Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A22: Focus Session: Spins in Group III-V and II-VI Semiconductors |
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Sponsoring Units: GMAG DMP FIAP Chair: Paul Crowell, University of Minnesota Room: 324 |
Monday, March 16, 2009 8:00AM - 8:36AM |
A22.00001: Electrically Injected Spin Polarized Lasers Invited Speaker: The ability to electrically modulate orthogonal polarization states in spin-polarized lasers opens up avenues for a wide range of applications such as photochemical spectroscopy, optical switches, and communications with enhanced security [1]. This has motivated us to investigate the properties of quantum well (QW) [2] and quantum dot (QD) [3] spin-polarized vertical cavity surface emitting lasers (spin-VCSELs). The laser heterostructures are grown by molecular beam epitaxy (MBE). The active region consists of In0.2Ga0.8As/GaAs QWs [2] or InAs QDs [3]. VCSELs are fabricated using standard micro-fabrication techniques. The FM Schottky tunnel contact is realized with Fe or MnAs re-grown by MBE. The QW spin-VCSELs exhibit a maximum threshold current reduction of 11 {\%} and output degree of circular polarization of 23 {\%} at 50 K. The corresponding values observed in QD spin VCSELs at 200 K are 8 {\%} and 14 {\%}, respectively. Inhibition of the D'yakonov-Perel spin scattering process results in higher operating temperatures for spin-lasers with QD active region. In addition, we have demonstrated electrical modulation of the output polarization with a peak modulation index of 0.6. The spin polarization of carriers in the active region of a spin laser gives rise to large gain anisotropy at biases near threshold. As a result, the output polarization can be much larger than the spin polarization of the injected carriers. This is contrary to the linear relation between carrier spin orientations in the active region and the polarization of photons emitted upon their radiative recombination in spin light emitting diodes. The exact magnitude of the output polarization in spin lasers and the parameters upon which it depends have been analytically determined and are in excellent agreement with those obtained from measurements. These results will be described and discussed. \\[4pt] References: \\[0pt] [1] M. Holub et al., J. Phys. D. 40, R179 (2007). \\[0pt] [2] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007). \\[0pt] [3] D. Basu et al., Appl. Phys. Lett. 92, 091119 (2008). \\[0pt] [4] M. I. D'yakonov et al., Sov. Phys Solid State. 13, 3023 (1971) [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A22.00002: Electrical Spin injection from Fe into ZnSe. Aubrey Hanbicki, G. Kioseoglou, M.A. Holub, O.M.J. van 't Erve, B.T. Jonker The wide bandgap semiconductor ZnSe is an opto-electronic material with a comparable spin lifetime and small lattice mismatch to GaAs. Novel spintronic devices that incorporate ZnSe/GaAs heterostructures will require the facile transport of spin information across several heterointerfaces including spin injection into the ZnSe. We have electrically injected spin-polarized electrons from a ferromagnetic Fe contact into a ZnSe epilayer grown on a GaAs heterostructure. The injected carriers proceed through 300 nm of ZnSe and recombine in the GaAs emitting light characteristic of the bulk GaAs exciton. We measure spin polarizations in excess of 40{\%} in the GaAs based on analysis of the circular polarization of the electroluminescence. We report results as a function of applied magnetic field, device current and temperature. The spin injection process and transport through the ZnSe layer sustains significant spin populations in this heterostructure. This work was supported by core programs at NRL. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A22.00003: Tuning spin-current across a Semiconductor/Ferromagnet junction by resonance tunneling Pengke Li, Hanan Dery We present a theory of spin-dependent transport in a hybrid semiconductor/ferromagnet system which includes an asymmetric double barrier region at the interface (e.g., GaAs/AlGaAs/GaAs/Fe). The system has two electron confinement regions with one being a thin quantum well between a heterostructure barrier and a Schottky barrier. The second confinement region is a two dimensional electron gas (2DEG) at the heterointerface with the bulk semiconductor generated by intentional doping. The I-V curve has two current peaks when electrons tunnel into the ferromagnet. These peaks are due to resonance tunneling of electrons whose energy matches the energy of the quasi-bound state in the quantum well. The first peak is governed by tunneling of delocalized electrons from the bulk semiconductor and the second by escape from the 2DEG. These resonances are met at different bias levels and correspond to opposite spin polarization of the current. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A22.00004: Triggering phase-coherent spin packets by pulsed electrical spin injection across an Fe/GaAs Schottky barrier B. Beschoten, L.R. Schreiber, C. Schwark, G. Guentherodt, C. Adelmann, C.J. Palmstrom, X. Lou, P.A. Crowell The precise control of spins in semiconductor spintronic devices requires an electrical means for generating spin packets with a well-defined initial phase. So far, ultrafast laser pulses have successfully been used to trigger the ensemble phase of optically generated spin packets. However, electrical methods for ensemble phase triggering remain challenging. Here, we use fast current pulses to inject phase triggered electron spin packets across an Fe/GaAs Schottky barrier into n-GaAs. We demonstrate phase coherence by the observation of multiple Larmor precession cycles for current pulse widths down to 500 ps at 20 K. We show that the current pulses are broadened by the charging and discharging time of the Schottky barrier. At high frequencies, the observable spin coherence is limited only by the finite band width of the current pulses, which is on the order of 2 GHz. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:48AM |
A22.00005: The importance of Fe interface states for ferromagnet-semiconductor based spintronic devices Invited Speaker: I present our recent theoretical studies of the bias-controlled spin injection, detection sensitivity and tunneling anisotropic magnetoresistance in ferromagnetic-semiconductor tunnel junctions. Using first-principles electron transport methods we have shown that Fe 3{\it d} minority-spin surface (interface) states are responsible for at least two important effects for spin electronics. First, they can produce a \emph{sizable} Tunneling Anisotropic Magnetoresistance in magnetic tunnel junctions with a \emph{single} Fe electrode. The effect is driven by a Rashba shift of the resonant surface band when the magnetization changes direction. This can introduce a new class of spintronic devices, namely, Tunneling Magnetoresistance junctions with a single ferromagnetic electrode that can function at room temperatures. Second, in Fe/GaAs(001) magnetic tunnel junctions they produce a \emph{strong} dependence of the tunneling current spin-polarization on applied electrical bias. A dramatic \emph{sign reversal} within a voltage range of just a few tenths of an eV is found. This explains the observed sign reversal of spin-polarization in recent experiments of electrical spin injection in Fe/GaAs(001) and related reversal of tunneling magnetoresistcance through vertical Fe/GaAs/Fe trilayers. We also present a theoretical description of electrical spin-detection at a ferromagnet/semiconductor interface. We show that the sensitivity of the spin detector has \emph{strong} bias dependence which, in the general case, is \emph{dramatically different} from that of the tunneling current spin-polarization. We show that in realistic ferromagnet/semiconductor junctions this bias dependence can originate from two distinct physical mechanisms: 1) the bias dependence of tunneling current spin-polarization, which is of \emph{microscopic} origin and depends on the specific properties of the interface, and 2) the \emph{macroscopic} electron spin transport properties in the semiconductor. Our numerical results show that the magnitude of the voltage signal can be tuned over a wide range from the second effect {\it alone} and thus identifies a universal method for enhancing electrical spin-detection sensitivity in ferromagnet/semiconductor tunnel contacts. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A22.00006: Three Terminal Spin Extraction Resistance in Fe/GaAs Heterostructures E.S. Garlid$^1$, T. Kondo$^1$, Q. Hu$^{1,2}$, C.J. Palmstr\O m$^{1,2}$, P.A. Crowell$^1$ Spin transport measurements have been difficult to interpret in two terminal Fe/GaAs/Fe devices where current flows in both the injector and detector electrodes. This is due to the strong non-monotonic dependence of the spin accumulation on the Fe/GaAs interface bias, which affects the spin injection and detection efficiencies. To address this, we measured the four terminal non-local spin valve resistance and the three terminal spin extraction resistance in epitaxial Fe/GaAs heterostructures with a systematically varied Schottky barrier doping profile. Lateral devices were fabricated from epitaxial Fe/n$^+$/n-GaAs (100) heterostructures in which the thickness of the n$^+$ layer (n$^+=5\times 10^{18}$) was varied from 5 to 50 nm while n $=5\times 10^{16}$ in the 2.5 $\mu$m channel. The three terminal resistance measured using a single contact as the injector and detector is $\approx 100\times$ larger than the non-local spin valve resistance, an effect which cannot be attributed to spin relaxation in the channel. In the case of a three terminal measurement, we obtain both a large spin accumulation as well as an enhanced detection sensitivity under forward bias conditions. This can be analyzed by considering the measured non-local spin polarization as a function of bias, as well as the electric fields at the Fe/GaAs interface in the presence of a charge current. Supported by ONR and the NSF MRSEC, and NNIN programs. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A22.00007: Contributions to oblique Hanle linewidths in Fe/GaAs non-local spin valve transport Chaffra Awo-Affouda, O. M. J. van 't Erve, G. Kioseoglou, A. T. Hankbicki, M. Holub, C. H. Li, B. T. Jonker The transport Hanle effect linewidth is commonly used to determine spin lifetimes in spin- polarized transport structures. We show that the magnetic domain structure of the ferromagnetic contacts used to inject and detect the spin current introduces asymmetries to the Hanle lineshape. In addition, the nuclear spin polarization can produce anomalous narrowing and broadening of the Hanle linewidth depending upon the orientation of the transport spin and the applied field. These contributions can significantly impact the apparent spin lifetime extracted from the Hanle curve, but are not included in the analysis typically applied. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A22.00008: High Optical Polarization from Electrical Spin Injection into an InGaAs QW C.H. Li, G. Kioseoglou, M. Holub, O.M.J. van 't Erve, B.T. Jonker, T. Ali, I. Khan, M. Yasar, A. Petrou We have fabricated spin light emitting diodes (LEDs) with Fe as the spin injector and 100{\AA} In0.1Ga0.9As/GaAs QWs as the detector. The emission efficiency from the InGaAs QW is extremely high, with a narrow linewidth of 4meV at 5K. The free exciton exhibits 25{\%} optical polarization due to the injection of spin polarized carriers from the reverse-biased Fe Schttky contact. At low biases, a feature 10meV below the free exciton appears which exhibits a much larger polarization with a peculiar magnetic field dependence. Similar to that of the free exciton, the circular polarization of this lower energy feature first increases with magnetic field, and reaches a maximum of 67{\%} at 2.5T, indicating injection from Fe. However, this behavior is superposed on a large diamagnetic background of 21{\%}/T which dominates above 2.5T. The intensity and polarization of this feature is strongly bias dependent, and the feature disappears above 15K, suggesting that it originates from a weakly bound complex. The origin of this feature and its dependence on the magnetic field will be discussed at the meeting. Supported by ONR, NRL core funds, and NSF. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A22.00009: Spin polarization control through resonant states in an Fe/GaAs Schottky barrier Atsufumi Hirohata, Shuta Honda, Hiroyoshi Itoh, Jun-ichiro Inoue, Hidekazu Kurebayashi, Theodossis Trypiniotis, C. H. W. Barnes, J. A. C. Bland We show that \textit{the IRSs }\textit{(Interface Resonant States)}\textit{ within the Schottky barrier }play an important role for the negative spin polarization of the current and its bias dependence, and compare with our experimental results [1]. We have calculated the spin polarization $P$ of the tunnel conductivity using a full-orbital tight-binding model, and have shown that the IRSs within the Schottky barrier in the GaAs layer influence significantly the spin-dependent tunneling across the interface. It has been clearly shown that the band matching of the IRSs plays a crucial role on the spin polarization. The theoretical results account well for earlier experimental results including the tunneling of photo-excited electrons. The present results suggest that the spin polarization can be controlled by the Schottky barrier heights, and that a spin-switch device with bias control may also be promising. Quantitative performance of the device, however, needs more quantitative calculations including effects of atomic disorder for example. [1] H. Kurebayashi \textit{et al.}, \textit{Appl. Phys. Lett.} \textbf{91}, 102114 (2007). [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A22.00010: Time dependent analysis of spin transport in lateral semiconductor/ferromagnet structures with non-collinear magnetization Yang Song, Hanan Dery We model the transport in lateral semiconductor channels beneath ferromagnetic contacts with non-collinear magnetization directions. We quantify the effects of the mixing conductance and of the spin polarization across the interface, of the electrical field in the channel and of the resistance ratio between the channel and the interface. We focus on a non-local spin valve geometry in which two contacts are biased and collinear and a third terminal is non-collinear and ``semi'' floating (connected in series with a capacitor). This structure can be used for memory devices with multi-valued stored bits by rotating the magnetization in one of the terminals and detecting the transient current signal that flows through the non-collinear terminal. The shape and magnitude of this current signal is strongly influenced by the relation between the non- collinear magnetization direction and the (2D) spin accumulation in the channel that is being set by the biased (collinear) contacts. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A22.00011: Magnetic force detection of non-equilibrium electron spin-polarization in n-GaAs Vidya Bhallamudi, Gang Xiang, Mark Brenner, Youngwoo Jung, Yuri Obukhov, Denis Pelekhov, Steve Ringel, P. Chris Hammel Magnetic Force Microscope (MFM) offers an alternative to optical and electrical techniques for detecting and imaging spin-polarized electron populations in semiconductor spintronic devices. Unlike other methods, MFM has the advantage of being material non-specific as it directly detects spins in the semiconductor through their magnetic dipole coupling to micromagnetic tip. However, it is challenging to achieve the high sensitivity required for sensing small non-equilibrium spin populations, orders of magnitude smaller than those in ferromagnetic materials. Here we present our progress developing a high sensitivity cryogenic MFM for imaging optically injected electronic spins in GaAs. Spins are created in an epitaxially grown n-GaAs membrane by circularly polarized laser shone from an optical fiber. A cantilever scans over the membrane and detects the magnetic force due to the optically injected spins. Micro-magnetic tip generating large field gradient is used for enhancing the signal. We will show simulation results for the expected forces, taking spin relaxation, diffusion and local tip field into account. The status of spin imaging experiment will also be presented. [Preview Abstract] |
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