Rolf Allenspach IBM Research
Christian H. Back Universitaet Regensburg
Bret Heinrich Simon Fraser University
J1: Spin Transfer and Magnetization Dynamics
Monday PM, November 26, 2007
Republic A (Sheraton)
9:15 AM - **J1.1
Effects of Lateral Constraint on Magnetic Domain Wall Motion.
Geoffrey Beach 1 , C. Knutson 1 , C. Nistor 1 , M. Tsoi 1 , J. Erksine 1 Show Abstract
1 Department of Physics, The University of Texas at Austin, Austin, Texas, United States
9:45 AM - **J1.2
Current-driven Domain Wall Dynamics in Magnetic Nanowires.
Luc Thomas 1 , Masamitsu Hayashi 1 , Rai Moriya 1 , Xin Jiang 1 , Bastiaan Bergman 1 , Charles Rettner 1 , Brian Hughes 1 , Stuart Parkin 1 Show Abstract
1 , IBM Almaden Research Center, San Jose, California, United States
New concepts for memory and logic devices based on magnetic domain walls (DWs) are made possible by the direct manipulation of the DWs using spin-polarized electrical current. The Racetrack Memory  takes advantage of the unique characteristics of current-driven domain wall motion through spin-transfer torque to allow for a novel magnetic domain wall shift register storage device. The detailed understanding of the dynamics of DWs excited by magnetic field and/or pulses of electric current is crucial for the successful realization of the Racetrack Memory. For this purpose, we have explored the DW dynamics in a wide variety of magnetic nanowires fabricated by electron beam lithography. The DW dynamics are probed by a combination of dc and time resolved resistance measurements with magnetic imaging [2-4].Our studies reveal the subtle interplay between DWs and spin-polarized currents. For example, we show that the dynamics of DWs driven by nanosecond-long current pulses is unexpectedly complex. Indeed, the probability of moving a DW out of a pinning site exhibits strong oscillations as a function of length of the current pulse . With the help of analytical modeling and micromagnetic simulations, this surprising behavior is understood as a signature of the precessional nature of the DW dynamics. We show that the oscillatory DW dynamics can be exploited to our advantage. By using trains of pulses whose lengths and separations are matched to the DW’s oscillation period, we have demonstrated a fivefold reduction of the threshold current needed to move a DW out of a pinning site . S. S. P. Parkin, US Patents 6834005, 6898132, 6920062, 7031178 (2004-5). M. Hayashi et al., Phys. Rev. Lett. 96, 197207 (2006); M. Hayashi et al., Phys. Rev. Lett. 97, 207205 (2006); M. Hayashi et al., Phys. Rev. Lett. 98, 037204 (2007); M. Hayashi et al., Nature Phys. 3, 21 (2007).  L. Thomas et al., Nature 443, 197 (2006) L. Thomas et al., Science 315, 1553 (2007).
10:15 AM - J1.3
Spin Dynamics in a Superconductor / Ferromagnet Proximity System.
Jan Aarts 1 , Chris Bell 1 , Martina Huber 1 , Sergey Milikisiyants 1 Show Abstract
1 Kamerlingh Onnes Laboratory, Leiden Institute of Physics, Leiden Netherlands
The ferromagnetic resonance of thin sputtered Ni80Fe20 (Permalloy, Py) films grown on Nb is measured. By varying the temperature and thickness of the Nb the role of the superconductivity on the damping of the macrospin is explored. The change in the spin transport properties below the superconducting transition of the Nb is found to manifest itself in the Py layer bya sharpening of the resonance of the ferromagnet, or a decrease in the effective Gilbert damping coefficient. We interpret this in terms of the spin pumping model combined with the occurrence of Andreev reflections. Moreover, the effects of spin scattering can be studied by inserting different layers such as Pt between the Py and Nb layers
10:30 AM - **J1.4
Switching a Vortex Core in a Ferromagnetic Disk by Electric Current.
Teruo Ono 1 Show Abstract
1 , Kyoto University, Uji Japan
The manipulation of magnetization by spin currents is a key technology for future spintronics. The underlying physics is that spin currents can apply a torque on the magnetic moment when the spin direc-tion of the conduction electrons has a relative angle to the local magnetic moment. This leads us to a general concept that any type of spin structure with spatial variation can be excited by a spin-polarized current in a ferromagnet. We confirmed this concept for two typical noncollinear spin structures: a mag-netic domain wall (DW) and a magnetic vortex. The direction of magnetic moments gradually changes in a DW. Since the spin direction of conduction electrons changes when the electrons cross the DW, spin transfer from electrons to the DW occurs and torque is exerted on the DW. In consequence, an electric current can displace the DW . Magnetic force microscopy observation of the current-driven DW displacement in submicron magnetic wires can provide quantitative information such as magnetic DW structure and the displacement as a function of the intensity and the duration of the pulsed-current [2, 3]. Uni-directional motion of a domain wall in a magnetic wire with asymmetric structures, which could be called a magnetic ratchet effect, is also pre-sented .The spin transfer torque is also expected to be active in a magnetic vortex, in which a curling magnetic structure with a nanometer-scale core is realized . We show that a magnetic vortex core in a ferro-magnetic circular dot can be resonantly excited by an AC current through the dot when the current fre-quency is tuned to the resonance frequency originating from the confinement of the vortex core in the dot . The core is efficiently excited by the AC current due to the resonant nature and the resonance fre-quency is tunable by the dot shape. We also demonstrate that the direction of a vortex core can be switched by utilizing the current-driven resonant dynamics of the vortex. . References L. Berger, J. Appl. Phys. 55, 1954 (1984). A. Yamaguchi et al., Phys. Rev. Lett. 92, 077205 (2004). A. Yamaguchi et al., Appl. Phys. Lett. 86, 012511 (2005). A. Himeno et al., Appl. Phys. Lett. 87, 243108 (2005). T. Shinjo et al., Science 289, 930 (2000). S. Kasai et al., Phys. Rev. Lett. 97, 107204 (2006). K. Yamada et al., Nature Materials 6, 269 (2007).
11:30 AM - **J1.5
Interaction of Spin Transfer Nanocontact Oscillators With AC Currents, Oersted Fields, and Spin Waves.
Matthew Pufall 1 2 , William Rippard 1 , M. Schneider 1 , Stephen Russek 1 Show Abstract
1 Electromagnetics Division, M/S 818.03, NIST, Boulder, Colorado, United States, 2 Dept. of Physics & Astronomy, University of Denver, Denver, Colorado, United States
Spin transfer-driven dynamics in nanocontacts is both a promising and challenging physical system. On the promising side, nanocontact-based spin transfer oscillators (STNOs) exhibit narrowband (1-10 MHz) oscillations that are tunable via current and field from 0.1 GHz to greater than 40 GHz. The frequency and amplitude of the STNO output is roughly understandable as resulting from large-angle ferromagnetic resonance-like precession of the free layer. Also, as current tunable oscillators, they can be easily modulated.On the challenging side, efforts to understand the details of the STNO output have met with only limited success. For example, the variation of frequency vs. current and field is not completely understood, with the frequency exhibiting discrete jumps with both current and field. The specifics of these jumps—the position with current, and magnitude of the jump in frequency—vary from contact to contact, and are observed in a variety of free layer materials. Current spin transfer theories have found it difficult to explain these jumps. One possible explanation is the interaction of the oscillating magnetization with variations in the magnetic microstructure. Another poorly-understood experimental detail is the small magnitude of the STNO linewidth and its variation with current and field. This result possibly indicates an effective oscillating volume larger than the contact region. Finally, current-hysteretic, narrowband oscillations can be observed in STNOs in zero external field that are reminiscent in frequency and field dependence of vortex dynamics in confined structures. These examples suggest that understanding the interaction of the oscillating region of the free layer magnetization with the surrounding environment—which includes the free layer outside the contact via exchange and spin waves, the Oersted field produced by the DC current itself, and other layers in the multilayer—is crucial to understanding the physics of the dynamics induced by the spin transfer effect in a nanocontact geometry. In my talk, I will review the basic characteristics of STNOs and describe measurements we have made to begin to probe the interactions of nanoscale oscillators with their surroundings, including mutual phase locking of closely-spaced oscillators, GMR detection of spin wave radiation, and injection locking to ac currents and fields. I will also discuss the possibility of employing other means of probing spin transfer dynamics, such as spin transfer driven magnetic resonance detected by nonlinear mixing. J.C. Slonczewski, JMMM,195, L261(1999); W.H. Rippard et al., Phys. Rev. B 70(10), 100406 (2004). M.R. Pufall et al., Appl. Phys. Lett. 86(8), 082506 (2005). W.H. Rippard et al., Phys. Rev. B 74(22), 224409 (2006). M.R. Pufall et al., Phys. Rev. B, 75(14), 140404 (2007). M.R. Pufall et al., Phys. Rev. Lett. 97(8), 087206 (2006).
12:00 PM - **J1.6
Spin Transfer Torque in Non-Uniform Magnetizations.
Mark Stiles 1 , Jiang Xiao 2 , Andrew Zangwill 2 , Wayne Saslow 3 , Michael Donahue 4 Show Abstract
1 Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Department of Physics, Georgia Institute of Technology, Atlanta, Georgia, United States, 3 Department of Physics, Texas A&M University, College Station, Texas, United States, 4 Mathematical and Computational Sciences Division , National Institute of Standards and Technology, Gaithersburg, Maryland, United States
When a spin polarized current passes through a non-uniform magnetization, it exerts a torque on the magnetization. In magnetic multilayers with non-collinear magnetizations this torque can be described as due to a transfer of angular momentum from the spin current to the magnetization. In particular, the part of the spin current that is transverse to the magnetization is transferred. This transfer is largely localized to the interfaces between the non-magnetic and the ferromagnetic layers. Here, the electron spins are responding to a sudden change in the exchange field. For spin polarized currents passing through a domain wall, the opposite limit largely holds, the electrons spins adiabatically follow the magnetization. The torque can then be described as the reaction torque from the rotation of the spin directions aligning themselves with the magnetization. These torques and their consequences can be largely understood from a series of simple models. However, experiments have become sophisticated enough to show that these simple models are not complete. In this talk, I will motivate the interest in these systems, describe the simple models that capture most of the physics, and highlight some the open questions that remain.
12:30 PM - **J1.7
Magnetic Tunnel Junction Nanopillars Using Low Resistance-Area Product MgO Barriers for Spin-Transfer MRAM.
Fred Mancoff 1 , Phil Mather 1 , Renu Dave 1 , Brian Butcher 1 , Jon Slaughter 1 , Nick Rizzo 1 Show Abstract
1 Technology Solutions Organization, Freescale Semiconductor, Inc., Chandler, Arizona, United States
Toggle magnetic random access memory (MRAM), now in commercial production, uses the magnetic fields generated by current carrying lines to program the bits. In contrast, spin-transfer MRAM (ST-MRAM) bits are programmed by flowing a spin-polarized current through a magnetic tunnel junction (MTJ) to reverse the free layer moment. An ST-MRAM is potentially much lower power and higher density than conventional toggle MRAM. In an ST-MRAM, the high spin-polarization in MTJ’s using an MgO tunnel barrier provides a large tunneling magnetoresistance (TMR) which leads to a large output read signal. The high polarization also gives efficient spin-transfer which leads to a low write current. Challenges for the development of ST-MRAM include achieving a low resistance-area product RA (with a tunnel barrier thickness only ≈ 1 nm), low switching current density, high MR, and high breakdown voltage, all with tight cross-wafer distributions. This talk will cover both material and device characteristics of MgO-based MTJ spin-transfer nanopillars with CoFeB free layers. First, we report the material properties of low RA MgO barriers fabricated either by plasma, radical, or natural oxidation of Mg. Unpatterned MTJ films were characterized electrically by in-plane TMR measurements, yielding low-bias TMR up to 120% at RA ≈ 6 Ω-μm2. Cross-section transmission electron microscopy and low-angle x-ray reflectivity showed an expansion factor from 1.06-1.17 for the MgO upon oxidation. Next, regarding devices, we fabricated MTJ nanopillars as small as 80 nm×160 nm using optical lithography on 200 mm diameter Si wafers. Low-bias TMR up to 90% at RA ≈ 6 Ω-μm2 was observed in the patterned bits. Measurement of several hundred bits across-wafer showed quasistatic spin-transfer switching currents of 0.5 mA (5×106 A/cm2) and a thermally-stable energy barrier of ≈ 45 kT, corresponding to a high-speed switching current density of Jc0 ≈ 9×106 A/cm2. The MTJ breakdown voltage Vbd is another critical parameter for ST-MRAM writing. The Vbd measured quasistatically increased logarithmically with RA, from 0.95 V at 6.5 Ω-μm2 to 1.2 V at 22 Ω-μm2 for the smallest bits. Decreases in Vbd with increasing bit size were explained by a model with a random distribution of barrier weak spots. The Vbd was then determined by the number of weak spots in the device and thus the device size. Requirements for a functioning ST-MRAM array and the necessary improvement of the MgO MTJ properties will also be discussed.
J2: Spin Injection into Silicon
Monday PM, November 26, 2007
Republic A (Sheraton)
2:30 PM - **J2.1
Electrical Spin Injection into Silicon from a Ferromagnetic Metal / Tunnel Barrier Contact.
Berry Jonker 1 , George Kioseoglou 1 , Aubrey Hanbicki 1 , Olaf van 't Erve 1 , Connie Li 1 , Phillip Thompson 1 Show Abstract
1 , Naval Research Laboratory, Washington, District of Columbia, United States
The electron’s spin angular momentum is one of several alternative state variables under consideration on the International Technology Roadmap for Semiconductors for processing information in the fundamentally new ways which will be required beyond end-of-roadmap CMOS technology (1). Electrical injection / transport of spin-polarized carriers is prerequisite for developing such an approach. While significant progress has been realized in GaAs (2), little has been made in Si, despite its overwhelming dominance of the semiconductor industry. We report successful injection of spin-polarized electrons from an Fe film into Si(001) n-i-p doped heterostructures, and spin transport across the Si/AlGaAs interface. The circular polarization of the electroluminescence (EL) due to radiative recombination in the Si tracks the Fe magnetization, confirming that the electrons originate from the Fe. The polarization reflects Fe majority spin, consistent with the common delta_1-symmetry of the Fe majority and Si(001) conduction band (CB). The spin polarization in the Si is ~30% at 5K, with significant polarization extending to at least 125K. In Si/AlGaAs/GaAs quantum well (QW) structures, the spin polarized electrons drift under applied field from the Si and recombine in the GaAs QW, where the polarized EL can be quantitatively analyzed, yielding an electron spin polarization of 10%. Spin transport occurs despite the poor crystalline quality of Si epilayers on GaAs, the 0.3 eV CB offset (Si band lower), and the fact that the sample surface was exposed to air before growth of the Si on the AlGaAs/GaAs. This approach injects spin-polarized electrons near the Si CB edge with near unity conversion efficiency and modest bias voltages. The realization of efficient electrical injection and significant spin polarization using a simple tunnel barrier compatible with "back-end" Si processing should greatly facilitate development of Si-based spintronics. This work was supported by ONR and core NRL programs.(1) International Technology Roadmap for Semiconductors, 2005 Edition, http://www.itrs.net/reports.html. See "Executive Summary" and "Emerging Research Devices."(2) for a recent review, see Jonker, B.T. & Flatte, M.E. in Nanomagnetism: Ultrathin Films, Multilayers and Nanostructures (in the series Contemporary Concepts of Condensed Matter Science, Elsevier, 2006), 227-272. ISBN 0-444-51680-8
3:00 PM - J2.2
Characterization of SiO2 and Al2O3 Tunnel Barriers for Spin Injection from Ferromagnet Electrode into Silicon.
Nicolas Bruyan 1 , Mehdi Kanoun 1 , Rabia Benabderrahmane 1 , Brayan Pang 1 , Claire Baraduc 1 , Ahmad Bsiesy 1 2 , Hervé Achard 3 Show Abstract
1 SPINTEC, CEA, Grenoble France, 2 , Université Joseph Fourier, Grenoble France, 3 , CEA/LETI, Grenoble France
In this work, we present an extensive study of the effect of the insulator nature on the diffusion of nickel or iron atoms and their contamination of the NiFe/Insulator/Si stack that can be used for spin injection into silicon. Indeed, TOF-SIMS (Time of Flight Secondary Ion Mass Spectroscopy) and electrical characterizations of NiFe/I/Si diodes were carried out. Samples with two different insulators (SiO2 and Al2O3) have been studied and it is demonstrated that the diffusion of the Ni and Fe is well reduced by taking the alumina as a tunnel barrier. In particular, TOF-SIMS measurements have shown that the contamination ratio of the silicon substrate, in the case of the SiO2 is more than ten times larger than in the case of Al2O3. Moreover, the Ni and Fe profiles in the NiFe/Al2O3/Si stack are comparable to the one obtained on a reference sample (without NiFe metal deposition). The conductance measurements performed on NiFe/SiO2/Si diodes have shown that the interface state density (Dit) is very large compared to the state of the art of the CMOS technology, indicating that the Fe and/or Ni may have diffused into the substrate. In contrast, for the NiFe/Al2O3/Si the Dit values recorded is comparable to those encountered in the literature suggesting that there is no diffusion of Ni or Fe. These results demonstrate that the alumina is a better barrier to the Ni and/or Fe diffusion compared to the SiO2 which makes Al2O3 a more suitable dielectric for spin injection since the absence of contaminants can suppress undesirable defects related spin flip.
3:15 PM - J2.3
Al2O3 Tunnel Barrier as a Good Candidate for Coherent Spin Injection into Silicon.
Rabia Benabderrahmane 1 , Mehdi Kanoun 1 , Nicolas Bruyant 1 , Claire Baraduc 1 , Ahmad Bsiesy 1 2 , Hervé Achard 3 Show Abstract
1 SPINTEC, CEA, Grenoble France, 2 , Université Joseph Fourier, Grenoble France, 3 , CEA/LETI, Grenoble France
This work is focused on the electrical characterisation of NiFe/SiO2/Si and NiFe/Al2O3/Si MIS diodes, used for spin injection into silicon. Studies of the effect of insulator nature on the device electrical properties have been done. Capacitance and conductance measurements have shown similar values of trap densities between SiO2/Si and Al2O3/Si interfaces. However, the extracted cross section values of these interface defects are different indicating traps of different nature. Current-Voltage analysis show a bulk oxide trap assisted tunnelling for NiFe/SiO2/Si diodes whereas a fowler Nordheim is observed on injection for the NiFe/Al2O3/Si structures. This result makes Al2O3 more suitable dielectric for spin injection devices based on silicon
4:00 PM - **J2.4
Tunable Spin Tunnel Contacts to Silicon using Low Work Function Ferromagnets.
Ron Jansen 1 Show Abstract
1 , MESA+ Institute for Nanotechnology, University of Twente, Enschede Netherlands
Semiconductor electronics has been the cornerstone of contemporary information technology for many decades. It is based on the manipulation, control and storage of electrical charge in circuits used for logic and memory applications, with the transistor as the central element. Semiconductor materials prevail because they allow power amplification, with silicon being by far the dominant material. However, as concerns are raised about the further advancements of semiconductor devices in future chip generations, alternative technologies are actively being explored. The field of spintronics aims to improve the performance and enhance the functionality of electronic circuits and systems by making use of the spin of electrons. While significant progress has been made in recent years in combining spin with III-V (GaAs) based semiconductor materials, the opposite is true for silicon.Spin-injection into silicon is potentially highly rewarding, but also challenging due to the difficulty of using optical techniques to probe the spin polarization injected into the semiconductor. The most obvious route to a breakthrough involves a fully electrical device, such as the spin-MOSFET, with a ferromagnetic (FM) source and drain. This requires careful consideration of the requirements for the FM injector and detector contacts. We show that Schottky barrier formation on Si is detrimental to the magnetoresistance of such a device. This is partly because of the resulting large resistance area (RA) product of the contacts (precluding conductivity matching with conventional FM electrodes), and partly because of the potential energy landscape, which affects spin flow across the interface.As a solution, we present a novel approach to control the Schottky barrier height and resistance-area (RA) product of spin tunnel contacts to Si using low work function materials. These include ferromagnets as well as non-magnetic materials, inserted as ultrathin (sub-nm) interfacial layers into FM/Al2O3/Si spin-tunnel contacts on either side of the tunnel barrier. We show that in this way the RA product of FM/Al2O3/Si contacts can be tuned over 8 orders of magnitude. Equally important, complementary tunnel magnetoresistance data show that a reasonable tunnel spin-polarization is simultaneously maintained over the full range. Interestingly, the Schottky barrier can even be inverted, in which case an interfacial accumulation layer (i.e. a 2 DEG) can be established, creating new device options. Such engineered spin-tunnel contacts with low work function materials qualify as conductivity-matched source and drain electrodes and raise prospects for Si-based spintronics.
4:30 PM - **J2.5
Coherent Spin Transport Through an Entire Silicon Wafer.
Biqin Huang 1 , Douwe Monsma 2 , Ian Appelbaum 1 Show Abstract
1 , University of Delaware, Newark, Delaware, United States, 2 , Cambridge NanoTech Inc., Cambridge, Massachusetts, United States
We use all-electrical methods to inject, transport, and detect spin-polarized electrons through an entire 350μm-thick undoped single-crystal silicon wafer. Spin precession measurements in a perpendicular magnetic field at different accelerating electric fields reveal high spin coherence with at least 8π precession angles. The magnetic-field spacing of precession extrema is used to determine the injector-to-detector electron transit time. These transit time values are associated with output magnetocurrent changes (from in-plane spin-valve measurements). Fitting the results to a simple exponential spin-decay model yields a conduction electron spin lifetime lower bound in silicon of over 200ns at 85K, and ~65ns at 150K.
5:00 PM - J2.6
Experimental Realization of a Silicon Spin Field-Effect Transistor.
Biqin Huang 1 , Douwe Monsma 2 , Ian Appelbaum 1 Show Abstract
1 Electrical and Computer Engineering Department, University of Delaware, Newark, Delaware, United States, 2 , Cambridge NanoTech Inc., Cambridge, Massachusetts, United States
A longitudinal electric field is used to control the transit time (through an undoped silicon vertical channel) of spin-polarized electrons precessing in a perpendicular magnetic field. Since an applied voltage determines the final spin direction at the spin detector and hence the output collector current, this comprises a spin field-effect transistor. An improved hot-electron spin injector providing about 115% magnetocurrent, corresponding to at least 38% electron current spin polarization after transport through 10μm undoped single-crystal silicon, is used for maximum current modulation.
Rolf Allenspach IBM Research
Christian H. Back Universitaet Regensburg
Bret Heinrich Simon Fraser University
J3: Spin Injection into GaAs and GaN
Tuesday AM, November 27, 2007
Republic A (Sheraton)
9:45 AM - J3.1
Transport Properties at the MgO/n-GaAs(001) Interface: Propositions for Improvement of Spin Injection.
Jean-Christophe Le Breton 1 , Philippe Schieffer 1 , Sylvain Le Gall 1 , Pascal Turban 1 , Bruno Lepine 1 , Yuan Lu 1 2 , Nicolas Bertru 3 , Guy Jezequel 1 Show Abstract
1 IPR- Equipe de Physique des Surfaces et Interfaces (EPSI), Université de Rennes 1, Rennes France, 2 , UMR CNRS/Thales, Palaiseau France, 3 Laboratoire d’Etude des Nano-Structures (LENS) , INSA de Rennes, Rennes France
Efficient injection of spin polarised current into semiconductors is essential for future magneto-electronics devices. This spin injection can be achieved by tunnel effect through a thin insulating barrier (I) intercalated between the ferromagnetic metal spin source (F) and the semiconductor (SC). Some model structures (MgO/GaAs , AlOx/Al(GaAs) , …) have qualitatively demonstrated high efficiency spin polarised current injection but the fundamental mechanisms governing the current injection in MIS (Metal/Oxide/Semiconductor) structures are not clearly understood so far. In this communication, we present a detailed study of the electrical and electronic properties of MgO thin layers (1 to 6nm) deposited on n-doped GaAs(001). We especially focus on the influence of the minority carriers on the transport properties of these tunnel structures.MgO layers are deposited under Ultra High Vacuum (UHV) or under low O2 pressure with a homemade Knudsen cell on GaAs(001). The GaAs buffer layers are n-doped and show a As(2x4) surface reconstruction. The RHEED (Reflection High Energy Electron Diffraction) patterns demonstrate a MgO barrier epitaxial growth with a “cube on cube” epitaxy relation: MgO(001)//GaAs(001). As observed by Transmission Electron Microscope the MgO films are continuous and the MgO/GaAs interface is fairly sharp.We analyze the transport properties across the MgO/GaAs(001) interface by I(V) and C(V) measurements on Au/MgO/GaAs(001) microstructures. Interface resistance values and bands alignment evolution in the polarized MIS structure were deduced from these experiments. We observe that the condition for efficient spin injection /detection as calculated by Fert et al. for a ferromagnet/semiconductor/ferromagnet structure  is not fulfilled. The MgO/n-GaAs interface contact resistance is indeed too large by 5 orders of magnitude. However we observe that this interface resistance value can be strongly reduced when the MgO tunnel barrier is grown on spin-Light Emitting Diode structures (n-AlGaAs/GaAs/p-AlGaAs). In that case, minority carriers are supplied by the p-doped region of the LED and accumulate at the MgO/GaAs interface. This accumulation of charges increases the voltage drop across the oxide barrier which causes the barrier to bend. Consequently the transmission coefficient of the bended MgO barrier is increased and the interface resistance approaches the theoretical optimal resistance for spin injection/detection. Finally we propose a possible heterostructure for spin injection through a n-type F/SC structure. X. Jiang et al. Phys. Rev. Lett. 94, 056601 (2005) Motsnyi, et al., Phys. Rev. B 68, 245319 (2003) A. Fert and H. Jaffres, Phys. Rev. B 64, 184420 (2001)
10:00 AM - **J3.2
Tuning the Sign and Sensitivity of Electrical Spin Detection and Injection in Lateral Fe/GaAs Devices.
Scott Crooker 1 , Darryl Smith 1 , Chris Palmstrom 2 , Paul Crowell 2 Show Abstract
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 , Univeristy of Minnesota, Minneapolis, Minnesota, United States
Together with electrical spin injection and manipulation, an all-electrical scheme for spin detection in semiconductors is an important and necessary component of many ‘semiconductor spintronic’ device proposals. In such devices, spin-dependent electrical signals are expected upon reversing the magnetization of a ferromagnetic detection electrode, or when manipulating the electron spin orientation in the underlying semiconductor. Using Fe/GaAs Schottky tunnel-barrier contacts on a lateral GaAs channel, we experimentally confirm spin-dependent electrical injection, transport, and detection using these two methods . More strikingly, we also demonstrate the ability to continuously tune both the magnitude and sign of the intrinsic spin-dependent electrical sensitivity, via the voltage bias applied across the Fe/GaAs detection electrode. Bias control of the sign and magnitude of electrical spin detection offers an alternative method of achieving functionality in real spin-transport devices.Using scanning magneto-optical techniques at low temperature , we first measure the spin polarization (P_GaAs) that is electrically injected into a lateral GaAs channel from a Fe/GaAs contact as a function of voltage bias. Both images and Hanle depolarization curves reveal that P_GaAs inverts sign when sweeping from small reverse bias (electrons flowing into GaAs) to small forward bias (electrons flowing into Fe), as expected from linear response. Surprisingly, P_GaAs also inverts sign again at higher bias across the Fe/GaAs contact. This crossover bias (< 0.1 V in the structures studied) can occur under either forward- or reverse-bias conditions, depending on sample. These data concur with recent all-electrical studies of P_GaAs in lateral spin transport devices having a source, drain, and a third ‘non-local’ detection electrode . Inverting the experiment and using the Fe/GaAs electrode as an electrical spin detector, we then show that both the sign and magnitude of its spin-sensitivity can be continuously tuned through zero in similar fashion with applied bias. The spin-detection sensitivity of some Fe/GaAs structures can be tuned by over an order of magnitude from the nominal zero-bias sensitivity. Although an understanding of this phenomenon is not yet complete, various theoretical models describing the effect will be discussed, and the data will be compared with related effects recently observed in all-metallic devices. This work is supported by the Los Alamos LDRD and NSF MRSEC programs, and ONR. X. Lou et al, Nature Physics v3, p197 (2007). S. A. Crooker et al, Science v309, p2191 (2005).
10:30 AM - J3.3
Spin Injection into Co2MnAl by Optical Absorption in GaAs.
Samih Isber 1 2 , Young Park 3 4 , Jagadeesh Moodera 4 , Don Heiman 1 Show Abstract
1 Physics, Northeastern University, Boston, Massachusetts, United States, 2 Department of Physics, American University of Beirut, Beirut Lebanon, 3 Nanodevice Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 4 Francis Bitter Magnet Laboratory, MIT, Cambridge, Massachusetts, United States
Transporting spin-polarized carriers between ferromagnetic metals and semiconductors with high efficiency is essential for semiconductor-based spintronic applications. With this goal in mind, we have made ferromagnet-semiconductor heterostructure devices containing half-metallic ferromagnets, which have larger spin polarizations than transition metal based ferromagnets. Spin polarized electrons generated in the semiconductor by circularly polarized light are injected across a Schottky barrier into the ferromagnet and detected as photocurrent. In this technique, the optically pumped semiconductor is the source of spin polarized electrons, and the ferromagnet is the detector of spin polarized electrons. It has been used to measure the efficiency of spin injection in transition metal based structures.[2,3]The heterostructures consist of MBE-grown Co2MnAl Heusler ferromagnets which are lattice matched to n+-GaAs, forming a Schottky barrier between the layers. Laser light having a photon energy slightly above the GaAs bandgap generates electrons in the semiconductor at room temperature. The circular polarization of the laser is modulated at a frequency of 50 kHz and the modulated photocurrent is detected by lock-in techniques. A magnetic field is applied parallel to the light and perpendicular to the ferromagnetic layer, allowing the magnetization to be switched from parallel to antiparallel to the light direction, thus switching the direction of the detected spin polarization.The spin-dependent photocurrent was measured for magnetic fields between B=-1.5 and +1.5T as a function of applied voltage bias across the Schottky barrier. At B=0 the magnetization direction lies in the plane of the ferromagnet, perpendicular to direction of the circularly polarized light and to the electron spin polarization, so the ferromagnet does not discriminate the spin polarized current. On the other hand, the perpendicular fields polarize the magnetization parallel to the electron spins, allowing the photocurrent to reverse sign. The spin polarized photocurrent was determined after subtracting the magnetic circular dichroism (MCD) effect. This research was supported by the National Science Foundation Grant, DMR-0305360. We thank the Arab Fund for Economic and Social Development for supporting S.I.===== A. Hirohata, Y.B. Xu, C.M. Guertler, and J.A.C. Bland, and S.N. Holmes, Spin-polarized electron transport in ferromagnet-semiconductor structures induced by photon excitation, Phys. Rev. B 63, 104425 (2001). S.J. Steinmuller, C.M. Gürtler, G. Wastlbauer, and J.A.C. Bland, Separation of electron spin filtering and magnetic circular dichroism effects in photoexcitation studies of hybrid ferromagnet/GaAs Schottky barrier structures, Phys. Rev. B 72, 045301 (2005). [3 Y.J. Park, M. van Veenhuizen, D. Heiman, C.H. Perry and J.S. Moodera, Efficient spin polarized charge transport in GaAs semiconductor, (to be published, 2007).
10:45 AM - J3.4
Electrically-induced Spin Coherence by Ultrafast Electrical Spin Injection.
Bernd Beschoten 1 , L. Schreiber 1 , J. Moritz 1 , C. Schwark 1 , S. Schulz 1 , M. Lepsa 2 , X. Lou 3 , C. Adelmann 4 , C. Palmstrøm 4 , P. Crowell 3 , G. Güntherodt 1 Show Abstract
1 II. Physikalisches Institut, and Virtual Institute for Spin Electronics (ViSel), RWTH Aachen University, Aachen Germany, 2 Institute of Bio- and Nanosystems (IBN-1), and Virtual Institute for Spin Electronics (ViSel), Research Centre Jülich GmbH, Jülich Germany, 3 School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota, United States, 4 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Electrical spin injection from a ferromagnet into a semiconductor has been demonstrated in various material systems [1,2]. High injection efficiency for electron spins has been achieved recently. Moreover, lateral spin transport devices with ferromagnetic Schottky contacts have been used to optically image emitted spin currents from the source contact and accumulated spins near the drain contact, respectively . Despite this progress, an essential ingredient for coherent spintronic devices is still missing: electrical injection of coherent spins packets. Here, we report on coherent electrical spin injection in devices consisting of a highly doped Schottky tunnel barrier formed between an epitaxial iron (Fe) and a (001)-oriented n-GaAs layer. Electron spin packets are electrically injected from the Fe layer through the Schottky barrier into n-GaAs by ultrafast current pulses. Spin coherence in the semiconductor is probed by subsequent spin precession in a transverse magnetic field using time-resolved Faraday rotation. We observe spin precession for current pulse widths down to 200 ps. The spin polarization of the spin packets is directly measured by Faraday rotation and is found to increase linearly with the current pulse width for pulses shorter than 3 ns at small magnetic fields. This finding together with independent measurements of the samples’ high frequency bandwidth indicate that even shorter than 200 ps pulses might be used for generating coherent spin currents in our devices. Work supported by BMBF, DFG and HGF. Y. Ohno et al. Nature 402, 790 (1999). H.J Zhu et al., Phys. Rev. Lett. 87, 016601 (2001). S.A. Crooker et al., Science 309, 2191 (2005).
11:30 AM - **J3.5
Spin-dependent Transport Across Fe/GaAs/Fe and Fe/GaAs/Au Tunneling Devices.
Dieter Weiss 1 Show Abstract
1 Experimentelle und Angewandte Physik, Universitaet Regensburg, Regensburg Germany
We investigated spin dependent transport across Fe/GaAs/Fe and Fe/GaAs/Au tunnel junctions. Both systems involve one Fe/GaAs interface which was grown epitaxially. In Fe/GaAs(001)/Fe devices we observed a pronounced TMR effect whose sign and size can be tuned by the applied bias voltage . This is ascribed to band structure effects at the epitaxial Fe/GaAs interface. Surprisingly, also in Fe/GaAs/Au tunneling structures the tunneling resistance depends on the magnetization direction of the iron layer. The observed two-fold anisotropy of the resistance is tunable by means of the applied bias voltage and the corresponding uniaxial axis can be swapped by 90° if the bias voltage is reversed. While the resistance changes are of order ~ 0.5 % at 4.2 K they change only little at elevated temperatures. The observed phenomena can be ascribed to the interplay of Rashba- and Dresselhaus-type of spin-orbit interaction  experienced by the tunneling electrons. A phenomenological model allows us to extract the bias dependent Rashba coupling constant at the epitaxial Fe/GaAs interface from experiment.
Work was done in collaboration with J. Moser, M. Zenger, C. Gerl, D. Schuh, R. Meier, P. Chen, G. Bayreuther, W. Wegscheider, C.-H. Lai, R.T. Huang, M. Kosuth, H. Ebert, A. Matos-Abiague, and J. Fabian. Support by the German Science Foundation (DFG) via SFB 689 is gratefully acknowledged.
 J. Moser, M. Zenger, C. Gerl, D. Schuh, R. Meier, P. Chen, G. Bayreuther, W. Wegscheider, C.-H. Lai, R.T. Huang, M. Kosuth, H. Ebert, and D. Weiss, Appl. Phys. Lett. 89, 162106 (2006)
 J. Moser, A. Matos-Abiague, D. Schuh, W. Wegscheider, J. Fabian, and D. Weiss, condmat/0611406
12:00 PM - J3.6
Spin-orbit Coupling in AlGaN/GaN 2-dimensional Electron Gases and Quantum Wires.
Thomas Schaepers 1 , Patrick Lehnen 1 , Nicolas Thillosen 1 , Nicoleta Kaluza 1 , Vitaliy Guzenko 1 , Hilde Hardtdegen 1 Show Abstract
1 Institute of Bio- and Nanosystems (IBN-1), Center of Nanoelectronic Systems for Information Technology (CNI), Virtual Institute of Spin Electronics (VISel), Research Centre Jülich, Jülich Germany
Despite the fact that GaN is a large band gap material, a relatively strong spin-orbit coupling was observed in AlGaN/GaN two-dimensional electron gases (2DEGs) . Spin-orbit coupling is one of the essential ingredients for the design of spin electronic devices, since it allows the manipulation of the spin orientation by means of spin precession. Information on the strength of spin-orbit coupling can be gained from weak antilocalization measurements . Here, quantum mechanical interference effects result in a distinct peak in the magnetoconductance at zero magnetic fields. In the present study we focused on the question how spin-dependent transport phenomena are modified if the dimension of the sample is changed from a 2DEG to wire structures.Our AlGaN/GaN heterostructures were grown on a (0001) sapphire substrate by metal-organic vapor phase epitaxy. Sample 1 consisted of a 3-µm-thick GaN layer followed by a 35-nm-thick AlGaN (20% Al) top layer, while in sample 2 a 40-nm-thick AlGaN (10% Al) layer was used as a top layer. Hall bar structures were prepared by optical lithography for the measurements on the 2DEGs, while electron beam lithography was used to define the quantum wire structures. Sets of 160 identical quantum wires connected in parallel with widths ranging from 1110 nm to 340 nm were investigated. All measurements were performed at temperatures below 1 K. Owing to the higher Al-content of the barrier layer in sample 1, the carrier concentration was more than twice as large as the one for sample 2, being 5.1x1012 cm-2 and 2.2x1012 cm-2, respectively. The magnetoconductance measurements of both 2DEG structures revealed a pronounced weak antilocalization effect. From a fit using the model of Iordanskii, Lyanda-Geller and Pikus  a relative short spin-orbit scattering length of about 300 nm was extracted for both samples. Regarding the quantum wire structures we only observed a weak antilocalization effect for wires wider than 700 nm. In contrast, for narrower wires only weak localization, indicated by a dip in the magnetconductance, was observed. By fitting our experimental curves to a model describing the transport in quasi one-dimensional structures , we found that the spin-orbit scattering length increases significantly with decreasing wire widths, i.e. by more than a factor of 3 for sample 1. Our finding might have important implications for the design of spin electronic devices using channels with reduced dimensions.  N. Thillosen, Th. Schäpers, N. Kaluza, H. Hardtdegen, V. A. Guzenko, Appl. Phys. Lett. 88, 022111 (2006). S. V. Iordanskii, Yu. B. Lyanda-Geller, G. E. Pikus, JETP Letters 60, 206 (1994). S. Kettemann, Phys. Rev. Lett. 98, 176808/1 (2007)
12:15 PM - **J3.7
Measurement of Rashba and Dresselhaus Spin-Orbit Magnetic Fields.
Gian Salis 1 , Lorenz Meier 1 2 , Ivan Shorubalko 2 , Emilio Gini 3 , Silke Schoen 3 , Klaus Ensslin 2 Show Abstract
1 Zurich Research Laboratory, IBM Research, 8803 Rüschlikon Switzerland, 2 Solid State Phyiscs Laboratory, ETH Zürich, 8093 Zürich Switzerland, 3 FIRST Center for Micro- and Nanosciences, ETH Zürich, 8093 Zürich Switzerland
A moving electron experiences an electric field as a magnetic field that interacts with its spin. This spin-orbit coupling can be used for spin manipulation in semiconductor structures. It has its origin in either bulk inversion asymmetry (BIA) or structural inversion asymmetry (SIA), and the corresponding coupling strengths are referred to as Dresselhaus and Rashba terms. We report on simultaneous measurements of the spin-orbit magnetic fields BSIA and BBIA in a two-dimensional electron gas confined in a GaAs/InGaAs quantum well. For this, the spin precession frequency of optically generated electron spins is monitored using time-resolved Faraday rotation. The electrons are brought into an in-plane oscillatory motion by an a.c. electric field oriented at an angle φ to the crystal's -direction, and an external magnetic field is applied at an angle θ. By analyzing the dependence of the spin precession frequency on φ and θ, the Dresselhaus and Rashba terms are extracted. Furthermore, we use the electrically-generated spin-orbit fields as tipping fields for electron-spin resonance and measure time-resolved Rabi precession of the QW electron spins.
J4: Graphene and Organic Semiconductors
Tuesday PM, November 27, 2007
Republic A (Sheraton)
2:30 PM - **J4.1
Electronic Spin Transport And Spin Precession In Single Graphene Layers At Room Temperature.
Bart vanWees 1 , Mihai Popinciuc 1 Show Abstract
1 , Zernike Institute for Advanced Materials, AG Groningen Netherlands
3:00 PM - J4.2
Spin Injection into Graphene Thin Films at Room Temperature.
Ryo Nouchi 1 , Megumi Ohishi 1 , Masashi Shiraishi 1 , Takayuki Nozaki 1 , Teruya Shinjo 1 , Yoshishige Suzuki 1 Show Abstract
1 Graduate School of Engineering Science, Osaka University, Toyonaka Japan
Graphene , an atomically flat layer of carbon atoms packed into a two dimensional (2D) honeycomb lattice, is currently one of the hottest topics in materials science and condensed-matter physics. A graphene thin film (GTF) formed from a small number of stacked layers of graphene behaves as a 2D conductor. A GTF exhibits gate-voltage-controlled carrier conduction and high field-effect mobilities  and it consists of only light elements (carbon atoms), which induce a small spin-orbit interaction. Therefore, it has potential to become a pivotal material for establishing a new research field of molecular spintronics in which the polarized spin current can be controlled not only by a magnetic field and a bias voltage but also by a gate voltage. In this study, we fabricated a lateral spin-valve device using a GTF and achieved spin injection from a ferromagnetic (FM) electrode into the GTF even at room temperature (RT).Graphene spin devices were fabricated by the following process. First we peeled flakes of graphite off from a bulk one by using adhesive tape, and pushed the flakes onto a surface of a SiO2/Si substrate. When we removed the tape, GTFs were absorbed onto the surface by van der Waals force. Then, non-magnetic electrodes (Au/Cr) and FM electrodes (Co) were fabricated onto the GTF by electron beam lithography. It should be noted that the absorption force between the GTF and the SiO2 was sufficiently strong that the GTF did not detach from the substrate in the lithographic processes. The widths of the two FM electrodes were different in order to generate different coercive forces (700 nm and 1000 nm). The gap width between FM electrodes was 200 nm. We employed a so-called ac lock-in non-local four-terminal measurement scheme at RT , which is able to separate a spin current path from a charge current path. Using this scheme, we can distinguish a spin-injection signal from any spurious signals such as anisotropic magnetoresistance. Depending on the magnetization directions of the FM electrodes, the non-local resistance was changed from 0.718 Ω to 0.720 Ω and displayed clear hysteresis loops in upward and downward sweeps of the magnetic field, when the injected electric current was set to be 100 μA. The observed variation in the non-local resistance clearly shows spin injection into the GTF at RT . In the presentation, we will report the dependence of the spin-injection signals on contact resistances at FM/GTF interfaces and on physical properties of GTFs. K. S. Novoselov et al.: SCIENCE 306 (2004) 666. F. J. Jedema et al.: Nature 416 (2002) 713. M. Ohishi et al.: Jpn. J. Appl. Phys. 46 (2007) L605.
3:15 PM - J4.3
Graphene Spin Transistor.
Michael Fuhrer 1 , Sungjae Cho 1 , Yung-Fu Chen 1 Show Abstract
1 Department of Physics and Center for Superconductivity Research, University of Maryland, College Park, Maryland, United States
We report spin-transport experiments on graphene contacted by ferromagnetic Permalloy electrodes in the non-local four-probe geometry. We observe sharp switching and often sign reversal of the non-local resistance at the coercive field of the electrodes, indicating definitively the presence of a spin current between injector and detector. The non-local resistance changes magnitude and sign quasi-periodically with back-gate voltage, and Fabry-Pérot-like oscillations are observed, consistent with quantum-coherent transport. The non-local resistance signal can be observed up to at least T = 300 K.
4:00 PM - J4.4
Preparation of Lateral Organic Spin-valve Devices with La0.7Sr0.3MnO3.
Tomonori Ikegami 1 , Iwao Kawayama 2 , Masayoshi Tonouchi 2 , Yoshiro Yamashita 3 , Hirokazu Tada 1 4 Show Abstract
1 Materials Physics, Osaka University, Toyonaka Japan, 2 Institute of Laser Engineering, Osaka University, Suita Japan, 3 , Tokyo Institute of Technology, Yokohama Japan, 4 , JST-CREST, Kawaguchi Japan
There is growing interest in spin injection into organic materials from ferromagnetic (FM) electrodes. Spin-valve characteristics were demonstrated for layered and lateral type sandwich structures composed of FM electrodes and organic materials. Layered structures using various organic films including Alq3, poly-3-hexylthiophene, and tetraphenyl porphyrin as spacers exhibited spin valve characteristics with a magnetoresistance (MR) ratio up to 40% at low temperature. As for lateral type devices, it has been reported that spins are injected and transported in carbon nanotubes bridging over 1μm gap FM electrodes. In the present work, we have studied spin injection and transport characteristics of various organic semiconductors such as pentacene and bis(l,2,5-thiadiazolo)-p-quinobis(l,3-dithiole)(BTQBT) utilizing lateral type spin-valve devices with half metal electrodes, La0.7Sr0.3MnO3(LSMO). The LSMO electrodes with a spacing from 50nm to 300 nm were prepared by electron-beam lithography and dry etching of epitaxial films grown on MgO. The devices showed clear spin-valve behaviors with a MR ratio up to 29 % at 5K. It was found that the MR ratio varied depending upon temperature, the gap spacing of the electrodes, bias voltage applied, and crystallinity of organic films. It was also affected by gas adsorption onto organic semiconductors. This work demonstrates the potentials of organic materials for application to spintronics devices.
4:15 PM - J4.5
Magnetoresistance in Spin Valves Based on Organic Semiconductors with Ferromagnetic Electrodes
Hirokazu Tada 1 3 , Motoyasu Kakita 1 , Tomonori Ikegami 1 , Iwao Kawayama 2 , Masayoshi Tonouchi 2 Show Abstract
1 Division of Meterials Physics, Osaka University, Toyonaka Japan, 3 CREST, Japan Science and Technology Agency, Kawaguchi Japan, 2 Institute of Laser Engineering, Osaka University, Suita Japan
Spin injection and transport in organic materials have recently attracted considerable attention. It has been demonstrated that sandwich structures composed of carbon nanotubes, Alq3 (tris-(8-hydroxyquinoline) aluminum), and P3HT (poly-3-hexylthiophene) with ferromagnetic electrodes such as La0.7Sr0.3MnO3 (LSMO), Co, and NiFe, showed spin-valve characteristics. Magnetic field effect onto electrical and optical properties has also been reported in organic light-emitting diodes with non-magnetic metal electrodes. In these devices, the interface between electrodes and organic materials plays a critical role to determine the spin-dependent properties.In the present work, we have prepared layered structures of Co/organic/LSMO with various materials such as Alq3, pentacene, C60 and phthalocyanine. The devices showed clear spin-valve characteristics with magentoresistance (MR) ratios of about 10 % at 5K. The MR ratios decreased with increasing temperature, bias voltages applied and film thickness. The devices with Alq3, C60 and phthalocyanine showed inverse MR behaviors in which the resistance decreased at zero magnetic field, while those with pentacene showed normal MR behaviors. The electronic structure of electrode/organic interfaces affects strongly to spin injection properties.
4:30 PM - J4.6
Spin Control Of Charge Transfer States In Organic Semiconductors.
Michael Segal 1 , Seth Difley 2 , Troy Van Voorhis 2 , Marc Baldo 1 Show Abstract
1 EECS, MIT, Cambridge, Massachusetts, United States, 2 Chemistry, MIT, Cambridge, Massachusetts, United States
Excitons localized to single molecules, and charge transfer (CT) states spread over adjacent molecules, possess either singlet or triplet spin symmetry. Both states exhibit very long decoherence times because localization enhances exchange interactions and lifts the spin state degeneracy. Consequently, the absorption and emission of light in organic semiconductors is spin dependent. Triplet excitons have lower energy than singlet excitons, but we demonstrate that this ordering may be inverted in the CT state. In the archetypal organic semiconductor, tris(8-hydroxyquinoline) aluminum we find a splitting of -(7 +- 3) meV , which exceeds the typical spin-orbit mixing interaction energy. Thus, singlet-triplet CT state mixing is usually negligible. We further show that it can be turned on by selectively engineering the spin orbit coupling of the CT and exciton states. We demonstrate an approximately three-fold increase in the quantum efficiency of a fluorescent organic light emitting device (OLED). Knowledge of the CT state spin energy ordering, and the ability to control CT state spin, may facilitate the use of long-lived CT states in spintronic devices.  M Segal, M. Singh, K. Rivoire et al., "Extrafluorescent Electroluminescence in Organic Light Emitting Devices," Nature Materials 6, 374-378 (2007).