Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A10: Focus Session: Spin Transport Devices |
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Sponsoring Units: DMP GMAG Chair: Roland Kawakami, UC Riverside Room: LACC 153B |
Monday, March 21, 2005 8:00AM - 8:12AM |
A10.00001: Magnetooptical Studies of Organic/Ferromagnetic Hybrid Structures Roland Kawakami, Richard Chiang, Ramesh Thamankar Organic semiconductors (OS) are desirable for spin-based devices because of their long spin coherence times due to low spin-orbit coupling. The use of ferromagnets (FM) to inject and detect spin polarization in OS has recently been demonstrated through experiments on organic spin valves (i.e. FM/OS/FM trilayer devices). An important issue for these devices is the spin-dependent properties of the FM/OS interface. To investigate this issue, layered FM/OS hybrid structures are fabricated using molecular beam epitaxy (MBE). Ultrathin single-crystalline Co films on Cu(100) substrate serve as the model FM layer, whose magnetic properties are characterized by in situ magneto-optic Kerr effect (MOKE). The structural properties of the Co film are characterized by RHEED and STM. Subsequently, wedged OS overlayers (e.g. Alq$_{3}$, Gaq$_{3})$ are deposited onto the FM film and in situ optical studies are performed. First, the effect of the OS on the magnetic properties of the Co layer is investigated using MOKE. Second, the effect of the FM layer on photo-excited carriers in the OS is investigated using polarization-resolved photoluminescence. [Preview Abstract] |
Monday, March 21, 2005 8:12AM - 8:24AM |
A10.00002: Spin-Valves Incorporating Magnetic and Nonmagnetic Organic Semiconductors J.D. Bergeson, R. Shima Edelstein, K.I. Pokhodnya, A.J. Epstein, Joel S. Miller A clear spin-valve effect is reported for stacked thin film devices constructed of two ferromagnets with differing coercivities, iron (Fe) (100nm) and iron-cobalt (Fe$_{50}$:Co$_{50})$ (30nm), which are magnetically decoupled by a layer of $\alpha $-sexithiophene ($\alpha $6T) (120nm). Coherent spin transport is expected to be facilitated by relatively low spin orbit coupling in $\pi $-conjugated materials$^{1-3}$. Spin-injection is aided by tunnel barriers at metal/semiconductor interfaces. A spin-valve effect of up to 20{\%}, with switching at the expected coercive fields, is observed at 4.5K and the effect persists up to 150K. The conduction electrons in vanadium tetracyanoethylene (V[TCNE]$_{x})$, an organic-based magnetic semiconductor with T$_{C}>$ 350K, are fully spin-polarized$^{4}$. In addition to the low field `conventional' spin-valve switching from 10 to 100K, an unusual background high field magnetoresistance is reported for the spin-valve device structure where $\alpha $6T (50nm) is the nonmagnetic spacing layer between V[TCNE]$_{x}$ ($<$1um), and cobalt (Co) (25nm). Supported by DOE Grant No. DE-FG02-01ER45931 and DARPA (ONR Grant No. N00014-02-1-0593). 1. Dediu, et al., Solid State Comm. \textbf{122} 181 (2002) 2. Epstein, MRS Bull. \textbf{28} 492 (2003) 3. Xiong, et al., Nature \textbf{427} 821 (2004) 4. Prigodin, et al., Adv. Mater. \textbf{14} 1230 (2002), Raju, et al., J. Appl. Phys. \textbf{93} 6799 (2003) [Preview Abstract] |
Monday, March 21, 2005 8:24AM - 8:36AM |
A10.00003: Modification of ferromagnetism in semiconductors by molecular monolayers Itai Carmeli, Francisco Bloom, Cheyne Scoby, Elisabeth Gwinn, Ted Kreutz, Ron Naaman, Art Gossard We report that adsorption of monolayers of organic molecules onto ferromagnetic semiconductor heterostructures can produce large changes in magnetic properties [1]. The digital-alloy heterostructures studied have 1/2 monolayer MnAs planes embedded in GaAs. We investigate effects on magnetic properties of self- assembly of various organic molecules onto the heterostructure surface. Depending on the molecular structure, the monolayers can either strengthen or suppress ferromagnetism. We attribute this chemical modulation of magnetic properties to electronic changes brought about by molecular binding to the semiconductor surface. \newline \newline [1] T.C. Kreutz, R. Artzi, E.G. Gwinn, R. Naaman, H. Pizem, C.N. Sukenik and A.C. Gossard, Applied Physics Letters 83, 4211(2003). [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 9:12AM |
A10.00004: Spin Transport in Organic Semiconductors Invited Speaker: Spin injection/detection and coherent transport are necessary ingredients in spin electronics or spintronics.~ Organic semiconductors are believed to have long spin coherence due to the weak spin-orbit interaction and hyperfine interaction; therefore, may be useful in spintronic device applications. Recently, we have successfully achieved electrical spin injection/detection and demonstrated coherent spin transport~in spin valve devices using an organic semiconductor spacer.~ The devices consist of LSMO and Co ferromagnetic electrodes and small molecule material Alq3 spacer.~ A large inverse spin valve magnetoresistance (up to 40{\%}) was observed in these devices, which entails coherent spin transport in organic semiconductors. In addition to the spin valve magnetoresistance effect at low fields, we have also found magnetoresistance and magneto-eletroluminescence at high magnetic fields. This latter high-field effect is due to the field-dependent carrier injection at the ferromagnetic/organic interfaces. It is the first experimental evidence of the anomalous chemical potential shift theoretically predicted for double exchange ferromagnets such as LSMO.~ In collaboration with Z.H. Xiong, D. Wu, and Z.V. Vardeny; work supported by DARPA, NSF, and DOE. [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:24AM |
A10.00005: Magnetoresistance Anomalies Across Domain Walls in Tensile Strained (Ga,Mn)As G. Xiang, A.W. Holleitner, B.L. Sheu, F.M. Mendoza, O. Maksimov, P. Schiffer, D.D. Awschalom, N. Samarth We describe measurements of the anomalous Hall effect (AHE), planar Hall effect and anisotropic magnetoresistance (AMR) in tensile-strained (Ga,Mn)As epilayers with relatively high Curie temperatures ($125 \rm{K} < T_{\rm{C}} < 135 \rm{K}$). Samples are grown on a strain-relaxed (Ga,In)As buffer layer deposited on (001) GaAs, creating an in-plane tensile strain that orients the easy axis of the magnetization along [001]. We measure magnetoresistance as a function of the magnetic field vector $\vec{H}$ and temperature ($4.2 \rm{K} < T < 150 \rm{K}$) using Hall bars oriented along $[110]$,$[1 \overline{1} 0]$ and $[100]$. AMR measurements reveal striking antisymmetric resistance anomalies as we sweep either the magnitude or angle of $\vec{H}$. These anomalies originate in a strong AHE contribution to the AMR when measurements are made across domain walls in the presence of slight sample misorientation, providing a sensitive probe of the nucleation and propagation of magnetic domain walls up to temperatures as high 120K. Work supported by DARPA, ONR and NSF. [Preview Abstract] |
Monday, March 21, 2005 9:24AM - 9:36AM |
A10.00006: Hysteretic resistance spikes in magnetic 2DEGs J. Carlos Egues, Henrique Freire We use spin-density-functional theory to study recently reported hysteretic magnetoresistance $\rho_{xx}$ spikes in Mn-based 2D electron gases [Jaroszy\'{n}ski \textit{et al.} Phys. Rev. Lett. \textbf{89}, 266802 (2002)]. We find hysteresis loops in our calculated Landau fan diagrams and total energies signaling quantum-Hall-ferromagnet phase transitions. Spin-dependent exchange-correlation effects are crucial to stabilize the relevant magnetic phases arising from \emph{distinct }symmetry- broken excited- and ground-state solutions of the Kohn-Sham equations. Besides hysteretic spikes in $\rho _{xx}$, we predict \textit{hysteretic dips} in the Hall resistance $ \rho _{xy}$. Finally, we note that our theory \textit{does not} include domain walls. While not ruling out the importance of these, our quantitative agreement with the experiments does highlight the relevance of spin-dependent exchange-correlation effects in magnetic 2DEGs. [Preview Abstract] |
Monday, March 21, 2005 9:36AM - 9:48AM |
A10.00007: Fabrication of a Ferromagnetic Semiconductor Spin Bipolar Transistor Mark Field, Bobby Brar, Brian Pierce, Chad Gallinat, David Awschalom, Art Gossard, James Speck We have fabricated a spin bipolar transistor that uses a bilayer of the ferromagnetic semiconductor Ga$_{(1-x)}$Mn$_{(x)}$As to provide a tunneling magnetoresistance (TMR) element in the emitter of the device. The two magnetic layers have a different manganese concentration that gives differing coercive fields and Curie temperatures. This allows the two magnetic layers to be set in parallel or anti-parallel configurations at low temperatures. TMR is clearly observed, and transistor action confirmed in the electrical characteristics of the device. [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:00AM |
A10.00008: Spin Gunn Effect Yunong Qi, Zhi-Gang Yu, Michael E. Flatt\'e Even in nonmagnetic semiconductors the electron drift velocity depends on the electron spin polarization. This effect, originating from the Pauli excusion principle, drives a novel phenomenon we call the spin Gunn effect. We predict that the flow of unpolarized current in electron-doped GaAs and InP at room temperature is unstable at high electric fields to the dynamic formation of spin-polarized current pulses. Spin-polarized current is spontaneously generated because the conductivity of a spin-polarized electron gas differs from that of an unpolarized electron gas, even in the absence of spin-orbit interaction. Magnetic fields are not required for the generation of these spin-polarization current pulses, although they can help align the polarization of sequential pulses along the same axis. We also find that the spin polarization amplification rate is the largest for electron mobilities dominated by LO-phonon scattering, and that the steady-state (saturation) spin polarization can exceed 80\% for both GaAs and InP at room temperature. Some possible applications to novel spintronics devices will also be suggested. This work was supported by DARPA/ARO, and more details are available in cond-mat/0407547. [Preview Abstract] |
Monday, March 21, 2005 10:00AM - 10:12AM |
A10.00009: Device Applications of Spin-Orbit Interaction in Semiconductor Heterostructures David Z.-Y. Ting, Xavier Cartoix\`{a}, Yia-Chung Chang We report recent progress in theoretical development of two classes of non-magnetic semiconductor heterostructure spin devices that exploit spin-orbit interaction in the presence of structural inversion asymmetry (SIA) or bulk inversion asymmetry (BIA). The first uses resonant tunneling to filter spins, and can be used to create a source of spin polarized current. We will provide an analysis on the origin of spin-dependent tunneling in these structures and discuss their applications. The second exploits the interplay between BIA and SIA to control spin lifetimes for device applications. We show that the D'yakonov-Perel' spin relaxation can be suppressed to first order in $k$ for one out three spin components in [001] and [011] heterostructures, and for all three spin components in [111] heterostructures. Our results suggest the use of [111] heterostructures as preferred channels for spin transport, as active regions in spin-LEDs, for spin lifetime transistor, and for spin storage. [Preview Abstract] |
Monday, March 21, 2005 10:12AM - 10:24AM |
A10.00010: Self-Consistent Non-equilibrium Greens Function Description of Spin Transport in Diluted Magnetic Semiconductors Alvaro S. Nu\~nez, Allan H. MacDonald We present and discuss some applications of a method for treating transport in semiconductor heterostructures which is based on the Kohn-Luttinger k.p model Hamiltonian and the non-equilibrium (Keldysh) Green's function formalism. The method is compatatible with self-consistent and time-dependent mean-field descriptions of magnetically ordered states and is capable of handling problems of current interest in semiconductor spintronics, including magnetoresistive and Spin Momentum Transfer related phenomena in ferromagnetic semiconductors. In the case of diluted magnetic semiconductors exchange interactions with local moments are included by means of the mean-field virtual crystal approximation and electron-electron interactions can be included at the self-consistent level, using either Hartree or local density approximations. We perform a calculation of the magnetoresistance in a GaMnAs/GaAlAs/GaMnAs diluted magnetic semiconductor heterostructure using the 4-band model. Spin Transfer effects are also studied by a direct calculation of the non-equilibrium spin density and its corresponding contribution to the exchange. [Preview Abstract] |
Monday, March 21, 2005 10:24AM - 10:36AM |
A10.00011: Photon assisted double quantum dot spin filter Ernesto Cota, Rafael Sanchez, Ramon Aguado, Gloria Platero We report on numerical and analytical studies of spin transport through a double quantum dot system in the Coulomb blockade regime, at zero bias. In the presence of a magnetic field and an AC field, a device is proposed where both spin filtering and spin pumping take place, in the sequential tunneling regime. We show that in such a device, the amount of polarization of the current can be controlled by the intensity of the AC field while the sign of the spin polarized current is controlled by its frequency. A master equation approach is used to study the time evolution of the reduced density matrix, in the Markov approximation, including spin relaxation and decoherence effects. The heights and widths of the current peaks obtained for one, two or more photon absorption processes, can be explained by a simple analytical study in the stationary regime. We also show that the decoherence time can be obtained from an analysis of the widths in frequency of the current. Finally, we include results on cotunneling effects on our spin filter and spin pump device. [Preview Abstract] |
Monday, March 21, 2005 10:36AM - 10:48AM |
A10.00012: Spin-polarized transport in a quantum wire controlled by a magnetic field Mahdi Zarea, Sergio Ulloa We investigate the effects of $p^3$-dependent Dresselhaus spin-orbit interaction on the transport properties of a quantum wire. The particles are subjected to a magnetic field along, and an electrict field normal to the wire. The spin-orbit term leads to the spin-dependent renormalization of the mass of particles as well as the energy level splitting for different spins. This, in turn leads to spin-polarized tunneling transport controlled by the strength of the spin-orbit interaction and the magnetic and electric fields. [Preview Abstract] |
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A10.00013: Spin-sensitive transient absorption measurements in CdTe high above the bad gap P. Nemec, P. Nahalkova, D. Sprinzl, P. Horodysky, E. Belas, J. Franc, P. Maly Spin-sensitive dynamics of carriers optically generated and probed high above the band gap in CdTe have been measured by time-resolved differential transmission experiments using 80 fs pump and probe pulses with the same ($\Delta $T/T)$_{++}$ and opposite ($\Delta $T/T)$_{+-}$ circular polarization. These experiments were motivated by the recent observation that in GaAs, depending on the photon excess energy \textit{h$\nu $} - $E_{g}$, the degree of circular polarization (DCP=[($\Delta $T/T)$_{++}$-($\Delta $T/T)$_{+-}$]/[($\Delta $T/T)$_{++}$+($\Delta $T/T)$_{+-})$ could be positive, negative or even zero [1]. In CdTe we observed that the results of pump-probe experiments are even more sensitive to the experimental conditions than in the case of GaAs. Namely, we show that for photoexcited carriers with certain excess energies and concentrations there is a sign change not only in DCP but also in $\Delta $T/T (an absorption bleaching changes to an induced absorption). We conclude that all these effects are a consequence of interplay between the state filling and the~spin-sensitive band gap renormalization. We acknowledge fruitful discussions with J. T. Devreese. This work was supported by the Ministry of Education of the Czech Republic (project 1K03022). [1] Y. Kerachian, P. Nemec, H. M. van Driel, A. L. Smirl, APS March meeting, paper H26 8, p. 407, Montreal, Canada, 2004. [Preview Abstract] |
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