] |
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