Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session A14: Focus Session: Perovskite Cobaltite & Titanate Heterostructures |
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Sponsoring Units: DMP GMAG Chair: Chris Leighton, University of Minnesota Room: 316 |
Monday, March 18, 2013 8:00AM - 8:12AM |
A14.00001: Orientation and Strain Dependence of the Magnetic Phase Separation at Perovskite Cobaltite Interfaces S. Bose, M. Sharma, M.A. Torija, J. Gazquez, M. Varela, H. Ambaye, R. Goyette, V. Lauter, M.R. Fitzsimmons, J. Schmitt, C. Leighton We recently showed that the degraded magnetic and electronic properties in very thin STO(001)/La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$CoO$_{3}$ films is due to a form of magnetic phase separation. This is primarily due to the strain driven accumulation of O vacancies near the interface. In this work we demonstrate how this understanding allows us to engineer these interfacial properties via crystallographic orientation and strain control. Using PNR, magnetometry and transport, we show how this degradation can be significantly mitigated by using LAO(001) and STO(110) substrates cf. STO(001). PNR on 400{\AA} x$=$0.28 films reveals an interfacial layer with suppressed magnetism on all three substrates. However, while this layer is 150{\AA} on STO(001), it extends at most to 30{\AA} on LAO(001) and STO(110). Transport measurements on x$=$0.5 films show that at a thickness of $\sim$ 55{\AA}, films on STO(110) and LAO(001) exhibit AMR whereas films on STO(001) are dominated by inter-cluster GMR. Finally, thickness dependent magnetometry shows that the magnetic order deteriorates more quickly on STO(001) than on LAO(001) and STO(110). Our work thus opens up a possible new route to tailor interfacial magneto-electronic properties in oxide heterostructures. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A14.00002: Synthesis of Epitaxially Strained Brownmillerite Strontium Cobaltate (SrCoO$_{2.5})$ Eric Jin, Matthew Marshall, Joseph Ngai, Charles Ahn, Fred Walker Strontium cobaltate (SrCoO$_{3})$ is a perovskite oxide predicted to have metal-insulator transitions and magnetic phases induced by epitaxial strain. The related brownmillerite phase SrCoO$_{2.5}$ has a similar structure, but contains alternating planes of oxygen vacancies in the octahedral oxygen cages of the perovskite structure. We demonstrate epitaxial growth of SrCoO$_{2.5}$ on both SrTiO$_{3}$ and LaAlO$_{3}$ substrates by molecular beam epitaxy using RF oxygen plasma. X-ray diffraction measurements show finite thickness oscillations that are characteristic of smooth films, and half-order diffraction peaks that are representative of the brownmillerite phase. We observe a single tetragonal domain when the film is deposited on SrTiO$_{3}$ with the planes of oxygen vacancies parallel to the interface. When grown on LaAlO$_{3}$, the film contains multiple orthorhombic domains. We conclude that the observed domain structures for SrCoO$_{3}$ grown on SrTiO$_{3}$ and LaAlO$_{3}$ are due to ordering of the oxygen vacancies to reduce strain. We will also present strategies to increase the oxygen content to that of SrCoO$_{3}$. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A14.00003: Magnetic Structure and Phase Separation in Epitaxial SrCoO$_{\mathrm{x}}$ Thin Films F.J. Rueckert, C. Abughayada, S.A. Sabok, F. He, H. Mohottala, J.I. Budnick, W.A. Hines, B. Dabrowski, B.O. Wells Bulk SrCoO$_{\mathrm{x}}$ separates into three distinct ferromagnetic phases as the oxygen content is increased from x $=$ 2.75 to 3.0, corresponding to T$_{\mathrm{C}} =$ 165 K (SrCoO$_{2.75})$, T$_{\mathrm{C}} =$ 220 K (SrCoO$_{2.88})$, and T$_{\mathrm{C}}$ $=$ 280 K (SrCoO$_{3.0})$. Over this composition, the lattice evolves smoothly and remains a single crystallographic phase. Using pulsed laser deposition and electrochemical oxidation, we have prepared epitaxial films of SrCoO$_{\mathrm{x}}$ of varying thickness and orientation on SiTiO$_{3}$ substrates. While in polycrystalline samples intermediate oxygen concentrations show a two-phase magnetic behavior, 100nm thick (0 0 1) films remain single phase but still favor the same ferromagnetic transitions. Thicker, 150 nm (1 1 1) films also order at comparable T$_{\mathrm{C}}$'s, but again show two-phase behavior during deoxidation. Resonant x-ray diffraction on these samples reveals both commensurate and incommensurate ordering dependent on the oxidation state. This implies a charge or orbital ordering which may be influenced by finite size effects. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A14.00004: Oxygen vacancy ordering in transition-metal-oxide LaCoO$_{3}$ films Neven Biskup, Juan Salafranca, Virat Mehta, Yuri Suzuki, Stephen Pennycook, Sokrates Pantelides, Maria Varela Oxygen vacancies in complex oxides affect the structure and the electronic and magnetic properties. Here we use atomically-resolved Z-contrast imaging, electron-energy-loss spectroscopy and densityfunctional calculations to demonstrate that ordered oxygen vacancies may act as the controlling degree of freedom for the structural, electronic, and magnetic properties of LaCoO$_{3}$ thin films. We find that epitaxial strain is released through the formation of O vacancy superlattices. The O vacancies donate excess electrons to the Co $d$-states, resulting in ferromagnetic ordering. The appearance of Peierls-like minigaps followed by strain relaxation triggers a nonlinear rupture of the energy bands, which explains the observed insulating behavior. We conclude that oxygen vacancy ordering constitutes a degree of freedom that can be used to engineer novel behavior in complex-oxide films. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A14.00005: Local atomic and electronic structure of LaCoO3/SrTiO3 thin films by HAADF STEM and EELS Albina Borisevich, Jae Hyuck Jang, Young-Min Kim, Liang Qiao, Michael Biegalski For perovskite films with several competing functionalities, magnetic and electronic properties can be affected both by structural order parameters and chemical factors. For example, in LaCoO3 (LCO) thin films, magnetic and transport properties are strongly dependent on strain state and oxygen content. For this study, LCO thin films were deposited by pulsed laser deposition method with different thicknesses (2, 5, 15 unit cell and 20 nm thickness) on SrTiO3 substrate. X-ray photoelectron spectroscopy studies of the grown films have demonstrated that Co 3p edges shift up to 2 eV for 15 u.c. and 20 nm films, indicating possible presence of 2D electron gas. The structure of the 5 u.c and 15 u.c LCO films was examined. Atomic position mapping from STEM HAADF and BF images can reveal lattice parameter and octahedral tilt behavior with atomic resolution. BF STEM imaging showed that octahedral tilts were active in the 15 u.c. film but not in the 5 u.c. film. A complex pattern of O K fine structure evolution at the interface was observed; results of the deconvolution of different contributions to this behavior using advanced simulations, as well as data on oxygen vacancy mapping, will be presented. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A14.00006: Dimensional control of cobalt spin state in oxide superlattices Da Woon Jeong, W.S. Choi, S. Okamoto, C.H. Sohn, H.J. Park, J.-Y. Kim, H.N. Lee, K.W. Kim, S.J. Moon, T.W. Noh Perovskite cobalt oxide is a very intriguing system with various spin states owing to the delicate balance between crystal field splitting and Hund exchange energy. In this talk, we show that its spin state can be altered through dimensional control, enabled by digital synthesis of perovskite cobalt oxide superlattices. We employed a few unit cells of LaCoO$_{\mathrm{3}}$ as an active magnetic layer, separated by LaAlO$_{\mathrm{3}}$ spacer layer. High quality [(LaCoO$_{\mathrm{3}})n$(LaAlO$_{\mathrm{3}})n$]8 ($n \quad =$ 2, 6, and 10) superlattices were fabricated using pulsed laser epitaxy. Spectroscopic tools including x-ray absorption spectroscopy and optical spectroscopy revealed clear evolution of the electronic structure and resultant spin state by changing dimensionality. Specifically, the spin state changed from a high to a low spin state with a larger optical band gap, as the dimension reduced from 3D to 2D. Dynamic mean field calculation supported the critical role of dimensionality on the spin state and electronic structure of LaCoO$_{\mathrm{3}}$. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A14.00007: Emergent phenomena and magnetism in high-density electron gases in SrTiO$_3$ Invited Speaker: Susanne Stemmer GdTiO$_3$/SrTiO$_3$ interfaces grown by molecular beam epitaxy exhibit mobile carrier densities that are remarkably well predicted by the electrostatic requirements of the compensation of the polar discontinuity at the interface. Carrier densities are $\sim$3x10$^{14}$~cm$^{-2}$, or $\sim$0.5 electron per surface unit cell. By sandwiching few-unit-cell-thick SrTiO$_3$ layers between GdTiO$_3$, carrier concentrations in the SrTiO$_3$ approach densities under which on-site Coulomb interactions may appear. By changing the width of the quantum well, the 3D electron density can be varied, which allows for a systematic study of interaction effects. In this presentation, we discuss evidence for short-range Coulomb interactions, and associated phenomena, in ultrathin, confined the SrTiO$_3$ quantum wells containing extreme charge densities. We show that narrow SrTiO$_3$ quantum wells exhibit ferromagnetism at low temperatures, as evidenced by a hysteresis in the magnetoresistance. The Curie temperature scales with the thickness of the SrTiO$_3$ quantum well. We discuss evidence for on-site Mott-Hubbard-type correlation physics in the temperature-dependent transport in metallic quantum wells. With increasing 3D carrier densities we observe a correlation-induced mass enhancement, followed by a transition to a correlated insulator at the highest 3D densities. We also discuss the role of disorder in the insulating state. This work was done in collaboration with Pouya Moetakef, Clayton A. Jackson, Leon Balents, Jim Allen, Jimmy Williams and David Goldhaber-Gordon. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A14.00008: Insulating room temperature ferromagnetic SrTiO$_3$ Agham Posadas, Chandrima Mitra, Chungwei Lin, Ajit Dhamdere, David Smith, Maxim Tsoi, Alex Demkov We report the epitaxial growth of ferromagnetic insulating material based on SrTiO$_{3}$ using molecular beam epitaxy (MBE). SrTi$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$O$_{3-\delta}$ films (x $=$ 0.1 to 0.5) were grown on Si(100) substrates via a buffer layer of four unit cells of undoped SrTiO$_{3}$. The crystalline structure was characterized by reflection high energy electron diffraction, x-ray diffraction, and cross-section transmission electron microscopy. Robust room-temperature ferromagnetism is confirmed in samples with composition 30-40{\%} Co. We also performed \textit{in situ }x-ray photoelectron spectroscopy of the Sr, Co, Ti, and O core levels to determine stoichiometry and cobalt oxidation state. In all single phase samples, an oxygen vacancy concentration of approximately equal to the amount of Co substitution was measured (compensated doping). In order to elucidate the origin of ferromagnetism, we also performed first-principles calculations of SrTiO$_{3}$ simultaneously doped with Co and an oxygen vacancy. We find that such a configuration at concentrations of $\sim$ 25{\%} can result in a ferromagnetic insulating state with high spin Co$^{2+}$. The ability to integrate an insulating ferromagnet on silicon in epitaxial form may potentially be useful for spin filtering and spin wave applications in the field of spintronics. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A14.00009: Linear magnetoresistance in non-degenerately doped SrTiO$_{3}$ Anand Bhattacharya I will present transport measurements on non-degenerately doped $n-$SrTiO$_{3}$ single crystals. The samples were doped by annealing at high temperatures in vacuum. The resistance decreases monotonically down to the lowest temperatures, for carrier densities as low as 3.85 x 10$^{15}$/cm$^{3}$. The magnetoresistance (MR) is found to be positive and linear at high fields, with R(9 T)/R(0 T) \textgreater\ 28 at 2 K for the lowest doping levels measured. The magnitude of the MR decreases with increasing temperature, and with increased doping. I will discuss the data in light of various mechanisms for linear magnetoresistance in the context of $n-$SrTiO$_{3}$. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A14.00010: The role of interface magnetic centers on the spin lifetime measured in doped SrTiO3 using Hanle technique Wei Han, Xin Jiang, Adam Kajdos, See-Hun Yang, Susanne Stemmer, Stuart Parkin Recently, the two dimensional electron gas that is formed at the surface of strontium titanate, SrTiO3 (STO), has attracted considerable attention, both concerning its origin as well as the many phenomena that it apparently displays: these include, gate tunable metallicity and superconductivity, and magnetic effects including Kondo scattering. Here, we report electrical injection and detection of spin currents in Nb doped STO substrates and La doped STO thin films using the Hanle technique and CoFe / MgO tunnel spin injectors. The spin lifetimes measured are on the order of 100 ps and vary little with temperature for temperatures varying from 10 K to 300 K, whereas the mobility of the STO has very strong temperature dependence. This suggests that the spin lifetime is limited by spin-dependent scattering at the MgO/STO interface, perhaps related to the formation of Ti3$+$ or other magnetic centers. Of considerable interest is that the spin lifetime decreases systematically with increasing dopant concentration, indicating that the number of magnetic centers at the interface increases with increasing dopant concentration. These results reveal a severe limitation of the Hanle technique for measuring spin lifetimes within the interior of the subject material. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A14.00011: Magnetocapacitance in surface-reconstructed LaMnO$_{3}$/SrTiO$_{3}$ multilayers Rainer Schmidt, Javier Garcia-Barriocanal, Norbert Nemes, Mar Garcia-Hernandez, Carlos Leon, Jacobo Santamaria We report on large magnetocapacitance (MC) effects in LaMnO$_{3}$/SrTiO$_{3}$ multilayer structures. Frequency, temperature and magnetic field dependent dielectric spectroscopy was employed using in-plane and out-of-plane measurement set-ups to investigate multilayers of LaMnO$_{3}$ (15 u.c.) and SrTiO$_{3}$ (2 u.c.) with a repetition rate of 8: (LMO 15/STO 2)$_{8}$. Such multilayer structures have been identified previously to display an electron transfer across the epitaxial interface from LMO to STO, orbital reconstruction and a considerable Ti$^{3+}$ magnetic moment near the LMO/STO interface [Garcia-Barriocanal et al. Adv. Mater. 22 (2010) p.627]. We demonstrate moderate in-plane magnetocapacitance (MC) of up to -5 {\%} associated with an intrinsic magneto-electric coupling (MEC) effect originating from magnetic STO layers. Massive out-of-plane MC of up to -52 {\%} was ascribed to current path changes due to magnetically active STO pin-holes and current path meandering. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A14.00012: Enhanced Magnetism in SrRuO$_{3}$ Thin Film by SrTiO$_{3}$ Capping Monolayers Sean Thomas, Bouwe Kuiper, Jeff Botimer, Elliot Persico, Gertjan Koster, Jing Xia Substrate induced mechanical strain is known to reduce the magnetism in itinerant ferromagnetic SrRuO$_{3}$ thin films. Here we show that monolayers of SrTiO$_{3}$ epitaxial capping film can be used to enhance the magnetism in ultra-thin SrRuO$_{3}$ films. For a device with a 6 monolayer thick SrRuO$_{3}$ film, a 2 monolayer thick SrTiO$_{3}$ capping layer can boost SrRuO$_{3}$'s magnetic Curie temperature by 15 Kelvin. Unlike thick substrates, the monolayers-thick SrTiO$_{3}$ capping layer can be patterned using standard lithography methods for making complex oxide electronic devices. We demonstrate a SrRuO$_{3}$ film device with regions of different Curie temperatures by patterning the SrTiO$_{3}$ capping layer. [Preview Abstract] |
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