Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L15: Focus Session: Spins in Semiconductors - Spin Torque and Spin Injection |
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Sponsoring Units: DMP GMAG FIAP Chair: Roberto Myers, Ohio State University Room: D171 |
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L15.00001: Spin Torque Switching in GaMnAs Magnetic Tunnel Junctions Lin Xue, R. A. Buhrman, D.C. Ralph, D.W. Rench, M.J. Wilson, P. Schiffer, N. Samarth We have fabricated and measured submicron magnetic tunnel junctions made from GaMnAs multilayers: GaMnAs/GaAs/GaMnAs/MnAs, where GaMnAs is a ferromagnetic semiconductor, GaAs serves as the tunneling barrier, and MnAs is a ferromagnet that provides an exchange bias to the upper GaMnAs (reference) layer. The devices have magnetoresistances of order 50{\%} at 4.2 K and exhibit clear spin-torque switching of the lower GaMnAs layer between parallel and antiparallel orientations relative to the reference layer. We report the switching phase diagram as a function of current and magnetic field. We also describe efforts to probe the high-speed magnetic dynamics in GaMnAs driven by spin torque from ns-scale current pulses and microwave-frequency currents that can drive ferromagnetic resonance. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L15.00002: Theory of current-induced torque in uniform ferromagnets Karel Vyborny, Liviu Zarbo, T. Jungwirth In a model ferromagnetic semiconductor (Ga,Mn)As with strong spin-orbit interaction, it has been experimentally shown that magnetization can be manipulated by injection of unpolarized currents [Chernyshov et al., Nat. Phys. 5, 656 (2009)]. We critically review the existing theoretical approaches to this phenomenon and present a model whose results are compared to more recent measurements of the current-induced torque driven by ferromagnetic resonance in (Ga,Mn)As. The results entail the dependence of the effect on carrier concentration, various types of strain, and temperature. [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L15.00003: Intrinsic spin-orbit coupling effects on spin and charge pumping in magnetic tunnel junctions with microwave-driven precessing magnetization Farzad Mahfouzi, Branislav Nikolic, Naoto Nagaosa We develop a microscopic quantum transport approach to the problem of spin pumping by precessing magnetization in one of the ferromagnetic layers within FIF or FIN (F-ferromagnet; N-normal metal; I-insulating barrier) magnetic tunnel junctions (MTJs) in the presence of intrinsic spin-orbit couplings (SOC) at the FI interface. Our approach evaluates the nonequilibrium Green functions (NEGFs) by starting from the time-dependent Hamiltonian of these junctions. To express the time-averaged charge current, or the corresponding dc pumping voltage in open circuits that was measured in recent experiments on MTJs [T. Moriyama {\em et al.}, Phys. Rev. Lett. {\bf 100}, 067602 (2008)], we construct a novel solution for the double-time-Fourier-transformed NEGFs where their two energy arguments are connected by the Floquet theorem describing emission and absorption of finite number of photons. Within this fully quantum-mechanical treatment of the conduction electrons, we find that only in the presence of the interfacial Rashba SOC non-zero dc pumping voltage in F$|$I$|$N junctions emerges at the adiabatic level (i.e., proportional to microwave frequency). [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L15.00004: Manipulation of ferromagnetic state by means of spin-orbit interactions Invited Speaker: The current state of information technology accentuates the dichotomy between processing and storage of information, with logical operations performed by charge-based devices and non-volatile memory based on magnetic materials. The major obstacle for a wider use of magnetic materials for information processing is the lack of efficient control of magnetization. Reorientation of magnetic domains is conventionally performed by non-local external magnetic fields or by externally polarized currents. Efficiency of the latter approach is greatly enhanced in materials where ferromagnetism is carrier-mediated. In such materials control of carriers' polarization provides an alternative mean to manipulate orientation of magnetic domains. In some crystalline conductors the charge current couples to spin via intrinsic spin-orbit (SO) interactions and generates electron spin polarization. Unlike the Oersted field, the SO-induced polarization is spatially localized and can be controlled by local electric fields. This non-equilibrium electron spin polarization couples to magnetic moments of magnetic ions and is capable of controlling magnetization of the ferromagnet. We show that magnetization can be reversibly manipulated by SO-induced polarization of carrier spins generated by the injection of unpolarized currents. We demonstrate domain rotation and hysteretic switching of magnetization between two orthogonal easy axes in a model ferromagnetic semiconductor GaMnAs. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L15.00005: Ferromagnetic resonance driven by current-induced torque in uniform ferromagnetic micro-structures H. Kurebayashi, D. Fang, J. Wunderlich, K. Vyborny, L.P. Zarbo, R.P. Campion, A. Casiraghi, B.L. Gallagher, T. Jungwirth, A.J. Ferguson We show that the recently demonstrated current induced torque (CIT) [A. Chernyshov, et al. Nat. Phys. 5, 656 (2009)] can excite magnetisation dynamics in micro-bars of uniform ferromagnetic semiconductors. Due to the combined effects of the spin-orbit and exchange interactions, a microwave current injected into (Ga,Mn)As (or (Ga,Mn)(As,P)) micro-bars generates an oscillating effective magnetic field. By using a sensitive electrical detection technique, we accurately measure the magnitude and direction of driving fields for samples under different strain. We confirm the observation of a field with the symmetry of the Dresselhaus spin-orbit interaction and observe an additional field with the symmetry of the Rashba spin-orbit interaction. Our work demonstrates a new scalable FMR technique which provides a sensitive method to study the nature of current-induced torques and to perform magnetic characterisation of uniform ferromagnetic micro-structures. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L15.00006: Non-local spin transport devices with a tunable channel J. Misuraca, J.-I. Kim, P. Xiong, S. von Molnar, K.K. Meng, J. Lu, J.H. Zhao The spin lifetime in GaAs is known to vary strongly with carrier density near the metal to insulator transition [1]. However, a detailed study to optimize this lifetime is complicated because many replica samples need to be made and measured. This difficulty can be circumvented by employing Si:Al$_{0.3}$Ga$_{0.7}$As, a persistent photoconductor, as the spin transport medium. This material, which is structurally similar to GaAs, has been characterized and shown to have an effective carrier density which can be tuned \textit{in situ} via photo-excitation from 10$^{14}$ to 10$^{18}$ cm$^{-3 }$[2]. Heterostructures (2-$\mu $m Si:Al$_{0.3}$Ga$_{0.7}$As, a thin epitxial Fe layer, and a GaAs graded junction to create linear contacts between them) have been grown by MBE and non-local spin devices have been patterned by photolithography and wet etching. Magnetic measurements on Fe micro-patterns demonstrated the possibility of controlling the coercivity of the Fe electrodes [3]. Electrical characterization of the devices will be presented. [1] J. Kikkawa et al., Phys. Rev. Lett. 80, 4313 (1998) [2] J. Misuraca et al., Phys. Rev. B. 82, 125202 (2010) [3] K. K. Meng et al., Appl. Phys. Lett. 97, 072503 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L15.00007: Electron Charge and Spin Transport in Ferromagnet/Semiconductor Schottky Barrier Heterostructures Qi Hu, Chris Palmstrom, Eric Garlid, Chad Geppert, Paul Crowell Previous studies of Fe/$n^+$-GaAs/$n$-GaAs heterostructures in the lateral non-local geometry have shown that spin accumulation is observed only when tunneling current across ferromagnet/semiconductor Schottky barrier is sufficiently large. The tunneling mechanism is modeled by band diagram simulations and transport calculation using the WKB approximation for the barrier. These suggest that tunneling from localized states in the quantum well (QW) which forms just inside the Schottky interface dominates over tunneling directly from the semiconductor bulk. Electron tunneling spectroscopy is utilized to probe the predicted localized states in the QW. Valleys in the derivative of tunneling conductance spectra were observed at discrete forward bias voltages which are attributed to localized 2-dimensional energy states. The spin escape time of the 2D energy states in the QW is calculated using a spin-dependent WKB approximation. The results are compared with the spin lifetime and escape time extracted from three-terminal spin Hanle measurements. This work was supported by the NSF MRSEC and ONR MURI programs, and NSF DMR-0804244. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L15.00008: Dependence of spin-injection spectra of CoFe/GaAs contacts on temperature and annealing conditions Gian Salis, Andreas Fuhrer, Santos F. Alvarado Spin injection from CoFe contacts into bulk GaAs epilayers is studied experimentally. Close to the metal/semiconductor interface the GaAs epilayer is highly n-doped, allowing efficient spin injection through the Schottky tunnel barrier. Spin polarization in the GaAs channel is measured as a non-local voltage at CoFe detection contacts. Similar to spin injection from Fe contacts, an inversion of the sign of injected spin polarization is found at a finite forward bias $U_c$ applied to the injection contact. We investigate the dependence of the nonlocal signal on $U_c$. From the data, the spin polarization of the differential interface conductance is obtained, providing spectral information on the spin-polarized density of states. The dependence of these spectra on measurement temperature as well as on annealing and growth conditions is discussed and compared to samples with Fe injection contacts. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L15.00009: Electrical Measurements of the Extrinsic Spin Hall Effect in Fe/In$_{x}$Ga$_{1-x}$As Heterostructures Chad Geppert, Eric Garlid, Mun Chan, Paul Crowell, Qi Hu, Chris Palmstr{\O}m We report on all-electrical measurements of the extrinsic spin Hall effect in Fe/In$_{x}$Ga$_{1-x}$As heterostructures with $n$-type channel doping (Si) and highly doped Schottky tunnel barriers. The spin Hall effect refers to the transverse spin current generated by application of a longitudinal unpolarized charge current. Complementary spin accumulation at opposing edges of the channel is detected via a Hanle effect in the voltage measured by pairs of ferromagnetic Hall contacts. The spin Hall conductivity is extracted by fitting the data to a drift-diffusion model incorporating spin precession and relaxation. Tuning the channel conductivity with applied bias allows the skew and side-jump contributions to be determined independently. The resulting magnitude is in agreement with models based on ionized impurity scattering. Further quantitative comparison to theoretical models is achieved by increasing the In concentration beyond previously reported values. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L15.00010: Ab initio Investigation of the Failure of Efficient Spin-Injection in Fe/GaAs Superlattices Sinead Griffin, Nicola Spaldin Magnetic metal-semiconductor systems have been widely studied for use as spintronic devices. The injection of elemental ferromagnetic Fe into GaAs shows great device potential because of the relativity high Curie temperature of Fe compared to other possible injection materials. However, spin-injection in Fe/GaAs has not been successful with several phases such as FeAs and Fe$_{2}$As forming at the interface. We perform Density Functional Theory calculations on bulk FeAs and Fe$_{2}$As to elucidate the structural and magnetic ground states. We then incorporate these Fe-As layers into GaAs/FeAs superlattices and investigate the resulting structures. Both the effects of Fe content and the number of layers in the heterostructure on the magnetic and electronic properties are considered. Our results show that the magnetic ground state of the FeAs compounds helps to explains the failure of spin-injection in these superlattices. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L15.00011: Lateral spin injection and detection through electrodeposited Fe/GaAs interfaces Sarmita Majumder, Anthony Arrott, Karen Kavanagh, Anthony Spring Thorpe We report results on spin injection and detection through epitaxial, electrodeposited Fe/GaAs tunnel barriers formed ex-situ on epitaxially grown GaAs (001). The BCC-Fe ?lms are predominantly single crystalline with large mosaic spread [1]. Tunnel junctions, fabricated with bulk GaAs wafers, and epitaxially grown MBE or MOCVD GaAs (001), showed the expected increase in tunneling current with increasing surface Si dopant concentration. Spin transport through in situ coherently strain, MBE Fe/GaAs interfaces have been reported at spin polarization levels as high as 42{\%} at 50K.[3]. In our experiments the design of the epitaxially grown GaAs substrates followed those used successfully for in situ MBE Fe spin contacts. [2]. A spin voltage (4 mV) has been detected for 2x10$^{18}$/cm3 doped tunnel junctions at liquid nitrogen temperatures (77 K) using an injection current of 20 A/cm2 while varying the applied in-plane magnetic ?eld ($\pm $ 300 Gauss) along a $<$100$>$ easyaxis of the Fe contacts. 1. Z. L. Bao, S. Majumder, A. A. Talin, A. S. Arrott, K. L. Kavanagh, JES 155 (2008) H841. 2. X. Lou, C. Adelmann, A. S. Crooker, E. S. Garlidi, J. Zhang, K. S. Reddy, S. D. Flexner, C. J. Palmstr{\o}m, and P.A. Crowell, Nature Phys. 3 (2007) 197. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L15.00012: All-electrical spin injection and detection in an AlGaN/GaN two-dimensional electron gas D.R. Hoy, Y. Pu, S.D. Carnevale, E. Johnston-Halperin, R.C. Myers Materials with low spin-orbit coupling, including wide band gap semiconductors, may allow practical semiconductor spintronics. Here we investigate all-electronic spin injection and detection using ferromagnetic Fe electrodes on a polarization doped AlGaN/GaN two-dimensional electron gas (2DEG) grown by molecular beam epitaxy. The ultrathin AlGaN cap provides polarization doped electrons and serves as a thin tunnel barrier for spins. The surface morphology is characterized by atomic force microscopy and the electron density, resistivity, and mobility are characterized by Hall measurements. Through the Hanle effect, we investigate the dependence of the spin injection efficiency and spin lifetime with temperature and bias. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L15.00013: Studies of spin injection into thin film InSb from CoFe Yong-Jae Kim, R.L. Kallaher, J.J. Heremans Spin-based electronics requires manipulation of spin-polarized carriers in materials. The narrow gap semiconductor InSb is a promising material for spin-based devices due to its strong spin-orbit interaction, allowing spin manipulation using electric fields. Yet, spin injection from spin-polarized electrodes into InSb has not yet been demonstrated. In order to electrically characterize spin injection and detection in InSb, we use InSb/CoFe lateral spin valve geometries studied at low temperatures and in tilted magnetic fields. The geometries are fabricated by depositing two non-epitaxial CoFe electrodes at mesoscopic separations on high-mobility InSb thin films through an insulator window. The anisotropy of the ferromagnetic electrodes provides parallel and anti-parallel configurations. We have observed two-state non-local output voltages, which are consistent with a spin injection signal. Interestingly, the switching signal is very sensitive to temperature in the range studied. The switching signal appears at low external fields due to CoFe magnetic anisotropy effects. The results are discussed in the light of the CoFe anisotropy and expected spin-coherence properties of InSb (partial support from DOE DE-FG02-08ER46532). [Preview Abstract] |
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