### Session D33: Focus Session: Complex Oxide Thin Films -- Magnetic Oxides

 Monday, March 15, 2010 2:30PM - 2:42PM D33.00001: Interfacial spin structure in \textit{epitaxial} \textit{ferromagnetic} oxide bilayers X. Ke , V. Lauter , C.B. Eom , M.S. Rzchowski Interfaces between the individual layers in magnetic multilayer systems play a key role in determining the properties of the systems. In oxide heterostructures the interface can become even more important, sometimes dominating the physical properties of the materials and introducing unexpected behavior. In this paper, we report exchange-bias phenomena in a unique \textit{epitaxial} exchange-biased \textit{ferromagnetic} bilayer, La$_{0.67}$Sr$_{0.33}$MnO$_{3}$~/ SrRuO$_{3}$, with an atomically sharp interface. Depending on the magnitude of the cooling fields, both negative and positive exchange-bias features are observed, in addition to a double hysteresis loop existing with intermediate cooling fields. Our previous work has shown that there is antiferromagnetic exchange coupling at the interface [1], despite both materials having ferromagnetic exchange coupling in bulk. Data of both bulk magnetometer and polarized neutron reflectometry (PNR) measurements will be presented, and the mechanism determining the interfacial spin structure of the bilayer will be discussed. Work supported by U.S. DOE. \\[4pt] [1]. X. Ke, M.S. Rzchowski, L.J. Belenky, and C.B. Eom, Appl. Phys. Lett. \textbf{84}, 5458 (2004). Monday, March 15, 2010 2:42PM - 2:54PM D33.00002: Magnetic properties of Fe substituted SrRuO$_{3}$ thin films and SrRuO$_{3}$/Fe$_{2}$O$_{3}$ superlattices Omar Chmaissem , Stanislaw Kolesnik , Bogdan Dabrowski , YongSeong Choi , Daniel Haskel In recent years, SrRuO$_{3}$ thin films have received considerable interest because of their potential for use as electrodes in oxide-based spintronic applications. SrRuO$_{3}$ bulk materials are known to exhibit good room temperature thermal and electrical conductivity, a stable perovskite crystal structure, and itinerant ferromagnetic properties at temperatures below 163 K. To the best of our knowledge, attempts to enhance the magnetic properties of SrRuO$_{3}$ through chemical substitutions of transition metal elements (e.g., Fe, Co, Mn, Cu, Zn, Ti, Cr, etc) at the Ru site, all failed except for the case of Cr substitutions in which T$_{C}$ was successfully raised to 190 K. In this work, we will demonstrate the drastically different effects of Fe on the magnetic properties of SrRuO$_{3}$ bulk materials and thin films. We will also show and discuss the magnetic properties of SrRuO$_{3}$/Fe$_{2}$O$_{3}$ superlattices. Work supported by the NSF (DMR-0706610) and the DOE-Office of Science (DEAC-02-06CH11357). Monday, March 15, 2010 2:54PM - 3:06PM D33.00003: The infrared complex magneto-optical conductivity tensor in Ca$_x$Sr$_{1-x}$RuO$_3$ films M.-H. Kim , G. Acbas , C.T. Ellis , M.-H. Yang , J. Cerne , P. Khalifah , I. Ohkubo , H. Christen , D. Mandrus , Z. Fang We explore the complex longitudinal ($\sigma_{xx}$) and Hall ($\sigma_{xy}$) conductivities, as well as the complex Hall angle ($\theta_H$) in a series of Ca$_x$Sr$_{1-x}$RuO$_3$ films by measuring the infrared ($E$ = 115 - 1400 meV) magnetization-induced complex Faraday and Kerr angles in the 10 - 300 K temperature ($T$) range. The Hall sign reversal with $T$ at 117 meV is observed up to 20\% Ca composition. It is similar to the sign reversal in the dc $\theta_H (T)$. The sign reversal $T$ decreases with increasing Ca composition. The infrared $\sigma_{xy}(E)$ and $\theta_{H}(E)$ show strong $E$ dependence at low energies. As Ca composition increases, the magnitude of the $\mathrm{Re}(\theta_H (E))$ dip decreases while the energy and width of the $\mathrm{Im}(\theta_H (E))$ dip increases. We analyze the results in terms of intrinsic and extrinsic anomalous Hall effect models. This work was supported by the Research Corp. Cottrell Scholar Award (UB), NSF-CAREER-DMR0449899 (UB), and an instrumentation award from the CAS (UB). Oak Ridge Natl. Lab is managed by UT-Battelle, LLC for the U.S. DOE (contract DE-ACO5-00OR22725). Monday, March 15, 2010 3:06PM - 3:18PM D33.00004: Ferroelectric field effect control of electronic and magnetic properties in ultrathin manganite films Zsolt Marton , Ho Nyung Lee , Takeshi Egami Electrostatic modulation of the carrier density in correlated electron oxides can be used to bring about drastic modifications of the electronic and magnetic properties. Therefore, in order to test the fascinating concept of electrostatic modulation of carrier population, we have fabricated field effect heterostructures composed of ferroelectric PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ (PZT) and magnetic La$_{1-x}$Sr$_{x}$MnO$_{3}$ (LSMO) thin films by pulsed laser deposition on SrTiO$_{3}$. The high polarization (80 $\mu$C/cm$^{2})$ in PZT is found to be crucial to effectively alter the carrier density in the vicinity of the interface between two layers. Note that the polarization of our PZT films corresponds to 0.8 e/interface unit cell. As an example, we have observed a clear metal-insulator-transition by changing the direction of ferroelectric polarization, resulting in a huge resistance change (up to about three orders of magnitude). In this talk, therefore, we will present a study on how effectively the transport and magnetic properties can be modulated by ferroelectric polarization by systematically changing the hole concentration in LSMO films. Furthermore, the influence of thickness and strain on the carrier modulation will be also discussed (Research sponsored by the LDRD Program of ORNL and by NSF DMR-0602876). Monday, March 15, 2010 3:18PM - 3:30PM D33.00005: Magnetization reversal in complex oxide magnetic tunnel junctions S.G.E. te Velthuis , Y.H. Liu , M. Zhernenkov , M.R. Fitzsimmons , Z. Sefrioui , C. Visani , J. Santamaria We have investigated the magnetization reversal of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (LCMO)/PrBa$_{2}$Cu$_{3}$O$_{7}$(PBCO)/LCMO magnetic tunnels junctions with Polarized Neutron Reflectivity (PNR) in order to better understand the observed tunneling magnetoresistance (TMR) behavior. The TMR initially increases with decreasing temperature but then reaches a maximum at a relatively high temperature (60 K) and decreases when the temperature is further decreased. PNR measurements reveal differences in the reversal behavior, in the temperature dependent magnetizations, and possibly in the anisotropy axis, between the thicker bottom and thinner top LCMO layers. Additionally a zero-moment layer of about 1.3nm is found at the top surface, which could explain an exchange bias behavior observed at low temperatures. These results are discussed in relation to the TMR behavior. Monday, March 15, 2010 3:30PM - 3:42PM D33.00006: Observation of Anti-parallel Spin Orientation at the LSMO/STO(100) Interface Xiaojing Tan , Jianxing Ma , Jing Shi , Harry Tom Perovskite ferromagnet La$_{1-x}$Sr$_{x}$MnO3 (0.2 $<$ x $<$ 0.5) is considered as one of the most potential candidates for tunneling magnetic resistance (TMR) device due to the almost 100{\%} spin polarization. However, there is still a problem which has been confusing people for a long time: the TMR goes to zero at a temperature about $\sim$100K lower than the $T_{c}$ of LSMO film, which is attributed to possible severe deterioration of ferromagnetism at LSMO/STO interfaces. So it is worthy carrying out interfacial measurement at such interface. Magnetic Second-Harmonic Generation (MSHG) is a well-established all optical tool for characterizing interfaces between centrosymmetric media. By carrying out MSHG measurements as a function of temperature at at various incidence angles, and utilizing the depth dependence of the MSHG susceptibility tensor, we found the interface is ferrimagnetic with the spins in most top layer oriented anti-parallel to that in underlying layers. And we also found the T$_{c}$ of the ferrimagnetic interface is 25K lower than that of the LSMO film. Monday, March 15, 2010 3:42PM - 3:54PM D33.00007: Oxygen Doping Study of Cuprate/Manganite Thin-Film Heterostructures Hao Zhang , Yi-Tang Yen , John Y.T. Wei Recent studies of thin-film heterostructures comprising superconducting cuprates and ferromagnetic manganites have revealed a range of novel physical phenomena [1].~ These phenomena are believed to involve complex interfacial interactions between competing order parameters [2], and appear to be highly sensitive to the doping of carriers [3]. To further examine these phenomena, we carry out a systematic oxygen-doping study of YBa$_{2}$Cu$_{3}$O$_{6+x}$/La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ multilayers, grown epitaxially by pulsed laser-ablated deposition. Our samples are characterized by electrical transport and magnetization measurements, as well as x-ray diffraction and various scanning microscopy probes.~ To assess the role of interfacial magnetism on the cuprate layer, YBa$_{2}$Cu$_{3}$O$_{6+x}$ /LaNiO$_{3}$ samples are also made and measured as a comparison.~ We also examine the effects of cation substitution in the YBa$_{2}$Cu$_{3}$O$_{6+x}$ layer, in order to determine the extent of carrier doping across the~interface.~ [1] for example, see Z. Sefrioui et al., Phys. Rev. B 67, 214511 [2] J. Hoppler et al., Nature Materials 8, 315 [3] V. Pe\~{n}a et al., Phys. Rev. Lett. 97, 177005 Monday, March 15, 2010 3:54PM - 4:06PM D33.00008: Atomic-resolution imaging of oxygen vacancy induced spin state superlattice in La$_{0.5}$Sr$_{0.5}$CoO$_{3-\delta }$ J. Gazquez , M. Varela , S. J. Pennycook , W. Luo , M.P. Oxley , M. Prange , M. Pantelides , M.A. Torija , M. Sharma , C. Leighton Certain complex cobalt oxides with perovskite structure are known to exhibit ordered Co spin states. The O K edge in electron energy loss spectroscopy (EELS) is sensitive to the spin state of Co atoms, and is used here to image such spin state superlattice in La$_{0.5}$Sr$_{0.5}$CoO$_{3-\delta }$ (LSCO) thin films in real space with atomic resolution. First principles calculations predict that a spin state ordering made of alternating planes with Co in high and low spin states can be stabilized in strained LSCO films through the ordering of oxygen vacancies. EELS images confirm that Co is significantly reduced and those Co atoms in the oxygen deficient CoO plane are in the high spin state, while the Co atoms in the fully oxygenated plane are in the low spin state. Research at ORNL sponsored by the Division of Materials Sciences and Engineering of the US DoE. Work at UMN supported by NSF DMR and DoE BES. Work at Vanderbilt supported by DoE BES grant DE-FG02-09R46554. Monday, March 15, 2010 4:06PM - 4:18PM D33.00009: Atomic and electronic structure of Fe$_2$O$_3$(0001) films on polar and non-polar substrates K. Pande , M. Gajdardziska-Josifovska , M. Weinert To address the role of polarity on the growth and stability of heterointerfaces, we present a first-principles study of the structural, electronic, and magnetic properties of the layer-by-layer growth of Fe$_2$O$_3$(0001) thin films on polar MgO(111) and metallic Ti(0001) substrates. The films show thickness-dependent properties, including metallic interface states even at the polar oxide-like'' interface between Fe$_2$O$_3$ and MgO. Drastic structural relaxations are, especially at thicknesses of three Fe-bilayers on both the polar and metallic substrates. The stability of the Fe and O surface terminations as a function of the chemical potential will also be discussed. Monday, March 15, 2010 4:18PM - 4:30PM D33.00010: Effect of the deposition field on the magnetic properties of epitaxial Fe$_{3}$O$_{4}$ thin films Jian Dou , Michael Pechan , Priyanga Jayathilaka , Daryl Williams , Chris Bauer , Casey Miller 50nm epitaxial Fe$_{3}$O$_{4}$ thin films grown on MgO (100) substrates have been investigated by ferromagnetic resonance. The in-plane magnetic properties are greatly influenced by the magnetic field applied during film deposition, which (1) decreases the effective magnetization, (2) introduces the uniaxial anisotropy along the field direction, (3) rotates the in-plane easy axis from [001] to [011], (4) increases the homogeneity dramatically, (5) decreases the magnetic saturation field (lower than 2KOe). These behaviors associated with crystalline structure and strain, as well as variation with temperature, will be described in detail. This work is supported by US Dept. of Energy at MU and NSF at USF. Monday, March 15, 2010 4:30PM - 4:42PM D33.00011: Electronic and Magnetic Properties of Co$_{1-x}$Fe$_{2+x}$O$_{4}$ Jarrett A. Moyer , Dario A. Arena , Victor E. Henrich New materials having both resistivities similar to those of semiconductors and high room temperature spin polarizations would greatly benefit spintronic devices. Cobalt ferrite (CoFe$_{2}$O$_{4})$ is predicted to have a high spin polarization, but is an insulator; however, it begins to conduct when doped with iron. In this work, epitaxial Co$_{1-x}$Fe$_{2+x}$O$_{4}$ thin films (0 $\le x\le$ 0.5) are grown by MBE on MgO (001). XPS and XMCD are used to determine the stoichiometry and cation valence states, and UPS measures the density-of-states near the Fermi energy. Transport measurements demonstrate the ability to tailor the conductivity by varying the value of $x$. Bulk magnetic moments are obtained with MOKE and SQUID magnetometry, while site specific magnetic moments are obtained with XMCD. These measurements enable us to determine which stoichiometry gives a material that is best suited for use as a spin-polarized source or detector in spintronic devices. Monday, March 15, 2010 4:42PM - 4:54PM D33.00012: Surface growth dependence in growth of half-metallic Sr2FeMoO6 epitaxial films fabricated by ultra-high vacuum sputtering A.J. Hauser , R.A. Ricciardo , P.M. Woodward , A. Genc , R.E. Williams , H.L. Fraser , F.Y. Yang Sr2FeMoO6, a double-perovskite half-metallic ferromagnet, has attracted much attention because of its high Tc of 420 K. However, simultaneously balancing the stoichiometry and ordering of a quaternary oxide is no trivial matter. Despite nearly a decade of research, the growth of a half-metallic thin film remains an open question. We have deposited pure-phase Sr2FeMoO6 epitaxial films on SrTiO3 substrates by ultrahigh vacuum off-axis magnetron sputtering, and found double perovskite ordering in excess of 90{\%}. We will discuss the effect of stoichiometry, oxygen content, growth pressure, and ordering in the initial stages of growth on the properties of the sample, by performing a variety of characterizations (HAADF TEM, XRD, XPS, RBS) on both 2-5 unit cell and 100 nm thick films. Monday, March 15, 2010 4:54PM - 5:06PM D33.00013: Fabrication of Phase-Pure Sr2CrOsO6 Epitaxial Films J.M. Lucy , A.J. Hauser , H.A. Seibel , P.M. Woodward , F.Y. Yang The newly discovered magnetic double perovskite Sr2CrOsO6 exhibits a combination of unique properties, including high Tc of 725 K, semiconducting band structure and nearly 100{\%} spin-polarized valence and conduction bands indicated by a number of theoretical calculations. We have grown epitaxial Sr2CrOsO6 thin films by off-axis ultrahigh vacuum sputtering using a stoichiometric Sr2CrOsO6 target made by multi-step solid state synthesis. Rietveld refinements of the x-ray diffraction (XRD) scans of the Sr2CrOsO6 target show mostly double perovskite Sr2CrOsO6 phase (a = 3.904 {\AA}) with less than 1{\%} secondary phases. Due to the perfect lattice match between Sr2CrOsO6 and SrTiO3, we chose LSAT substrates (a = 3.868 {\AA}) to grow Sr2CrOsO6 films. XRD results demonstrated phase-pure, fully epitaxial Sr2CrOsO6 (100) films on LSAT with a rocking curve FWHM of 0.32\r{ }. Magnetic measurements for bulk Sr2CrOsO6 show an extremely large magnetic anisotropy with a coercivity of 2T at 385 K. Monday, March 15, 2010 5:06PM - 5:18PM D33.00014: Ferromagnetism in ZnO-CoO multilayers Chandran Sudakar , Ambesh Dixit , Gavin Lawes , Ratna Naik , Brian Kirby , Sanjiv Kumar , Vaman Naik The magnetic properties of CoO-ZnO heterostructures are examined to elucidate the origin of the ferromagnetic signature in Co doped ZnO. We used RF magnetron sputter deposition to prepare superlattice films with alternating layers of CoO and ZnO on sapphire substrates. The CoO and ZnO layer thickness were varied from 20 nm to 100 nm and from 75 to 225 nm, respectively. Bulk magnetization measurements show that the multilayers exhibit a ferromagnetic moment at 300 K. Saturation magnetization decreases by two orders from 5 x 10$^{-4}$ emu/cm$^{2}$ for CoO with no interface layer to 1.6 x 10$^{-5 }$emu/cm$^{2}$ with ten layers of CoO/ZnO interfacial area. To study the chemical and magnetic interaction between ZnO and CoO, the CoO/ZnO interface has been probed with Rutherford backscattering and polarized neutron reflectometery. We will present thickness measure of any magnetized interface region, and the Co magnetic moment within those regions, which allow us to test specific models for the origin of ferromagnetism in this system.