Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A48: Focus Session: Spin-Dependent Phenomena in Semiconductors: Spin Injection and Transport in Semiconductors |
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Sponsoring Units: GMAG DMP FIAP Chair: Hanan Dery, University of Rochester Room: Mile High Ballroom 1A |
Monday, March 3, 2014 8:00AM - 8:36AM |
A48.00001: Spin-pumping-induced spin transport in Si and graphene at room temperature Invited Speaker: Masashi Shiraishi Spin transport in Si is one of the quite significant research targets in semiconductor spintronics, since Si is expected to possess long spin coherence because of its lattice inversion symmetry and spin transistors using Si can be a potential beyond CMOS device. By now, much effort has been paid to realize room temperature spin transport in n-type and p-type Si, however there was no report on it in p-type Si. Here, our recent success on spin transport in p-type Si at room temperature [1] by using spin pumping is presented. Spin pumping is well known as a potential method for spin injection into materials with a large spin-orbit coupling, resulting in successful conversion from a pure spin current to a charge current [2]. Simultaneously, spin pumping is also potential for generating spin-wave spin current in YIG [3]. Now, we used this attractive method for generating a conventional pure spin current and for transporting spin angular momentum in solids [1,4,5]. A number of control experiments for p-Si spin devices corroborated our claim, and the spin coherence at room temperature was estimated to be ca. 120 ps in the simplest model. This method can be used in graphene [4] and Al [5], and they will be also introduced in the presentation. \\[4pt] [1] E. Shikoh, M. Shiraishi et al., Phys. Rev. Lett. 110, 127201 (2013).\\[0pt] [2] E. Saitoh et al., Appl. Phys. Lett. 88, 182509 (2006).\\[0pt] [3] Y. Kajiwara, E. Saitoh et al., Nature 464, 262 (2010).\\[0pt] [4] Z. Tang, M. Shiraishi et al., Phys. Rev. B87, 140401(R) (2013).\\[0pt] [5] Y. Kitamura, M. Shiraishi et al., Sci. Reports 3, 1739 (2013). [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A48.00002: Origin of three-terminal Hanle-type signals in low-temperature ferromagnet-silicon structures with direct Schottky contacts Lan Qing, Hanan Dery, Yuichiro Ando, Shinya Yamada, Kenji Kasahara, Kohei Masaki, Masanobu Miyao, Kohei Hamaya, Kentarou Sawano We analyze three-terminal electrical Hanle-type measurements in CoFe/Si devices. We show that at low temperatures there exists a Lorentzian-like dependence of the voltage signal on external magnetic field that does not correspond to the spin lifetime. The signal stems from spin-dependent scattering of electrons by neutral impurities in the bulk Si channel. The measured signal amplitude is explained by exchange interactions between free (injected) and localized electrons, while the ``Lorentzian" width by exchange between localized electrons on adjacent impurities. The theory reproduces the observed dependencies on temperature and injected current density. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A48.00003: Mobility and spin lifetime enhancement in thin silicon films by shear strain Dmitri Osintsev, Viktor Sverdlov, Siegfried Selberherr We investigate numerically the spin lifetime and mobility enhancement in (001) silicon films. Surface roughness and electron-phonon scattering is taken into account. To find the wave functions and scattering matrix elements we use the ${\mathbf{k\cdot p}}$ Hamiltonian with spin-orbit interaction for the relevant [001] valleys [1]. Knowing the wave functions at the center of the two-dimensional Brillouin zone is sufficient for mobility calculations. When shear strain increases the [110] mobility is enhanced due to the transport mass lowering and the usually ignored wave functions' dependence and the corresponding matrix elements' reduction. For spin relaxation calculations the in-plane momentum dependence of the subband wave functions due to spin-orbit coupling responsible for spin admixture must be preserved. This significantly increases demands for computational resources and requires extensive code parallelization. The spin lifetime is mostly determined by the spin-flip processes between the opposite [001] valleys strongly coupled by the effective spin-orbit interaction. Shear strain mitigates this coupling by lifting the valley degeneracy. This results in a strong increase of the spin lifetime with shear strain.1.P.Li and H.Dery, {\it Phys.Rev.Lett.}{\bf 107}, 107203 (2011). [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A48.00004: Real time electrical detection of coherent spin oscillations in silicon Hans Huebl, Felix Hoehne, Christian Huck, Martin S. Brandt In this presentation we demonstrate that the bandwidth of pulsed electrically detected magnetic resonance (EDMR) can be increased to at least 80 MHz using a radio frequency-reflectometry scheme based on a tank circuit and homodyne detection. Using this technique, we measure Rabi oscillations of phosphorus donors and Si/SiO2 interface states in real time during a resonant microwave pulse. We find that the observed signal is in quantitative agreement with simulations based on rate equations modeling the recombination dynamics of the spin system under study. The increased bandwidth demonstrated opens the way to study faster spin-dependent transport processes and could therefore significantly broaden the range of spin systems studied by EDMR. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A48.00005: Spin Relaxation Theory in Amorphous Silicon and Germanium Nicholas Harmon, Michael E. Flatt\'e Research into spintronic devices using amorphous inorganic semiconductors has seen little attention despite the surge of interest in amorphous organic spintronics. In many ways the two materials are similar - for instance hopping transport is observed in both for certain regimes. Amorphous semiconductors such as silicon and germanium offer advantages such as the ability to greatly reduce and control hyperfine field effects by the process of hydrogenation, and considerably higher mobilities. We present a theory of spin relaxation in amorphous semiconductors based on the theory of a continuous-time random walk, and obtain analytic results in several regimes. We also calculate the spin relaxation with a Monte Carlo simulation. We find that the spin-orbit coupling is the primary limit to long spin lifetimes in amorphous silicon and germanium. The theory we introduce is very general and can also be applied to amorphous organic semiconductors. We compare our results for amorphous inorganic and amorphous organic materials. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 10:00AM |
A48.00006: A graphene solution to conductivity mismatch: spin injection from ferromagnetic metal/graphene tunnel contacts into silicon Invited Speaker: Olaf van 't Erve New paradigms for spin-based devices, such as spin-FETs and reconfigurable logic, have been proposed and modeled. These devices rely on electron spin being injected, transported, manipulated and detected in a semiconductor channel. This work is the first demonstration on how a single layer of graphene can be used as a low resistance tunnel barrier solution for electrical spin injection into Silicon at room temperature. We will show that a FM metal / monolayer graphene contact serves as a spin-polarized tunnel barrier which successfully circumvents the classic metal / semiconductor conductivity mismatch issue for electrical spin injection. We demonstrate 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, confirming that the spin accumulation measured occurs in the Si and not in interface trap states. An ideal tunnel barrier should exhibit several key material characteristics: a uniform and planar habit with well-controlled thickness, minimal defect / trapped charge density, a low resistance-area product for minimal power consumption, and compatibility with both the FM metal and semiconductor, insuring minimal diffusion to/from the surrounding materials at temperatures required for device processing. Graphene, offers all of the above, while preserving spin injection properties, making it a compelling solution to the conductivity mismatch for spin injection into Si. Although Graphene is very conductive in plane, it exhibits poor conductivity perpendicular to the plane. Its sp$^{\mathrm{2}}$ bonding results in a highly uniform, defect free layer, which is chemically inert, thermally robust, and essentially impervious to diffusion. The use of a single monolayer of graphene at the Si interface provides a much lower \textit{RA} product than any film of an oxide thick enough to prevent pinholes (1 nm). Our results identify a new route to low resistance-area product spin-polarized contacts, a crucial requirement enabling future semiconductor spintronic devices, which rely upon two-terminal magnetoresistance, including spin-based transistors, logic and memory. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A48.00007: Efficient spin injection in Co$_{2}$Mn$_{x}$Fe$_{1-x}$Si/GaAs heterostructures Kevin Christie, Chad Geppert, Lee Wienkes, Sahil Patel, Chris Palmstr{\O}m, Paul Crowell Several Heusler alloys that are well lattice-matched to the In$_y$Ga$_{1-y}$As family of semiconductors are also candidates for half-metallic ferromagnets. We investigate here their potential for generating near unity spin polarizations in a semiconductor channel. We report on all-electrical measurements of the spin transport properties of epitaxial Co$_{2}$Mn$_{1-x}$Fe$_x$Si / $n$-GaAs heterostructures. The FM/$n$-GaAs interface is degenerately doped to form a narrow Schottky barrier as in previous work on Fe-based devices. The heterostructures were patterned into lateral spin-valve devices, and spin accumulation has been detected at temperatures up to 200 K using both spin-valve and Hanle techniques over a contact separation of 10 $\mu$m. In Co$_2$MnSi devices, a spin splitting of the chemical potential on the order of the Fermi energy (over 2 mV) is observed at 30 K. This is the largest spin accumulation observed to date in a FM/III-V system. We observe a change in sign of the spin accumulation at high Fe concentrations. The connection of this sign inversion to either the bulk of the ferromagnet or interfacial band structure is being investigated. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A48.00008: Electrical Spin Injection and Detection in Mn$_{5}$Ge$_{3}$/Ge/Mn$_{5}$Ge$_{3}$ Nanowire Transistors Jianshi Tang, Chiu-Yen Wang, Kang L. Wang, Lih-Juann Chen We report the electrical spin injection and detection in Ge nanowire transistors with single-crystalline Mn$_{5}$Ge$_{3}$ as the ferromagnetic source/drain contacts. The magnetoresistance (MR) of the Mn$_{5}$Ge$_{3}$/Ge/Mn$_{5}$Ge$_{3}$ nanowire transistor was found to be largely affected by the applied bias. Specifically, negative and hysteretic MR curves were observed under a large current bias from 2 K up to 50 K, clearly indicating successful spin injection into the Ge nanowire. In addition, the MR amplitude was found to exponentially decay with the Ge channel length. The fitting of MR further revealed a spin diffusion length of about 480 nm and a spin lifetime exceeding 244 ps at 10 K in $p$-type Ge nanowires, which are much larger than those reported for bulk $p$-type Ge. Our study of the spin transport in the Ge nanowire transistor points to a possible realization of spin-based transistors, and it may also open up new opportunities to create novel nanowire-based spintronic devices. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A48.00009: Model for the Spin Seebeck Effect in InSb in a Magnetic Field Nicholas Pike, David Stroud The spin Seebeck effect is the generation of a voltage due to spin currents in the presence of a temperature gradient. We have developed a theory for this effect in the semiconductor InSb in a magnetic field. We consider spin-$1/2$ electrons in the conduction band of InSb with a temperature gradient parallel to the applied magnetic field. A Boltzmann equation approach leads to a spin current parallel to the field and proportional to the temperature gradient. The spin-orbit interaction induces a canting of the electronic spin which produces an electric field perpendicular to the temperature gradient via the inverse spin Hall effect. This effect is measured in experiments as the spin Seebeck coefficient [1]. We find that the spin current exhibits oscillations as a function of magnetic field which arise when the Fermi energy crosses the bottom of a Landau band. These oscillations resemble those seen in measurements of the spin Seebeck coefficient in the semiconductor InSb. \\[4pt] [1] C.M. Jaworski, et al. Nature {\bf 487}, 210-213 (2012). [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A48.00010: Spin injection in LaAlO$_{3}$/SrTiO$_{3}$ heterostructures Adrian Swartz, Satoshi Harashima, Yanwu Xie, Bongju Kim, Takashi Tachikawa, Christopher Bell, Yasuyuki Hikita, Harold Hwang There are new device opportunities at the interface of complex oxide heterostructures due to the interplay of charge, orbital, and spin degrees of freedom. A model system is the low dimensional conducting layer generated at the interface of LaAlO$_{3}$ and SrTiO$_{3}$, which has demonstrated high mobilities and tunable carrier densities. However, little has been explored towards employing these high mobility interfaces as spin transport channels. Such conducting interfaces could be practical routes for realizing efficient spin transistors in which spin manipulation functionality could be epitaxially incorporated. First, spin injection, a key requirement of the spin transistor, must be explored. Here, we report our investigations of spin injection into the LaAlO$_{3}$/SrTiO$_{3}$ interface in a three-terminal geometry. Complex oxide films are grown by pulsed laser deposition and patterned into devices through lithography and hard-mask techniques. Using Hanle spin precession, we have observed spin lifetimes in the range of 80 - 100 ps. Notably, the devices exhibit unusual bias dependence in the Hanle signal and high field magnetoresistance. These results provide a building block in the field of oxide-based spintronics. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A48.00011: A differential spin current detection scheme Bryan Hemingway, Ian Appelbaum We offer an alternative scheme to detect spin polarization of conduction electrons injected into a nonmagnetic metal or degeneratively doped semiconductor using transport to two oppositely polarized ferromagnetic metal contacts. We show that, as in the well-known spin injection problem, detection efficiency can be amplified by the addition of spin-selective tunneling barriers. Considering the appropriate geometry and achievable injection rates, we estimate that the differential current can be as high as 1-10?nA for reasonable design parameters. We will also discuss the realization of this detection scheme in laboratory set-ups. [Preview Abstract] |
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