Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S32: Focus Session: Magnetic Oxide Films and Heterostructures: Electrostatic and Strain Effects |
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Sponsoring Units: GMAG DMP Chair: Pavel Lukashev, University of Northern Iowa Room: 207B |
Thursday, March 5, 2015 8:00AM - 8:12AM |
S32.00001: Electric-field control of magnetism in multiferroic heterostructures Yonggang Zhao, Sen Zhang, Peisen Li, Aitian Chen, Dalai Li, Lifeng Yang, S. Rizwan, Y. Liu, Xia Xiao, Yizheng Wu, Xiaofeng Jin, Xiufeng Han, Huiyun Zhang, Meihong Zhu We have studied electric-field control of magnetism in different multiferroic heterostructures, composed of ferromagnetic (FM) and ferroelectric (FE) materials such as Co40Fe40B20(CoFeB)/Pb(Mg1/3Nb2/3)0.7Ti0.3O3(PMN-PT) and magnetic tunnel junctions (MTJ) on PMN-PT, etc. A giant electric-field control of magnetization as well as magnetic anisotropy was observed in a CoFeB/PMN-PT structure at room temperature with a maximum relative magnetization change up to 83 percent and a 90$^{\circ}$ rotation of the easy axis. In MTJ of CoFeB/AlOx/CoFeB grown on PMN-PT, we demonstrate a reversible, continuous magnetization rotation and manipulation of tunneling magnetoresistance at room temperature by electric fields without the assistance of a magnetic field. These results show the interesting new physics and potential applications of the FM/FE multiferroic heterostructures. [Preview Abstract] |
Thursday, March 5, 2015 8:12AM - 8:24AM |
S32.00002: Effects of Ferroelectric Polarization and Strain on Magneto-Crystalline Anisotropy of SrRuO$_3$ Jeevaka Weerasinghe, Tula R. Paudel, Evgeny Y. Tsymbal Magnetoelectric properties of materials have recently been extensively investigated due to their potential application in magnetic data storage, spintronics and high-frequency magnetic devices. Among those properties is the electric field effect on magneto-crystalline anisotropy (MCA) which allows the control of magnetization orientation and thus has a direct relevance to magnetic memory applications. In this work, we explore the magneto-crystalline anisotropy of ferromagnetic metal oxide SrRuO$_3$ using first-principles density functional calculations. Due to the presence of Ru atoms this material has a relatively strong spin-orbit coupling resulting in high MCA. We investigate how epitaxial compressive and tensile strains affect the bulk anisotropy of this material. We also explore epitaxial SrRuO$_3$/BaTiO$_3$ heterostructures where ferroelectric polarization of BaTiO$_3$ affects the interface MCA energy and thus may be used as a control parameter to switch the magnetization orientation. We discuss the physical origins of the effects predicted in terms of the modulation of the electronic structure of SrRuO3 by polarization charge screening and strain. [Preview Abstract] |
Thursday, March 5, 2015 8:24AM - 8:36AM |
S32.00003: Strain control of magnetic structure in layered iridates via strong orbital-lattice coupling Choong Hyun Kim, Craig Fennie We have studied from first principles the structural, electronic, and magnetic properties of the layered-perovskite iridates A$_2$IrO$_4$ and A$_3$Ir$_2$O$_7$ (A=Sr,Ba) as a function of epitaxial strain. In most of perovskite iridates, due to their large spin-orbit coupling and cubic crystal field, ground state could be described by an effective total angular momentum state $J_{\rm eff}=1/2$ within $t_{2g}$ manifold. In contrary to what is usually assumed, we find that $d_{x^2-y^2}$ orbital plays a crucial role to determine a magnetic ground state of iridates if the cubic crystal field is not big enough compared to band width. For instance Ba$_2$IrO$_4$ with tensile strain induces a situation in which magnetization is reversed. Our first-principles results show how A-site cation, dimensionallity, and strain are correlated with the band width and crystal field to control magnetic ground states. [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 9:12AM |
S32.00004: Electrostatic Chemical Strain: An Approach to Electronic Structure Engineering in Layered Oxides Invited Speaker: James Rondinelli Traditional approaches to create and control functional electronic materials have focused on new phases in previously unknown bulk minerals. More recently, interlayer physics has spawned interest in known materials in unexplored atomic scale geometries, especially in complex transition metal oxides (TMO), where heterostructures and superlattices with abrupt interfaces can be created on demand. The interfaces between TMO overs a handle to direct the electrostatic field exerted on the transition metal centers via the coordinating oxygen ligands, which alter the M cation's $d$-orbital occupancies and spin state, thereby imparting desirable electronic functionality. In this talk, I describe an atomistic engineering approach that makes use of long-range electrostatic interactions between atomic metal-monoxide planes ($A$O and $A^\prime$O) in naturally occurring superlattices, e.g., Ruddlesden-Popper (RP), phases, to tune interlayer atomic structure, orbital degeneracies, and magnetic properties. Using first-principles electronic structure calculations, I show how this electrostatic chemical strain (ECS) effect can be used to tune both crystal field energies and the frontier orbital structure in correlated (La,$A$)NiO$_4$ RP phases at fixed stoichiometry. I describe how to enhance the Ni $e_g$ orbital polarization, resulting in NiO$_6$ units that exhibit a single $d(x^2-y^2)$ band at the Fermi level---electronic features similar to the layered superconducting cuprates. This approach is generic in construction, making it applicable to any layered topology supporting heterovalent cation substitutions. I conclude by showing it is a realistic strategy to tailor the electronic properties of known materials, and discover yet-to-be realized novel functional oxides without resorting to complex assembly of multi-component heterostructures. [Preview Abstract] |
Thursday, March 5, 2015 9:12AM - 9:24AM |
S32.00005: First principles study of the origin of Strain-tunable extraordinary magnetocrystalline anisotropy in Sr$_{2}$CrReO$_{6}$ epitaxial films M.R. Ball, J.M. Lucy, O.D. Restrepo, A.J. Hauser, J.R. Soliz, J.W. Freeland, P.M. Woodward, W. Windl, F. Y. Yang Magnetocrystalline anisotropy (MCA) has significant implications in a range of applications such as power generation and magnetic data storage. We report the discovery of extraordinarily large anisotropy fields and strain-tunable MCA in Sr$_{2}$CrReO$_{6}$ epitaxial films. These films grown on (LaAlO$_{3})_{0.3}$(Sr$_{2}$AlTaO$_{6})_{0.7}$ (LSAT), SrTiO$_{3}$, and~SrCr$_{0.5}$Nb$_{0.5}$O$_{3}$/LSAT substrates undergo dramatic changes in MCA shown by a shift in easy axis from in-plane to out-of plane. To find the origin of this, we determine the strain-induced distortions and octahedral rotations by performing density functional theory (DFT) calculations using VASP. Correlation effects were treated within GGA$+$U. In DFT, the change in easy axis under strain seen in experiment can be examined through the difference in total energies for magnetic orientation along different crystalline axes known as the magnetic anisotropy energy (MAE). The MAE is directly related to the moment anisotropy which is the difference between hard- and easy-axis orbital moments. When a sign change in the moment anisotropy is present, a change in easy axis is indicated. We indeed find this sign change with increasing $c/a$ ratio which is in agreement with experiments. The origin of the MAE resides in the strain-induced changes in spin-orbit coupling on the Re-atoms. This interplay between structural deformations and magnetism leads to a giant MCA [Preview Abstract] |
Thursday, March 5, 2015 9:24AM - 9:36AM |
S32.00006: Depth-resolved magnetic and structural analysis of relaxing epitaxial Sr$_2$CrReO$_6$ Jeremy Lucy, Fengyuan Yang, Adam Hauser, Yaohua Liu, Hua Zhou, Yongseong Choi, Suzanne G.E. te Velthuis, Daniel Haskel Structural relaxation in a Sr$_2$CrReO$_6$ epitaxial film, with strong spin-orbit coupling, leads to depth-dependent magnetism. We combine a couple of depth-resolved synchrotron x-ray techniques, including two-dimensional reciprocal space mapping and x-ray magnetic circular dichroism experiments, to demonstrate this effect. An 800 nm film of Sr$_2$CrReO$_6$, grown with tensile epitaxial strain on SrCr$_{0.5}$Nb$_{0.5}$O$_3$(200 nm)/LSAT, relaxes away from the Sr$_2$CrReO$_6$/SrCr$_{0.5}$Nb$_{0.5}$O$_3$ interface. Grazing incidence x-ray diffraction measurements of the film elucidate the in-plane strain relaxation while depth-resolved x-ray magnetic circular dichroism at the Re \textit{L} edge reveals the magnetic contributions of the Re site. The smooth relaxation of the film correlates with a systematic change in the magnetism.This provides an interesting and powerful way to probe the depth-varying structural and magnetic properties of a complex oxide with synchrotron-source x-ray techniques. [Preview Abstract] |
Thursday, March 5, 2015 9:36AM - 9:48AM |
S32.00007: Effects of Strain and Buffer Layer on Interfacial Magnetization in Sr$_2$CrReO$_6$ Films Yaohua Liu, S.G.E. te Velthuis, A. Glavic, H. Ambaye, V. Lauter, J.M. Lucy, F.Y. Yang Magnetic double-perovskite Sr$_2$CrReO$_6$ (SCRO) has several functional properties including a T$_C >$ 500 K, high spin polarization, large spin-orbit interaction, and semiconducting behavior in highly ordered films. However, fabrication of highly ordered films is still challenging, and progress toward device applications requires an in-depth understanding of the electronic and magnetic properties, especially at interfaces. We have investigated how the Cr/Re antisite disorder and strain affect the interfacial magnetization in SCRO films via x-ray and polarized neutron reflectometry [1]. We find that the magnetization of SCRO films is reduced near the interface with the substrate. The width of this interfacial layer weakly depends on the strain and decreases when a SrCr$_{0.5}$Nb$_{0.5}$O$_3$ (SCNO) buffer layer is used to reduce the antisite disorder. Interestingly, for the SCRO film deposited on a SCNO buffer layer, the region with reduced magnetization is wider than the antisite disorder region at the SCRO/SCNO interface, suggesting that antisite disorder is not the only mechanism reducing the magnetization. [1] Yaohua Liu {\it et al.}, Phys. Rev. B 90, 104416 (2014). [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S32.00008: Strong Enhancement of Magnetization in Fully Strained SrRuO$_{3}$ films on Sr$_{2}$RuO$_{4}$ single crystal substrates Seungran Lee, Y.J. Shin, M.C. Lee, C.H. Sohn, S.J. Kang, M.S. Anwar, Y. Sugimoto, S. Yonezawa, Y. Maeno, T.W. Noh We have investigated epitaxal growth and magnetic properties of SrRuO$_{3}$ (SRO113) films on single crystal Sr$_{2}$RuO$_{4}$ (SRO214) substrates. X-ray diffraction shows only SRO113(00$l)$ peaks indicating epitaxial deposition; rocking scan at SRO112(002) exhibits a sharp curve with a full width at half maximum of \textless 0.05$^{\circ}$ implying high crystallinity of our films. Transmission electron microscopy further verifies fully strained of SRO113 films with sharp interface. Surprisingly, the magnetic properties of SRO113/SRO214 show strong enhancement of magnetization (M \textgreater 3 $\mu _{\mathrm{B}}$/Ru), which has never found SRO113(001) material systems. In addition, the Curie temperature of our films is identical to that of a bulk SRO113. Magnetic properties of SRO113 films are known to decrease under strain, attributed to RuO6 octahedral distortion. In comparison with varying strain of SRO113 films employing various perovskite substrates, we also found such enhancement is not coming from strain effect; M does not exceed 2 $\mu_{\mathrm{B}}$/Ru consistent with previous results due to the low spin configuration nature of SRO113. Possible origins of unique magnetic properties of SRO113/214 will be further discussed. [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S32.00009: Strain dependence of interfacial antiferromagnetic coupling in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/SrRuO$_{3}$ superlattices Sujit Das, Andreas Herklotz, Eckhard Pippel, Er-Jia Guo, Diana Rata, Kathrin D\"orr We have investigated the magnetic response of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/SrRuO$_{3}$ superlattices to biaxial in-plane strain applied \textit{in-situ}. Superlattices grown on piezoelectric substrates of 0.72PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$-0.28PbTiO$_{3}$(001) (PMN-PT) show strong antiferromagnetic coupling of the two ferromagnetic components. The coupling field of $\mu_{0}H_{AF\thinspace }=$ 1.8 T is found to change by $\mu_{0}\Delta H_{AF}/\Delta \varepsilon \sim $ -520 mT {\%}$^{-1}$ under reversible biaxial strain ($\Delta \varepsilon )$ at 80 K in a [La$_{0.7}$Sr$_{0.3}$MnO$_{3}$(22 {\AA})/SrRuO$_{3}$(55 {\AA})]$_{15}$ superlattice. This reveals a significant strain effect on interfacial coupling. The applied in-plane compression enhances the ferromagnetic order in the manganite layers which are under as-grown tensile strain. It is thus difficult to disentangle the contributions from strain-dependent antiferromagnetic Mn-O-Ru interface coupling and Mn-O-Mn ferromagnetic double exchange near the interface, since the enhanced magnetic order of Mn spins leads to a larger net coupling of SrRuO$_{3}$ layers at the interface. We discuss our experimental findings taken into account both the strain-dependent orbital occupation in a single--ion picture and the enhanced Mn order at the interface. [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S32.00010: Epitaxial strain induced atomic ordering in stoichiometric LaCoO$_{3}$ thin films Woo Seok Choi, Ji-Hwan Kwon, Hyoungjeen Jeen, George A. Sawatzky, Vladimir Hinkov, Miyoung Kim, Ho Nyung Lee Heteroepitaxial strain imposed in complex transition metal oxide thin films is recognized as an effective tool for identifying and controlling emergent physical phenomena. Stoichiometric LaCoO$_{3}$ is particularly interesting, since the thin film form of the material exhibits a robust macroscopic ferromagnetic ordering, while the bulk form of the material is a zero spin, nonmagnetic insulator. In this work, we show that the ferromagnetic ordering observed in LaCoO$_{3}$ thin films is related to a lattice modulation in the atomic scale, originating from the epitaxial strain. The possibility of oxygen vacancies have been carefully ruled out using various macroscopic and microscopic spectroscopic techniques, and an unconventional strain relaxation behavior identified by strip-like lattice modulation pattern was responsible for the non-zero spin ground state of Co$^{3+}$ ions [1,2]. We further note that the unconventional strain relaxation did not involve any uncontrolled misfit dislocations. \\[4pt] [1] W. S. Choi \textit{et al.}, \textit{Nano Lett.} \textbf{12}, 4966 (2012).\\[0pt] [2] J.-H. Kwon \textit{et al.}, \textit{Chem. Mater.} \textbf{26}, 2496 (2014). [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S32.00011: Effect of periodicity on order parameters of multiferroic superlattices Shalini Kumari, Nora Ortega, Ashok Kumar, Ram Katiyar Superlattice (SL) structures with alternating perovskite oxide layers have attracted enormous attention due to involved fascinating physics and technology. The half-metallic oxide La$_{0.67}$Sr$_{0.33}$MnO$_{3}$(LSMO) and multiferroic Pb(Zr$_{0.53}$Ti$_{0.47})_{0.60}$(Fe$_{0.5}$Ta$_{0.5})_{0.40}$O$_{3}$(PZTFT) materials have been chosen to fabricate SLs by pulsed laser deposition technique on cubic LSAT substrates with LSMO or LaNiO$_{3}$ as bottom electrodes. X-ray diffraction studies revealed superlattice structure with satellite peaks modulated around main peaks. Atomic force microscopy studies disclosed a systematic decrease in grain size with decrease of modulation periodicity ( $\Lambda )$ in SLs. Piezo force microscopy studies of SL films confirmed ferroelectricity at a nanoscale level. XPS studies of SLs with $\Lambda =$5 nm confirmed the existence of all elements in the films. A relatively small reduction in saturation magnetization from 28 to 20 emu/cm$^{3\, }$at H$=$5 kOe, remanant polarization from 21 to 10 $\mu $C/cm$^{2}$ and increase in dielectric constant from 530 to 743 were observed with decrease of $\Lambda $.The observed features will be explained in context of finite size, interfaces, stress, lattice distortion, and grain sizes effects. [Preview Abstract] |
Thursday, March 5, 2015 10:36AM - 10:48AM |
S32.00012: Electric-field control of magnetization in (Co(t)/Pt)n/Pb(Mg1/3Nb2/3)1-xTixO3 multiferroic heterostructures Ying Sun, Yonggang Zhao, Aitian Chen, Yan Liu, Lvkuan Zou, Xiaoli Zheng, Qintong Zhang, Jianwang Cai, Xiufeng Han, Wenbo Wang, Weida Wu A promising way to control magnetism via electric fields is using the converse magnetoelectric effect(CME) in heterostructures composed of ferromagnetic and ferroelectric materials. So far there are few reports on electric-field (E) control of magnetic materials with perpendicular magnetic anisotropy(PMA) which is important in information storage because of its high density and thermostability. In this work, we have systematically studied the CME in heterostructures formed by growing (Co(t)/Pt)n multilayers with different Co thicknesses and n on (011)-orientated ferroelectric Pb(Mg1/3Nb2/3)1-xTixO3 substrates. By tuning Co thickness to the vicinity of the spin reorientation critical thickness, samples with PMA, in-plane magnetic anisotropy and crossover were obtained. They showed dramatic different behaviors of E control of magnetization. The results can be understood by considering the interaction between the piezostrain induced magnetic anisotropy and Co thickness-dependent magnetic anisotropy. [Preview Abstract] |
Thursday, March 5, 2015 10:48AM - 11:00AM |
S32.00013: Manipulation of magnetic phase separation and orbital occupancy in manganites by strain engineering and electric field Bin Cui, Cheng Song, Feng Pan The modification of electronic phases in correlated oxides is one of the core issues of condensed matter. We report the reversible control of ferromagnetic phase transition in manganite films by ionic liquid gating, replicating the La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$MnO$_{3}$ (LSMO) phase diagram. The formation and annihilation of an insulating and magnetically hard phase in the soft magnetic matrix, which randomly nucleates and grows across the film, is directly observed under different gate voltages ($V_{\mathrm{G}})$. The realization of reversible metal-insulator transition in colossal magnetoresistance materials can lead to the development of four-state memories. (Adv Funct. Mater. DOI: 10.1002/adfm.201402007) The orbital occupancy and magnetic anisotropy of LSMO films are manipulated by $V_{\mathrm{G}}$ in a reversible and quantitative manner. Positive and negative $V_{\mathrm{G}}$ increases and reduces the occupancy of the orbital and magnetic anisotropy that were initially favored by strain (irrespective of tensile and compressive), respectively. This finding fills in the blank of electrical manipulation of four degrees of freedom in correlated system. [Adv Funct. Mater. (revised)] [Preview Abstract] |
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