Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C18: Focus Session: Spin-Dependent Phenomena in Semiconductors - Spin Injection and Transport |
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Sponsoring Units: GMAG DMP FIAP Chair: Paul Crawell, University of Minnesota Room: 320 |
Monday, March 18, 2013 2:30PM - 2:42PM |
C18.00001: Optical spin injection into Ge at room temperature Yuhsuke Yasutake, Shuhei Hayashi, Susumu Fukatsu A realistic opto-spintronic device draws on the knowledge and control over the optical orientation at room temperature. We investigated circularly polarized photoluminescence of tensilely strained Ge-on-Si in an attempt to inject spins into Ge by optical means at ambient temperature. More than 10-\% polarization was observed up to 300 K. The absence of indirect-gap luminescence facilitated spectral analysis. Meanwhile, very fast decay was observed, unlike bulk Ge with little spin polarization. This indicates that intervalley scattering has diminished in Ge-on-Si due presumably to dissipative channels introduced during growth. It is concluded that spin coherence remains in a short time scale. This led us to optical injection of spins, O(1\%), in bulk Ge at room temperature, which was observed in the prompt decay. [Preview Abstract] |
Monday, March 18, 2013 2:42PM - 2:54PM |
C18.00002: Spin accumulation in Ge at room temperature A.T. Hanbicki, S.-F. Cheng, R. Goswami, O.M.J. van 't Erve, B.T. Jonker We have investigated spin injection into n-type Ge(001) from Fe through a sputter-deposited MgO tunnel barrier using 3-terminal Hanle measurements[1]. Unlike Si, spin research in Ge is hampered by Fermi level pinning at the Ge interface, which makes it difficult to efficiently inject carriers. We observe here precessional dephasing of the spin accumulation in an applied B-field (Hanle effect) in Fe/MgO/Ge for both forward and reverse bias. At room temperature, spin lifetimes range from $\tau$$_{s}$ = 50 ps to 123 ps as the carrier concentration is reduced from n=8x10$^{17}$cm$^{-3}$ to 2x10$^{16}$cm$^{-3}$. The measured spin resistance-area product is in good agreement with values predicted by theory for samples with carrier densities below the metal-insulator transition (MIT), but 100x larger for samples above the MIT. These data demonstrate that measured spin accumulation occurs in the Ge, although dopant-derived interface or band states may enhance the measured spin voltage above the MIT. The observed room temperature injection of spins shows that despite persistent Fermi level pinning, spin accumulation is possible in the surface region of Ge. This work was supported by core programs at NRL.$\\$ [1] A.T. Hanbicki, et al., $\it{Solid State Comm.}$ $\textbf{152}$, 244 (2012). [Preview Abstract] |
Monday, March 18, 2013 2:54PM - 3:06PM |
C18.00003: Crossover from Spin Accumulation into Interface States to Spin Injection in the Germanium Conduction Band Juan-Carlos Rojas-Sanchez, Abhinav Jain, Murat Cubukcu, Julian Peiro, Jean-Christophe Le Breton, Eric Prestat, C\'eline Vergnaud, Pascale Bayle-Guillemaud, Laurent Vila, Jean-Philippe Attan\'e, Emmanuel Augendre, Serge Gambarelli, Henri Jaffr\`es, Jean-Marie George, Matthieu Jamet Spin injection into semiconductors is crucial for exploring spin physics and new spintronic devices. Ge is of great interest for high carrier mobilities, long spin diffusion length and large spin-orbit coupling to perform electric field spin manipulation. However the exact role of interface states in spin injection mechanism in n-Ge has not been clarified yet. Here we show a clear transition from spin accumulation into interface states to spin injection in the Ge conduction band. For this purpose, we have grown CoFeB/MgO as a spin injector on Germanium On Insulator. We observe spin signal amplification at low temperature due to spin accumulation into interface states. At 150 K, we find a clear transition to spin injection in the conduction band up to room temperature: the measured spin signal is compatible with the spin diffusion model. We could in particular demonstrate spin signal modulation applying a back gate voltage and spin-pumping by the ferromagnetic resonance of the CoFeB layer which are clear manifestations of spin accumulation in the Ge conduction band. [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:18PM |
C18.00004: Electrical Spin Injection and Detection in Silicon Nanowires Shixiong Zhang, Shadi Dayeh, Yan Li, Scott A. Crooker, Darryl L. Smith, S. T. Picraux We report on the electrical injection and detection of spin-polarized electrons from cobalt contacts into n-type Si nanowires through Al2O3 tunneling barriers. Analysis of local (2 terminal) and non-local (4 terminal) spin valve signals at 4 K on the same nanowire device using a standard spin-transport model suggests that high spin injection efficiency ( about 30{\%}) and long spin diffusion lengths (about 6 micron) are achieved for these nanoscale structures. These values compare favorably to those reported for spin transport devices based on comparably-doped bulk Si. The spin valve signals are strongly influenced by temperature, bias current and by the geometry of the ferromagnetic [Preview Abstract] |
Monday, March 18, 2013 3:18PM - 3:30PM |
C18.00005: Electrical spin injection into Si with Ni/graphene contacts Connie H. Li, Olaf M. J. van 't Erve, Jeremy T. Robinson, Berry T. Jonker Graphene, a single layer of sp2 bonded C atom, provides a highly uniform barrier with well-controlled thickness and minimal defect, has been shown to be a viable tunnel barrier in magnetic tunnel junctions [1]. More recently, we have further demonstrated that it also circumvents the conductivity mismatch between a FM metal and semiconductor, whilst lowering the resistance area product [2]. Excellent spin filtering has also been predicted across graphene-magnetic metal (e.g., Ni, Co) interfaces, due to electronic structure overlap for only the minority spin [3]. This spin filtering effect is also accumulative, with predicted spin polarization reaching 100{\%} with multiple layers (\textgreater 5) of graphene, and is also robust against interface roughness and disorder [3]. Here we explore electrical spin injection into Si utilizing this spin filtering effect in a Ni/graphene/Si structure. We observe Hanle precession of the electron spin accumulation in the semiconductor, where the extracted spin lifetime from the Lorentzian fit to the Hanle data, $\sim$ 160 ps, is consistent with those observed for this Si carrier density (1E19) with other FM contacts (NiFe) and tunnel barriers (SiO2, Al2O3, graphene) [2,4], confirming spin injection and accumulation in the Si. Results comparing single and multiple layer graphene tunnel barriers will also be presented.\\[4pt] [1] Cobas et al., Nano Lett., 12, 3000 (2012)\\[0pt] [2] van `t Erve et al., Nat. Nano., 7, 737 (2012)\\[0pt] [3] Karpan et al., Phys. Rev. Lett. 99, 176602 (2007); Phys. Rev. B. 78, 195419 (2008); Phys. Rev. B. 84, 153406 (2011).\\[0pt] [4] Li et al., Nat. Comm., 2, 245 (2011) [Preview Abstract] |
Monday, March 18, 2013 3:30PM - 3:42PM |
C18.00006: A graphene solution to conductivity mismatch: spin injection into Silicon Olaf van 't Erve, Adam Friedman, Enrique Cobas, Connie Li, Aubrey Hanbicki, Kathy McCreary, Jeremy Robinson, Berend Jonker The classic FM/semiconductor conductivity mismatch prevents spin injection into silicon. Typically, an oxide barrier such as MgO, AlOx or SiO2 is used to overcome this mismatch. These oxide tunnel barriers introduce defects, trapped charge, interdiffusion and add significant resistance, which compromise device performance. Here we will show that a FM/graphene contact serves as a spin-polarized tunnel barrier. [1] Although graphene is very conductive in plane, it exhibits poor conductivity perpendicular to the plane. Its sp2 bonding results in a highly uniform, defect free layer, which is chemically inert, thermally robust, and impervious to diffusion. The use of a monolayer of graphene at the Si interface provides a much lower RA product than any oxide film thick enough to prevent pinholes. We will present electrical injection and detection of spin accumulation in Si above room temperature, and show that the corresponding spin lifetimes correlate with the Si carrier concentration. The RA products are three orders of magnitude lower than achieved with oxide tunnel barrier contacts on identical Si substrates. Our results identify a new route to low RA-product spin-polarized contacts, a crucial requirement enabling future semiconductor spintronic devices, which rely upon two-terminal MR. [1] van `t Erve et al., Nature Nanotechnology, DOI 10.1038/nnano.2012.161 (2012) [Preview Abstract] |
Monday, March 18, 2013 3:42PM - 3:54PM |
C18.00007: Analysis of 3-terminal Hanle signals in Si-based spintronic devices Shoichi Sato, Ryosho Nakane, Masaaki Tanaka We have investigated 3-terminal Hanle (3TH) signals and inverted 3-terminal Hanle (Inv3TH) signals [1] obtained in a temperature range of 5 -- 300 K in devices with a Si channel and Fe/SiO$_{2}$/Si tunnel junctions. The Hanle signals were measured with a magnetic field applied in various directions from parallel (Inv3TH) to normal (3TH) to the plane. The shape of the 3TH signals was a positive Lorentzian, whereas that of the Inv3TH signals was composed of two negative Lorentzians whose centers are at around $\pm$ 1000 Oe. We analyzed the signals using the Eq.(2) in [1], in which the effective magnetic field acting on electron spins is assume to be composed of the following fields; an applied field, a constant stray field, and a stray field proportional to the magnetization of the Fe electrode. Note that the last two fields are introduced in the present analysis. All the experimental signals in any applied field directions were well fitted by the formula. From the fitting parameters at various measurement temperatures, we found that the FWHM of the 3TH signals is determined by the stray field of the Fe electrode, and that the spin-polarized electrons accumulate in electronic states between the Fe electrode and the Si channel.\\[4pt] [1] S. P. Dash et al., Phys. Rev. B84, 054410 (2011). [Preview Abstract] |
Monday, March 18, 2013 3:54PM - 4:06PM |
C18.00008: Correlation of electrical spin injection and non-linear charge-transport in Fe/MgO/Si Jonas Beardsley, Yong Pu, Patrick Odenthal, Adrian Swartz, Roland Kawakami, P. Christopher Hammel, Ezekiel Johnston-Halperin, Jairo Sinova, Jon Pelz The three-terminal (3T) Hanle method has recently been used by several groups to measure electrical spin injection into Si up to 500K, with signals that can be orders of magnitude larger than expected for spin injection into bulk Si states. While much discussion has centered on the possible origin of the enhanced spin signal, there has been little discussion of the anomalously-strong bias dependence of the spin resistance area product (SRA) often measured at low-temperatures. We report 3T-Hanle measurements MBE grown Fe/MgO/Si tunnel diodes which show an SRA up to seven orders larger at low temperature than the bulk prediction, which is both strongly bias dependent and highly correlated with the differential resistance area product, dV/dJ, over a large range of bias and temperature. This cannot be explained by current theories that assume energy-independent tunneling into localized or bulk states. We show that a simple model with strongly energy-dependent tunneling can explain the strong bias-dependences and correlation of the SRA and dV/dJ, and suggests that the intrinsic spin-injection properties may have little bias dependence even though the measured SRA decreases by three orders of magnitude with increasing bias. [Preview Abstract] |
Monday, March 18, 2013 4:06PM - 4:18PM |
C18.00009: Spin filtering with EuO: Insight from a complex band structure Pavel Lukashev, Aleksander Wysocki, Julian Velev, Mark van Schilfgaarde, Sitaram Jaswal, Kirill Belashchenko, Evgeny Tsymbal Spin-filter tunneling is a promising way to create highly-spin-polarized currents. So far the understanding of the spin-filtering effect has been limited to a free-electron description based on the spin-dependent tunneling barrier height. In this work we employ density-functional calculations to explore the complex bands of EuO as a representative ferromagnetic insulator used in spin-filter tunneling experiments and show that the mechanism of spin filtering deviates significantly from the standard free-electron picture and involves effects associated with the symmetry of spin-dependent evanescent states and the dependence of the decay constant on the transverse wave vector. We demonstrate the importance of the multiorbital band structure with an indirect band gap for spin-filter tunneling. By analyzing the symmetry of the complex bands and the decay rates for different wave vectors and energies we draw conclusions about spin-filter efficiency of EuO. Our results provide a new insight into spin-filter tunneling and may help to design tunnel junctions with enhanced spin polarization. [Preview Abstract] |
Monday, March 18, 2013 4:18PM - 4:30PM |
C18.00010: Doping dependence of the inverse spin-Hall effect in $n$-GaAs near the metal-insulator transition Chad Geppert, Kevin Christie, Mun Chan, Sahil Patel, Chris Palmstr\O m, Paul Crowell We present measurements of the inverse spin-Hall effect in $n$-GaAs at various doping levels ranging from one to five times the Mott criterion ($n_{c}\approx2\times10^{16}/$cm$^3$). Spin currents are generated electrically (from either Fe or Co$_2$MnSi) which give rise to a Hanle effect in the local Hall voltage. The observed magnitude corresponds to a spin Hall angle of $\approx10^{-3}$ in the regime of small electron polarization, yet exhibits a dramatic enhancement at low temperatures and/or high biases due to the presence of polarized nuclei. This enhancement is largest for intermediate dopings, reverses sign as the samples become more metallic, and exhibits an asymmetry with respect to the absolute sign of the polarization. These features are all indicative of conduction electrons resonantly scattering from localized states in the impurity band. Further confirmation of this picture is provided by quantitative modeling of the observed magnetic field dependence at oblique angles, where the Overhauser effect partially compensates the applied field. The resulting phenomenological form demonstrates that in addition to conventional spin-orbit effects, asymmetry in the spin-flip scattering may be directly mediated by the local nuclear spin system. Supported by NSF DMR-1104951. [Preview Abstract] |
Monday, March 18, 2013 4:30PM - 4:42PM |
C18.00011: Large magnetoresistance on spin injection in InSb/CoFe junctions Martin Rudolph, Y.J. Kim, J.J. Heremans During electrical spin injection from micron-sized ferromagnetic CoFe structures into the strongly spin-orbit coupled semiconductor InSb, we observe an unanticipated magnetoresistance at low temperatures. For temperatures below 3.5 K, the interface resistance falls abruptly, by up to 25{\%}, at a critical magnetic field. The functional dependence on magnetic field of the magnetoresistance and the temperature and angular dependence of its amplitude and width are not consistent with previously observed magnetoresistive semiconductor/ferromagnet junction phenomena. A differential conductance map of the density of states shows a symmetric minimum at zero current bias corresponding to the magnetoresistance maximum. The experimental observations appear consistent with formation of a quasiparticle gap of width approximately 0.5 meV during a phase transition in spin polarized InSb for temperatures below 3.5 K and magnetic fields below a critical field (DOE DE-FG02-08ER46532). [Preview Abstract] |
Monday, March 18, 2013 4:42PM - 4:54PM |
C18.00012: Carrier density dependence of the spin lifetime in the persistent photoconductor Si:Al$_{0.3}$Ga$_{0.7}$As Joon-Il Kim, J. Misuraca, K. Kountouriotis, S. von Molnar, P. Xiong, K. Meng, J. Lu, L. Chen, X. Yu, J. Zhao Electrical spin injection/detection experiments have been performed on Si:Al$_{0.3}$Ga$_{0.7}$As, a persistent photoconductor. The carrier density of this material can be tuned in situ via photo excitation across the insulator-metal transition (IMT) [1], which enables spin accumulation and transport measurements in one and the same sample over orders of magnitude variation in carrier density, thus circumventing the difficulties of making many replicas to realize different doping levels. Fe/AlGaAs heterostructures were grown by MBE, in which AlGaAs and GaAs graded Schottky junctions were tested for optimum spin injection. Spin transport devices, suitable for 3-terminal and non-local 4-terminal Hanle-type measurements and on-chip determination of the carrier density, were fabricated from the wafers. The spin lifetimes, determined from fits of the Hanle curves to a Lorentzian or the spin drift-diffusion model, range from 0.5 ns to 2.8 ns and exhibit a nonmonotonic carrier density dependence possibly peaked at the IMT.\\[4pt] [1] J. Misuraca et al., Phys. Rev. B82, 125202 (2010). [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C18.00013: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 5:06PM - 5:18PM |
C18.00014: Determination of Spin Polarization of Fe$_{65}$Si$_{5}$ Using Andreev Reflection Spectroscopy Jonathan Martinez, Charles Snider, Jessica Gifford, Tingyong Chen Ferromagnetic Fe$_{\mathrm{x}}$Si$_{\mathrm{1-x}}$ alloys have been proposed as potential spin injectors into silicon with a substantial spin polarization. Experimentally, however, the observed spin polarization of the alloys still remains low. Ideally, spin polarization of a metal is defined as the imbalance of density of states at the Fermi level, but in amorphous alloys it is different since the Fermi level is not well defined. Recently, it has been found that the magnetic properties of the amorphous Fe$_{\mathrm{x}}$Si$_{\mathrm{1-x}}$ alloys are very different from the crystalline phase. In this work, we utilize Andreev Reflection Spectroscopy (ARS) to determine the spin polarization of both amorphous and crystalline Fe$_{65}$Si$_{35}$ alloys. We show that the additional resistance in ARS is quite high because of large resistivity of these alloys and must be taken into account to correctly extract the spin polarization. The obtained spin polarization values are very different: the amorphous phase has a significantly higher spin polarization than that of the crystalline phase. [Preview Abstract] |
Monday, March 18, 2013 5:18PM - 5:30PM |
C18.00015: Spin Transistor Action from Hidden Onsager Reciprocity I. Adagideli, V. Lutsker, M. Scheid, Ph. Jacquod, K. Richter We investigate generic Hamiltonians for confined electrons with weak inhomogeneous spin-orbit coupling [1]. Using a local gauge transformation we show how the SU(2) Hamiltonian structure reduces to a U(1) $\times$ U(1) structure for spinless fermions in a fictitious orbital magnetic field, to leading order in the spin-orbit strength. Using an Onsager relation, we further show how the resulting spin conductance vanishes in a two-terminal setup, and how it is turned on by either weakly breaking time-reversal symmetry or opening additional transport terminals, thus allowing one to switch the generated spin current on or off. We numerically check our theory for mesoscopic cavities as well as Aharonov-Bohm rings.\\[4pt] [1] Adagideli et al., Phys. Rev. Lett. 108, 236601 (2012) [Preview Abstract] |
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