Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J9: Focus Session: Spin Transport/Magnetism Theory |
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Sponsoring Units: DCOMP DMP GMAG Chair: Oleg Mryasov, Seagate Research Center Room: LACC 153A |
Tuesday, March 22, 2005 11:15AM - 11:51AM |
J9.00001: A Full-Potential Linearized Augmented Plane-Wave Method for Calculating Transport Properties: Application to Fe/MgO/Fe Tunnel Junctions Invited Speaker: In order to calculate on the basis of the single particle picture as provided by the density-functional theory (DFT), the spin-dependent tunneling through barriers and interfaces of materials with increasing chemical and structural complexity, an extention of the full-potential linearized augmented plane- wave method (FLAPW) as realized in the {\tt FLEUR} code is introduced. The volume in which the electrons scatter is sandwiched between two semi-infinite leads. The leads and the scattering volume are described by an embedding Green function formalism. Different scenarios of electron transport such as sequential and coherent tunneling is formulated and will be compared. Several applications will be presented. The method is used to understand the spin-polarized scanning tunneling microscope. For a three- layer heterosystem SrRuO$_3$/SrTiO$_3$/SrRuO$_3$, the effect of different orbital characters of the states at the Fermi level on the tunneling conductance was investigated. The main focus is on the Fe/MgO/Fe system for which we show that very small changes at the interface can have drastic effects on the conductance. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J9.00002: \emph{Ab-initio} scattering-state based method for calculating transport in nanostructures Ivan P. Daykov, Tom\'as A. Arias We will present an \emph{ab-initio} computational method for calculating the transport properties of nanostructures. In contrast to the commonly employed scattering-state approaches it directly utilizes supercells with periodic boundary conditions, which makes it ideal for use with planewave density functional codes. [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J9.00003: Spin Torque in Magnetic Tunnel Junctions Alan Kalitsov, Ioannis Theodonis, Nicholas Kioussis, William Butler The effect of current-induced switching in the orientation of magnetic moments has attracted much attention both experimentally[1] and theoretically in the past several years due to its potential application to spin electronics. The origin of the current-induced switching is the spin torque due to the local exchange interactions between the conduction electrons and the magnetic moments. Using a simple tight-binding Hamiltonian[2] for the magnetic metal-insulator-magnetic metal tunneling junction and the Keldysh formalism for the non-equilibrium Green functions, we have calculated the spin torque and the tunneling current in non-collinear magnetic junctions. We have studied the effect of the bias voltage, the thickness of the barrier and the angle between the magnetizations of the ferromagnetic electrodes on the spin torque and the tunneling current. \begin{enumerate} \item E. Myers, D. Ralph, J. Katine, R. Louie, R. Buhrman, Science \textbf{285,} 867 (1999). \item C. Caroli, R. Combescot, P. Nozieres, D. Saint-James, J. Phys. C: Solid St. Phys., \textbf{4}, 916 (1971). \end{enumerate} *Supported through the NSF grant No. DMR-0097187 [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J9.00004: Electronic Structure and Schottky Barrier Formation in Fe/GaAs Magnetic Junctions Denis Demchenko, Amy Liu In the most successful experiments, spin injection efficiencies of 2\% at room temperature and 30\% at low temperature were achieved [1,2]. The role of intrinsic Schottky barriers in controlling spin-dependent tunneling through the interface is crucial since the barriers significantly reduce the conductivity mismatch, which would otherwise eliminate the possibility of spin injection almost entirely. In this work electronic and magnetic properties of Fe/GaAs magnetic junctions are investigated using the first-principles plane wave based pseudopotential method. It has been shown that the properties of such junctions can differ significantly depending on the interface structure [3]. Therefore, here we calculate wide range of properties of Fe/GaAs junctions, including Schottky barrier heights, magnetization profiles, charge distributions, potential profiles, and equilibrium structures of such junctions, and show how these properties depend on the interface structure. [1] H.J.Zhu et al., {Phys. Rev. Lett.} {\bf 87}, 016601 (2001). [2] A.T.Hanbicki et al.,{Appl. Phys. Lett.} {\bf 80}, 1240 (2002). [3] S.C.Erwin et al., {Phys. Rev. B} {\bf 65}, 205422 (2002). [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J9.00005: {\it Ab inito} study of molecular spin valves Alexandre Reily Rocha, Stefano Sanvito The combination of spin and molecular electronics poses exciting perspectives both for basic science and technological applications. By manipulating the spin degree of freedom at the atomic level we enter a new and exciting era where entire spin devices can be substituted by single a molecule performing analogous tasks. In this talk, we will present a through theoretical study of electronic transport through molecular spin valves \footnote{Towards Molecular Spintronics, A.~Reily Rocha, V.~M.~Garcia~Su\'arez, S.~W.~Bailey, C.~J.~Lambert, J.~Ferrer and S.~Sanvito, submitted to Nature Materials.} obtained by sandwiching a molecule between two Ni electrodes. The calculations are performed with our novel code Smeagol (www.smeagol.tcd.ie), which combines density functional theory with non-equilibrium Green function transport method. We will show results for two types of molecules with distinct transport mechanisms, namely: tunneling and metallic conductance. In both cases we analyze the effects of the contacts on the molecule and the particular states contributing to the transport. We will demonstrate that it is not only possible to obtain large magnetoresistance effects in both types of molecules, but also to engineer the signal by an appropriate choice of end-groups. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J9.00006: Coulomb Correlation Effects in Variable-Range Hopping of Magnetic Polarons Michael Foygel, Andre Petukhov We study electrical conductivity due to spin polaron hopping in disordered solids with wide distributions of the localized electron energies and polaron shifts taking into account the Coulomb correlation effects. By means of the percolation theory we demonstrate that in such materials a hard polaron gap does not manifest itself while the soft Coulomb gap persists at low temperatures. As a result, the variable-range \textit{polaron }hopping conductivity, $\sigma $, as a function of temperature, $T$, obeys the stretched-exponent law: $\ln \left( \sigma /\sigma _{0}\right) =-\left( \widetilde{T}/T\right) ^{p}$ , where $p=$ 4/7 (3/5) for $3D\;(2D)$ case. It differs from the standard Shklovskii-Efros law for which $p=1/2$. In addition, parameter $\widetilde{T}$ is shown to depend on the dispersion of the polaron shift distribution. Therefore, it decreases with the application of an external magnetic field thus leading to giant negative magnetoresistance in the variable-range hopping regime where for paramagnetic materials $p=$ 5/7 (4/5). [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J9.00007: Tunable spin filter and molecular hybridization in a quantum dot molecule F. Mireles, E. Cota, F. Rojas, S.E. Ulloa Spin filtering using few electron semiconductor quantum dots formed in two-dimensional electron gas systems has attracted much recent attention in spintronics. Spin filtering has been achieved in a quantum dot via universal conductance fluctuations and electron magnetic focusing [1]. A bipolar spin filter (\textbf{SF}) has been realized recently using a semiconductor quantum dot which can operate practically as a perfect SF, provided there is a large enough Zeeman splitting [2]. In this work we present calculations showing that the tunable (molecular) hybridization between two quantum dots with few electrons and connected ``in parallel,'' produces a singlet-triplet transition in the ground state which can be used as a robust bipolar SF in both the linear and non-linear regimes of transport. The bipolar SF is found to be \textit{fully tunable by only electrical gating} at low temperatures. We show that a singlet-triplet transition in the energy spectrum gives rise to the natural spin selectivity in the odd-to-even electron number transition in Coulomb blockade experiments. The competition between the Zeeman, Coulomb, and tunneling energies is studied in detail to determine the optimal conditions to achieve the singlet-triplet transition, so that it becomes broadly useful as a bipolar SF. [1] J. A. Folk et al., Science \textbf{299}, 679 (2003). [2] R. Hanson et al., cond-mat/0311414 (2003). *Supported by DGAPA-UNAM project 1N114403, CONACYT, projects J40521F and 143673F, and NSF-IMC. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J9.00008: Ferromagnetic Resonance in FeZrN Animesh Chakraborty, Prasanta Dutta, Mohinder Seehra, Keith Mountfield We report here the in-plane ferromagnetic resonance in FeZrN . The parameters measured were intensity I$_{0 }$, linewidth $\Delta $H, and resonance field H$_{r }$.The FMR studies were performed using a conventional X-band ($\mu $=9.1 GHz) ESR spectrometer equipped with a TE$_{011 }$cavity. The samples about 3 m.m. by 3 m.m were placed onto a quartz sample holder, put into the cavity and held in either parallel or perpendicular orientation to the sample plane of the applied magnetic field. The samples were prepared by R.F. sputtering with Zr chips bonded to the target. The films were approximately 0.4 micron thick on glass substrates. [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J9.00009: Entanglement and Quantum Phase Transition in Low Dimensional Spin Systems Yan Chen, Paolo Zanardi, Zidan Wang, Fuchun Zhang Entanglement of the ground states in $XXZ$ and dimerized Heisenberg spin chains and in two-leg spin ladder is analyzed by using spin-spin concurrence and the entanglement entropy between a selected block of spins and the rest of the system. Quantum critical points as well as phase boundaries can be in some cases identified straightforwardly by analyzing the local extreme of the entanglement. We show that various subsystem partitions may provide complementary description of a quantum phase diagram. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J9.00010: An LAPW Study of d - Plutonium and the (001) Surface A. K Ray, X. Wu The electronic structure properties of bulk fcc $\delta $-Plutonium and the quantum size effects in the surface energies and the work functions of the (001) ultra thin films (UTF) up to 7 layers have been investigated with periodic density functional theory calculations within the full-potential linearized augmented-plane wave approach.$^{1}$ Several levels of theory, namely NSP-NSO, NSP-SO, SP-NSO, and SP-SO, have been examined and our calculated equilibrium atomic volume of 178.3 a.u.$^{3}$ and bulk modulus of 24.9 GPa at the fully relativistic level of theory are in good agreement with experimental results. The energy difference brought by spin-orbit coupling, about 7-8 eV, is dominant, but the energy difference brought by spin-polarization, from a few tenths to 2 eV, has a stronger dependence on the atomic volume. Density of states show that 5f electrons are more itinerant when the volume of $\delta $-plutonium is compressed and they are more localized when the volume is expanded. The surface energy converges rapidly and the semi-infinite surface energy is predicted to be 0.692eV. Quantum size effects for the work function is not pronounced for (001) surface. $^{\ast }$Work supported by the Department of Energy (Grant No. DE-FG02-03ER15409) and the Welch Foundation (Grant No. Y-1525). $^{1 }$P. Blaha, K. Schwarz, G. K. H. Madsen, D. Kvasnicka and J. Luitz, \textit{WIEN2k} (Technische Universitat Wien, Austria, 2001) [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J9.00011: A First Principles Electronic Structure Study of Quantum Size Effects in (111) Films of FCC Plutonium A. K. Ray, J. C. Boettger First principles linear combinations of Gaussian type orbitals -- fitting function (LCGTO-FF) electronic structure calculations are used to study thickness dependencies in the surface energies and work functions of ultra-thin (111) films of fcc Pu, up to five layers thick. The calculations are carried out at both the scalar- and fully-relativistic (with and without spin-orbit coupling) levels of approximation. The surface energy is shown to be rapidly convergent, while the work function exhibits a strong quantum size effect for all thicknesses considered. The surface energy and work function of the semi-infinite solid are predicted to be 1.12 J/m$^{2}$ and 2.85 $\pm $ 0.20 eV, respectively, for the fully-relativistic case. These results are in substantial disagreement with results from previous electronic structure calculations. The present predictions are in fair agreement with the most recent experimental data for polycrystalline fcc Pu, namely 0.91 J/m$^{2 }$and 3.1-3.3 eV, for the surface energy and work function, respectively. \textbf{*}The work of AKR is supported by DOE (Grant No. DE-FG02-03ER15409) and the Welch Foundation (Grant No. Y-1525). The work of JCB is supported by DOE under contract W-7405-ENG-36 and the LDRD program at LANL. [Preview Abstract] |
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J9.00012: An Ab Initio Study of Molecular Oxygen Adsorption on Pu (111) Surface M. N. Huda, A. K. Ray We will present a study of oxygen molecule adsorption on a Pu (111) surface using the generalized gradient approximation to density functional theory using the DMol3 suite of programs [1]. Horizontal approaches on center site, with and without spin polarization, were found to be the highest chemisorbed sites among all the cases studied here with chemisorption energies of 8.365eV and 7.897eV, respectively. The second highest chemisorption energy occurs at the vertical approach on bridge site with chemisorption energy of 8.294eV (non-spin-polarized) and 7.859eV (spin-polarized). In general, with spin polarization, dissociative adsorption with a layer by layer alternate spin arrangement of the plutonium layer is found to be energetically more favorable compared to molecular adsorption. Non-spin-polarized chemisorption energies are usually higher than the spin-polarized energies. We also find that 5f electrons are more localized in spin polarized case, than the non-spin polarized counterparts. The ionic part of O-Pu bonding plays a significant role along with the Pu 5f-O 2p hybridization. [1] B. Delley, J. Chem. Phys. \textbf{92}, 508 (1990); J. Chem. Phys. \textbf{113}, 7756 (2000). [Preview Abstract] |
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J9.00013: First-principles study of alloy formation at Fe/GaAs interfaces Igor Zutic, Steven Erwin The combination of the high Curie temperature of Fe, high-quality epitaxial growth of Fe on GaAs, and demonstrated high-efficiency spin injection in GaAs, together make Fe/GaAs heterojunctions very attractive candidates for room-temperature spintronic applications. However, little is known about the structure of Fe/GaAs interfaces, and there is a range of conflicting experimental results describing them---including findings of magnetically dead layers [1], an intermediate FeGaAs phase [2], and bulk-like magnetic moments at the interface [3]. Motivated by these findings, we use density-functional theory to study the structural and magnetic properties of interfaces involving possible alloyed phases occurring between Fe and GaAs. From the calculation of interface formation energies we report results on the stability of various structural and magnetic configurations, and provide microscopic parameters which may be used in studies of spin transport to assess the device potential of these heterostructures. [1] J. J. Krebs, B. T. Jonker, and G. A. Prinz, J. Appl. Phys. 61, 2596 (1987). [2] J. Deputier, R. Guerin, B. Lepine, A. Guivarc'h, and G. Jezequel, J. Alloys Comp. 262, 416 (1997). [3] J. S. Claydon, Y. B. Hu, M. Tselepi, J. A. C. Bland, and G. van der Laan, Phys. Rev. Lett. 93, 037206 (2004). [Preview Abstract] |
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