Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session D19: Focus Session: Semiconductor Spin Injection and Detection |
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Sponsoring Units: GMAG DMP Chair: Berry Jonker, Naval Research Laboratoy Room: Baltimore Convention Center 316 |
Monday, March 13, 2006 2:30PM - 2:42PM |
D19.00001: Onsager relations and the detection of current-induced spins by Ferromagnetic contacts Inanc Adagideli The recent prediction of intrinsic spin Hall currents raised many questions about methods of detection. We focus on the theoretical issues related to the detection of spin currents and accumulations via ferromagnetic contacts. Along the way, based on Onsager-like relations for the conductance, we show the unfeasibility of detecting spin current from a non-magnetic conductor in a two probe geometry within linear response. It is nevertheless possible to detect spin current in a four probe geometry, in which the spin current flowing into the ferromagnet leads to a potential difference between the Hall contacts. We calculate the Hall conductivity and compare this effect to the Anomalous Hall effect. Work done in collaboration with G.E.W. Bauer and B.I. Halperin [Preview Abstract] |
Monday, March 13, 2006 2:42PM - 2:54PM |
D19.00002: Low temperature annealing and spin injection in Fe/AlGaAs Spin-LEDs A.T. Hanbicki, G. Kioseoglou, R. Goswami, T.J. Zega, C.H. Li, R.M. Stroud, G. Spanos, B.T. Jonker With the Fe/AlGaAs/GaAs spin-LED system, it is now routine to obtain electron spin polarizations of up to 32{\%} in the GaAs quantum well. Because the nature of the interface between a magnetic contact and a semiconductor is expected to influence the spin-injection efficiency we have undertaken an annealing study to isolate the role of the Fe/AlGaAs interface on the overall spin injection efficiency. Optimized annealing can improve the measured spin polarization, therefore multiple pieces of a single sample can be annealed to generate a systematic dataset. Enhancement in polarization is seen with anneals as low as 175$^\circ$C, and the maximum increase in polarization occurs for anneals above 200$^\circ$C. Upon annealing, there is a net gain in measured spin polarizations of 8 to 10 percentage points independent of the starting value. Details on changes in parameters affected by annealing such as the magnetization, polarization, I-V characteristics, and interface structure will be presented. The role of the interface on changes in these parameters will be discussed. This work was supported by the DARPA SpinS program and ONR. [Preview Abstract] |
Monday, March 13, 2006 2:54PM - 3:06PM |
D19.00003: Spin Injection from Fe and FeGa into side-emitting GaAs spin LEDs G. Kioseoglou, O.M.J van 't Erve, A.T. Hanbicki, C.H. Li, B.T. Jonker Most of the spin LEDs used as spin detectors are of the surface type. In this type of LEDs, using Fe as spin polarized source, electron spin polarizations up to 40 {\%} have been reported. Since Fe has its magnetization easy axis in the substrate plane, a large magnetic field B (2.5 tesla) along the surface normal is required to saturate the magnetization. A side-emission geometry, on the other hand, enables magnetic switching fields of $\sim $ 100 Oe, a much more practical range for applications. We present a study of electrical spin injection from Fe and FeGa into side-emitting GaAs spin LEDs in which the applied magnetic field is parallel to electroluminescence emitted from the cleaved side. Several samples with different quantum well (QW) widths were investigated. The optical polarization for the 100{\AA} QW spin LED is zero, as expected due to hole spin orientation. For wider QWs, the confinement energy diminishes and the magnetic field rather than the confinement defines the quantization axis. An optical circular polarization P = 6 {\%} is measured for the 1000 {\AA} QW LED which corresponds to an electron spin polarization of 12 {\%}. The non-volatile character of the side-emitting spin LED will be also discussed using FeGa as a spin injector. This work was supported by DARPA and ONR. [Preview Abstract] |
Monday, March 13, 2006 3:06PM - 3:18PM |
D19.00004: Atomic structure of the interface between Fe and AlGaAs and its impact on spin transport Thomas Zega, Aubrey Hanbicki, Steven Erwin, Igor Zutic, George Kioseoglou, Connie Li, Berend Jonker, Rhonda Stroud We combined density-functional theory (DFT) with experimental- and simulated- transmission-electron-microscope (TEM) imaging to determine the atomic structure of the interface between Fe and AlGaAs in light-emitting diodes (LEDs) used to investigate the transport of spin-polarized electrons. We show that a 44{\%} increase in the spin-injection efficiency occurs after a low-temperature anneal, which produces an interface consisting of a single atomic plane of alternating Fe and As atoms. Our results provide the structural information required to develop accurate models of charge transport and spin injection at complex interfaces between dissimilar materials. [Preview Abstract] |
Monday, March 13, 2006 3:18PM - 3:30PM |
D19.00005: Resonant X-Ray Emission investigation of Fe/GaAs Spin Devices Jessica McChesney, C. Adelmann, Per-Anders Glans, K.E. Smith The ability to produced spin polarized electrons has important consequences to electronics, as we know it. One such spintronics system is Fe on GaAs. Fluorescence measurements, using synchrotron radiation, were used to probe the electronic structure of the Fe at the interface of a working device structure. These ferromagnetic-semiconductor heterostructures were grown via molecular beam epitaxity. Absorption spectroscopy (NEXAFS) and energy-resolved fluorescence (RIXS) both on and off resonance were done on the Fe L-edge. Subtle differences between samples with differing degrees of spin polarization were observed. These differences are discussed in terms of bonding at the interface. Supported by ONR and DOE. [Preview Abstract] |
Monday, March 13, 2006 3:30PM - 3:42PM |
D19.00006: Molecular Beam Epitaxy of Fe$_{3}$Ga/Ga$_{1-x}$Al$_{x}$As Heterostructures: Growth, Properties and Spin Transport C. J. Palmstr{\O}m, C. Adelmann, X. Lou, S. K. Srivastava, P. A. Crowell Single crystal Fe$_{3}$Ga thin films have been grown on Ga$_{1-x}$Al$_{x}$As (001) by molecular beam epitaxy. The films are found to be pseudomorphic on Ga$_{1-x}$Al$_{x}$As and grow in a tetragonally-distorted Heusler-like D0$_{3}$ crystal structure. The Fe$_{3}$Ga films are ferromagnetic above room temperature with a saturation magnetization of 1200 emu/cm$^{3}$. They exhibit a perpendicular magnetic anisotropy due to the strain-induced tetragonal distortion. Rutherford backscattering spectrometry finds no interfacial reactions of Fe$_{3}$Ga/GaAs heterostructures, suggesting that Fe$_{3}$Ga is thermally stable on GaAs. Spin injection from Fe$_{3}$Ga into GaAs has been assessed by using Fe$_{3}$Ga/Ga$_{1-x}$Al$_{x}$As spin light-emitting diode (LED) structures. A steady-state spin polarization of 20{\%} is obtained at 2K. The bias dependence of spin injection is found to be very similar to that of Fe/Ga$_{1-x}$Al$_{x}$As spin LEDs [1]. This work was supported in part by ONR and NSF-MRSEC. [1] C. Adelmann \textit{et al.}, Phys. Rev. B \textbf{71}, 121301 (2005). [Preview Abstract] |
Monday, March 13, 2006 3:42PM - 4:18PM |
D19.00007: Imaging Spin Injection and Accumulation in Lateral Ferromagnet/Semiconductor Devices Invited Speaker: We have directly imaged electrical spin injection and accumulation in the GaAs channel of a lateral spin transport device.$^\dagger$ The ferromagnetic source and drain tunnel-barrier contacts at each end of the channel are epitaxial Fe Schottky barriers separated by a distance of 300 $\mu$m, which is much longer than the spin diffusion length. Emission of majority-spin electrons from the Fe source contact is observed using scanning Kerr microscopy. A majority spin polarization in the channel is also detected near the drain electrode, and we show that this accumulation is due to a spin current flowing away from the drain, against the unpolarized incident electron current. This result implies that the transmission of electrons through the forward-biased Schottky barrier is spin-sensitive. This is confirmed by demonstrating that the electrical conductance of the device can be modulated by controlling the spin orientation of optically injected electrons flowing through the drain. These experiments are conducted in a geometry sensitive only to spin precession and allow for detailed modeling of spin transport in the channel. We show how this approach can be used to detect the spin accumulation at the drain electrode using purely electrical means. $^\dagger$S. A. Crooker {\it et al.}, Science {\bf 309}, 2191 (2005). [Preview Abstract] |
Monday, March 13, 2006 4:18PM - 4:30PM |
D19.00008: Electrical Detection of Spin Accumulation at a Ferromagnet-Semiconductor Interface X. Lou, C. Adelmann, M. Furis, S.A. Crooker, C.J. Palmstr$\O$m, P.A. Crowell We report a direct electrical transport measurement of spin accumulation in epitaxial Fe/GaAs heterostructures. This result is obtained in lateral transport devices consisting of two Fe Schottky tunnel barriers at the opposite ends of a lightly $n$- doped GaAs channel. Polarized spin accumulates near the forward- biased GaAs/Fe Schottky interface, resulting in an additional voltage that is observed between the channel and the ferromagnetic drain contact. We establish that the spin accumulation, which can also be observed with Kerr microscopy, occurs when unpolarized electrons are incident on the interface. The voltage due to the electron spin accumulation is suppressed in a small transverse magnetic field as a result of precession. The width of the voltage peak is determined by a combination of spin diffusion and relaxation and can be modeled accurately using parameters determined from transport and optical experiments. These results demonstrate that Fe/GaAs Schottky barriers can be used as electrical spin detectors as well as injectors. This work was supported by DARPA, ONR, NSF- MRSEC, and LANL. [Preview Abstract] |
Monday, March 13, 2006 4:30PM - 4:42PM |
D19.00009: Scanning Kerr Rotation Microscopy of Lateral Spin Transport Devices M. Furis, D.L. Smith, S.A. Crooker, X. Lou, C. Adelmann, C.J. Palmstr$\o$m, P.A. Crowell Scanning Kerr-rotation microscopy is employed to image electrical spin injection and accumulation in the GaAs channel of lateral spin transport devices having ferromagnetic Fe/GaAs Schottky barrier source and drain contacts [1]. Because electrically-injected spins are initially oriented in the $xy$ sample plane ($S_0 \parallel \hat{x}$), we measure the Kerr rotation ($\theta_K \propto S_z$) as a function of small in- plane magnetic field ($B_y$). We specifically investigate how these ``Hanle curves" vary across a series of devices with systematically increasing electron density $n_{e}$ in the n-type GaAs channel ($2\times10^{16}$ cm$^{-3} < n_{e} < 3\times10^{17} $ cm$^{-3}$). The width of the Hanle curves near the source contact increases with $n_{e}$, reflecting the decreasing electron spin lifetime. The amplitude of the Hanle curves (a measure of the spin polarization) decays on a lengthscale related to the $n_{e}$-dependent spin diffusion constant, lifetime, and mobility. [1] S.A. Crooker, M. Furis, X. Lou, C. Adelmann, D.L. Smith, C.J. Palmstr$\o$m, P.A. Crowell, Science \textbf{309}, 2191 (2005). [Preview Abstract] |
Monday, March 13, 2006 4:42PM - 4:54PM |
D19.00010: Complete non-equilibrium spin polarization of electrons in degenerate semiconductors A. G. Petukhov, V. V. Osipov, V. N. Smelyanskiy We show that spin polarization of electrons in nonmagnetic degenerate semiconductors near specially tailored forward- biased ferromagnet-semiconductor junctions can achieve 100\%. This effect is realized even at moderate spin polarization of the interfacial contact resistance. Our conclusions are based on detailed analysis of non-linear spin diffusion equation with realistic boundary conditions. The effect of complete spin extraction occurs at relatively strong electric fields and arises from a reduction of spin penetration length due to the drift of electrons from a semiconductor towards the spin- selective tunnel junction. We further demonstrate that non- equilibrium electrons in thin nonmagnetic semiconductor layers can be almost fully spin polarized by means of simultaneous electrical spin injection and extraction. The complete spin polarization is achieved if the thin layer is placed between two ferromagnetic metal contacts with moderate spin injection coefficients and antiparallel magnetizations. The sign of the spin polarization is determined by the direction of the current. Applications of this effect in spintronics and quantum information processing are discussed. [Preview Abstract] |
Monday, March 13, 2006 4:54PM - 5:06PM |
D19.00011: Ballistic spin injection spectroscopy of a (Ga,Mn)As spin Esaki diode Makoto Kohda, Tomohiro Kita, Yuzo Ohno, Fumihiro Matsukura, Hideo Ohno A p-type ferromagnetic semiconductor (Ga,Mn)As is one of the promising materials for spin injector by interband tunneling [1]. In this work, we investigated injection of spin-polarized electrons in a (Ga,Mn)As/n$^+$-GaAs Esaki diode (ED) by using a three-terminal device integrating a (Ga,Mn)As ED and a light emitting diode (LED), which allows us to apply bias voltages to the ED and LED independently. Electroluminescence polarization ($P_{\rm EL}$) from the LED was measured under the Faraday configuration as a function of bias voltages. The maximum $P_{\rm EL}$ of 32.4\% was observed when the valence electrons near the Fermi energy of (Ga,Mn)As are ballistically injected into the LED. The experimental results proved high spin polarization of valence electrons ($>85$\%) for 100 meV below the Fermi energy of (Ga,Mn)As. [1] M.~Kohda et al., Jpn. J. Appl. Phys. Part2 {\bf 40}, L1274 (2001); E.~Johnston-Halperin et al., Phys. Rev. B {\bf 64}, 041306(R) (2002); P.~Van Dorpe et al., Appl. Phys. Lett. {\bf 84}, 3495 (2004). [Preview Abstract] |
Monday, March 13, 2006 5:06PM - 5:18PM |
D19.00012: Electrical Spin Injection into Two Dimensional Electron Gas. Jonghwa Eom, Hyun Cheol Koo, Donghwa Jung, Hyunjung Yi, Joonyeon Chang, Jae Beom Ko, Suk-Hee Han While electrical spin injection has been observed in a variety of metal films, the signal of the electrical spin injection into semiconductors has been hardly observed. We have overcome the difficulties of spin injection by developing a well-controlled interface treatment between the ferromagnetic films and the InAs semiconductor surface. We have found the evidence of electrical spin injection and accumulation in permalloy(Py)/InAs 2DEG/permalloy(Py) lateral devices. Ferromagnetic metal electrodes with different aspect ratio being separated by various separations were fabricated on top of the InAs 2DEG channels. Our multi-terminal geometry allows us to measure non-local spin valve effect as well as local spin valve effect. In the non-local spin valve measurement, we were able to completely isolate the spin valve signal from other spurious effects and observed clear spin accumulation signals. Based on the spin accumulation signal depending on the channel length, we have evaluated the spin relaxation length in the InAs based 2DEG. [Preview Abstract] |
Monday, March 13, 2006 5:18PM - 5:30PM |
D19.00013: Fabrication of GaAs spin injection devices Y. Jung, O. Kwon, V. P. Bhallamudi, R. Yu, D. V. Pelekhov, S. A. Ringel, P. R. Berger, P. C. Hammel We report on the progress towards fabrication of a ferromaget-GaInP(or AlGaAs)-GaAs devices. The devices are designed for use in future experiments to electrically detect, and spatially map spin accumulation in GaAs using a high sensitivity magnetic force microscope. The device consists of n-GaAs channel capped with a 3-5nm thick highly doped GaInP or AlGaAs layer. Both the GaAs channel and GaInP( or AlGaAs) layers are MBE grown. The ferromagnetic injector and detector are deposited ex-situ over the GaInP (or AlGaAs) barrier layer. We will discuss the electrical characteristics and transport behavior of the devices. [Preview Abstract] |
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