Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L35: Focus Session: Spins in Semiconductors -- Spin Injection |
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Sponsoring Units: GMAG DMP FIAP Chair: Ron Jansen, University of Twente Room: E145 |
Tuesday, March 16, 2010 2:30PM - 2:42PM |
L35.00001: Electrical spin injection into Si(001) through a SiO$_{2}$ tunnel barrier C.H. Li, G. Kioseoglou, O.M.J. van 't Erve, P.E. Thompson, B.T. Jonker We recently demonstrated successful injection of spin-polarized electrons from an Fe film through an Al$_{2}$O$_{3}$ tunnel barrier into Si (001) [1]. However, the utilization of SiO$_{2}$ as the spin tunnel barrier can prove technologically important owing to the prevalence of SiO$_{2}$ and the SiO$_{2}$/Si interface in the CMOS industry. Here we demonstrate spin polarized tunneling from Fe through a SiO$_{2}$ tunnel barrier into a Si $n-i-p$ heterostructure [2]. Transport measurements indicate an effective barrier height of $\sim $1.7 eV and barrier thickness of $\sim $21 {\AA} for 10 K, and that single step tunneling is the dominant transport mechanism. The electroluminescence from the Si exhibits circular polarization that tracks the Fe magnetization, confirming electrical injection from Fe. A rate equation analysis provides a lower bound of 30{\%} for the electron spin polarization in the Si at 5 K. These results demonstrate that an ultra thin layer of SiO$_{2}$, readily fabricated on Si through UV-ozone oxidation, can be used as a viable tunnel barrier for electrical spin injection from a ferromagnetic contact into Si.\\[4pt] [1] B. T. Jonker et al., Nature Phys. 3, 542 (2007); G. Kioseoglou et al., APL 94, 122106 (2009).\\[0pt] [2] C. H. Li et al. APL 95, 172102 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 2:42PM - 2:54PM |
L35.00002: Spin Polarized Electron Transport near the Si/SiO2 Interface H.-Jae Jang, Gardner Swan, Ian Appelbaum Using long-distance lateral devices, spin transport near the interface of Si and its native oxide (SiO2) is studied by spin-valve measurements in an in-plane magnetic field and spin precession measurements in a perpendicular magnetic field at 60K.[1] As electrons are attracted to the interface by an electrostatic gate, we observe shorter average spin transit times and an increase in spin coherence, despite a reduction in total spin polarization. This behavior, which is in contrast to the expected exponential depolarization seen in bulk transport devices, is explained using a transform method to recover the empirical spin current transit-time distribution and a simple two-stage drift-diffusion model. [2,3] We identify strong interface-induced spin depolarization (reducing the spin lifetime by over two orders of magnitude from its bulk transport value) as the consistent cause of these phenomena. In addition, we will discuss the novel spin transport phenomena near Si/SiO2 interface in high magnetic field regime. References [1] H.-J. Jang and I. Appelbaum, Phys. Rev. Lett. 103, 117202 (2009) [2] J. Li and I. Appelbaum, Appl. Phys. Lett. 95, 152501 (2009) [3] I. Appelbaum, arxiv:condmat/0910.2606 (2009) [Preview Abstract] |
Tuesday, March 16, 2010 2:54PM - 3:06PM |
L35.00003: Spin injection in Silicon Louis Grenet, Matthieu Jamet, Pierre No\'e, Patrick Warin, Yves Samson Spintronic aims at manipulating both charge and spins in semiconductors. It thus requires a robust scheme for injecting polarized current into these materials. Spin injection into GaAs has already been demonstrated optically. However few works have been published so far dealing with spin injection into Silicon although it is the core material for potential application and a very promising material for spin polarized transport because of very long spin lifetimes expected. We present results obtained on spin injection from a ferromagnetic layer to Silicon. A SiGe quantum well embedded in a Si diode is used to optically detect spin-polarized electrons. The spin of the electrons are aligned when going through a CoPt electrode with perpendicular magnetization and are injected through an alumina tunnel barrier to prevent depolarization due to conductance mismatch with Silicon. The remanent configuration of the electrode magnetization allows us to study spin injection without applying a magnetic field, avoiding spurious effects such as Faraday rotation of the light in Silicon. Light polarization of more than 3 percent has been measured. [Preview Abstract] |
Tuesday, March 16, 2010 3:06PM - 3:18PM |
L35.00004: Ferromagnetic proximity polarization in magnetite/GaAs Yan Li, A. Swartz, K. Pi, W. Han, J. J. I. Wong, K. M. McCreary, S. Mack, D. D. Awschalom, R. K. Kawakami Magnetite is an attractive material for spintronics because of its half metallic property and a very high Curie temperature. However, it is not easy to get stoichiometric magnetite films with high quality interface, namely no mixture of other oxides and boundary phases. One limitation is that there are sparse tools for characterizing the interface. We probe the interface of magnetite/GaAs by spin sensitive method, ferromagnetic proximity polarization (FPP), and study the growth condition for best spin filtering effect of the interface. The magnetite film is grown by molecular beam epitaxy, post oxidation of epitaxial Fe film on GaAs. We observed that the electron spins generated by the FPP process are antiparallel to the magnetization of magnetite in magnetite/GaAs, which is opposite to that of Fe/GaAs. This is consistent with the fact that magnetite has fully negative spin polarization at the Fermi level. Detailed study on how the spin dependent performance the interface changes with the growth conditions, e.g. temperature, oxidation time, will be discussed. Supported by NSF and ONR. [Preview Abstract] |
Tuesday, March 16, 2010 3:18PM - 3:30PM |
L35.00005: Novel All-Spin Devices Behtash Behin-Aein, Deepanjan Datta, Sayeef Salahuddin, Supriyo Datta We propose a spintronic device that uses spin at every stage of its operation: information manipulation, transport, storage, input and output are all accomplished with magnets and spin-coherent channels. The all-spin device could potentially find use for low-power digital logic since it should satisfy the five essential requirements for logic applications namely nonlinearity, gain, concatenability, feedback prevention and a complete set of Boolean operations. Moreover it could provide a basis for unconventional approaches. For example the spin accumulation in a semiconducting channel underneath a magnetic contact could provide a weighted average of different inputs that makes it switch (fire) when it exceeds a threshold like neural networks. Alternatively the magnetic contacts on top of the channel could possibly serve as Input-Output interface for spin-based quantum computing. [Preview Abstract] |
Tuesday, March 16, 2010 3:30PM - 3:42PM |
L35.00006: Gate Controlled Spin Precession Effect Abu Naser Zainuddin, Lutfe Siddiqui, Seokmin Hong, Supriyo Datta A two-dimensional (2D) non-local lateral spin-transport model is developed based on the non-equilibrium Green's function (NEGF) formalism for ballistic carriers in mode space approach. The effect of gate controlled Rashba spin-orbit (RSO) interaction in modulating the non-local spin voltage has been explicitly taken into account. We found a quantitative agreement with the recent observation on non-local controlled spin-precession by Koo \textit{et. al }[1]. The phase shift observed in the voltage according to the analytical equation used in [1] is found to be the consequence of both multichannel effect and the effect of injecting and detecting ferromagnetic contact length. In such structures we predict that a short length contact as well as an etched out channel can improve the non-local voltage significantly. \textbf{[1] }H. C. Koo \textit{et. al.} Science, 325, 1515 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 3:42PM - 3:54PM |
L35.00007: Electrical Control of Magnetoresistance In a InP-Based Lateral Spin Valve with a Two-Dimensional Electron Gas (2-DEG) Channel Hyun Kum, Debashish Basu, Pallab Bhattacharya, Wei Guo Electrical field control of spin transport has been of recent interest. The control of magnetoresistance of a lateral InP-based spin valve consisting of a InAs/In$_{0.53}$Ga$_{0.47}$As/In$_{0.52}$Al$_{0.48}$As 2-DEG channel with a gate electrode is demonstrated. The polarizer and analyzer are made with MnAs/In$_{0.52}$Al$_{0.48}$As Schottky tunnel barriers and the Ti/Au gate electrode is placed outside the channel region, of length 0.6 $\mu $m, and alongside the polarizer. The magnetoresistance changes from 0.14 to 4{\%} at 10 K, when the gate bias is changed from 0 to 8V, in a device in which the magnetization of the polarizer and analyzer is in the direction of spin transport in the 2-DEG. The effect is absent in a GaAs channel spin valve and other control devices, indicating that the observed change in magnetoresistance in the 2-DEG spin valve is due to Rashba spin-orbit coupling. The results will be compared with those obtained from spin valves in which the gate is placed directly above the local spin valve channel region. [Preview Abstract] |
Tuesday, March 16, 2010 3:54PM - 4:06PM |
L35.00008: Determining the bandtail shape of highly Si-doped Al$_{0.3}$Ga$_{0.7}$As for investigation as a spin transport channel Jennifer Misuraca, Peng Xiong, Stephan von Molnar, Jelena Trbovic, Jun Lu, Jianhua Zhao, Hideo Ohno Knowledge of the band structure near the metal insulator transition (MIT) is essential to understanding spin transport and coherence in a semiconductor. Highly Si-doped Al$_{0.3}$Ga$_{0.7}$As can be driven through the MIT using persistent photoconductivity [1] via photodoping the insulating sample at 5K. This provides a way to tune the carrier concentration of the sample\textit{ in situ}.~The carrier concentration as a function of temperature for various illumination times is measured as the Fermi energy is tuned systematically. The critical carrier concentrations for the MIT and the deep state Hall activation energies of two differently Si-doped, MBE grown, Al$_{0.3}$Ga$_{0.7}$As samples have been established. Furthermore, the bandtail shape of the DOS has been inferred using the shallow activation energies [2] in a 16 meV range between the Fermi energy of the unilluminated sample and the mobility edge. [1] S. Katsumoto, et al. J. Phys. Soc. Jpn. 56, 2259 (1987). [2] I. Terry, et al. Solid State Commun. 84, 235 (1992). [Preview Abstract] |
Tuesday, March 16, 2010 4:06PM - 4:18PM |
L35.00009: Spin Dependent Transport in an InAs Two Dimensional Electron Gas Olaf van 't Erve, Greg Jones, Connie Li, Aubrey Hanbicki, Chaffra Awo-Affouda, Mike Holub, Brian Bennett, Berend Jonker Semiconductors, such as InAs, that exhibit a large spin orbit interaction (SOI), provide a mechanism to manipulate the spins in the semiconductor by precession in a Rashba field caused by an electric field generated with a gate. However a large SOI also implies short spin lifetimes, due to spin-relaxation caused by the interaction of impurities and crystal structure with the electron's spin orbit. To make a practical device out of a material with a short spin lifetime, you would need a high mobility in order to get a spin diffusion length that is on the length scales of your device. We designed a high mobility InAs 2deg (45 000 cm$^{2}$/V-sec @ 5K) and used tunnel barrier contacts (CoFe/Al$_{2}$O$_{3}$ and NiFe/Al$_{2}$O$_{3})$ to electrically inject and detect electron spin in the 2deg. We will show spin dependent transport in the 2DEG both in the spin-charge region and in the pure spin diffusion region using 3 terminal and nonlocal devices respectively. [Preview Abstract] |
Tuesday, March 16, 2010 4:18PM - 4:30PM |
L35.00010: Temperature dependence of spin polarization in a lateral Fe/GaAs spin injection device G. Salis, S.F. Alvarado, A. Fuhrer, L. Gross, R. Schlittler The size of electrically-detected electron spin polarization in a non-local Fe/GaAs lateral spin device is studied as a function of temperature and injection bias. The non-local spin signal decays approximately exponentially with temperature, and remains observable up to room temperature. With Hanle measurements in a perpendicular magnetic field, the diffusion constant and the spin lifetime are determined. These parameters characterize the diffusive spin transport in the GaAs channel. It is found that the decrease of the signal with temperature is mainly attributed to the strongly increasing spin-decay rate in GaAs, whereas a temperature-dependence of the spin-injection efficiency is less pronounced. Differences for spin injection and spin filtering are discussed. [Preview Abstract] |
Tuesday, March 16, 2010 4:30PM - 4:42PM |
L35.00011: Direct detection of spin chemical potential shift through spin filtering effect Guoxing Miao, Jagadeesh Moodera Spin filtering (SF) effect is a unique way to generate highly spin-polarized tunnel currents from nonmagnetic electrodes. Magnetic tunnel junctions based on pure SF effect have been realized recently [1] as a clear demonstration of principle for the spin manipulation through SF effect. The next challenge is the readout of spin information. In this work, we present the direct detection of the spin chemical potential shift in an Al nano cluster sandwiched between two SF EuS tunnel barriers. The spin channels are split by depositing Al directly onto EuS, and the indirect exchange interaction between the Al conduction electrons and the localized Eu 4f electrons gives rise to an effective Zeeman splitting with the strength of a few mV. EuS on the readout side is isolated from the Al clusters with a natural Al$_{2}$O$_{3}$ barrier. In a vertical measurement geometry with no transport current, we directly detected the spin dependent voltage levels by aligning the detection SF barrier parallel or antiparallel to the first SF barrier, corresponding to the equilibrium up- and down-spin chemical potentials. A simple analysis treating the barriers as a set of resistors revealed that the observed voltage difference is the actual chemical potential shift modulated by the SF efficiency.\\[4pt] [1] G.X. Miao, M. Muller, J.S. Moodera,PRL102,076601(2009) [Preview Abstract] |
Tuesday, March 16, 2010 4:42PM - 4:54PM |
L35.00012: Distinctly Different Switching Characteristics of Interface and Bulk Magnetization in Fe/AlGaAs(001) and Fe/MgO(001) Yichun Fan, Haibin Zhao, Gunter Luepke, Aubrey Hanbicki, Connie Li, Berry Jonker The interface magnetization processes of Fe/AlGaAs(001) and Fe/MgO(001) are investigated by magnetization induced second harmonic generation (MSHG). The interface switching characteristics of Fe/AlGaAs(001) are distinctly different from the bulk Fe film due to magnetization curling in nano-islands located at the interface. We estimate the thickness of Fe interface to be a few atomic layers thick and the interface exchange stiffness to be three orders of magnitude less than the bulk. In Fe/MgO(001) interface, the cubic magnetic anisotropy is dominant, but the interface magnetization exhibits an exchange bias which is absent in the bulk. The implications of these findings for spintronic applications will be discussed. [Preview Abstract] |
Tuesday, March 16, 2010 4:54PM - 5:06PM |
L35.00013: Spin Lifetimes at Fe/GaAs and Fe/AlOx/GaAs interfaces A.T. Hanbicki, C.A. Awo-Affouda, O.M.J. van 't Erve, C.H. Li, G. Kioseoglou, M.A. Holub, B.T. Jonker Three terminal Hanle measurements are useful for probing the spin lifetime at magnetic contact/semiconductor interfaces. In this technique, a spin polarized charge current is injected into a transport channel and the resulting spin accumulation under the injecting contact leads to a split in the chemical potential between majority and minority spins in the semiconductor. We inject a spin polarized current from Fe into GaAs across several different tunnel barriers and use the Hanle effect to determine spin lifetimes by monitoring the voltage drop as a function of perpendicular magnetic field. The measured Hanle curves are modeled to extract the spin lifetime under the magnetic contacts. By fitting experimental data using the spin lifetime as the only fitting parameter we obtain a spin lifetime of 4 ns for spin injection from Fe into GaAs through a Schottky contact. These results will be compared with data we obtained on Fe/AlOx/GaAs using the same measurement geometry. This work was supported by core programs at NRL. [Preview Abstract] |
Tuesday, March 16, 2010 5:06PM - 5:18PM |
L35.00014: Spin Modulation in Semiconductor Lasers Jeongsu Lee, William Falls, Rafal Oszwaldowski, Igor Zutic We study analytically dynamical operation of semiconductor lasers with injection (pump) of spin-polarized electrons, previously considered in the steady-state regime [1-5]. Using complementary approaches of quasi-static and small signal analyses, by carefully including the presence of holes [4,6], we elucidate how the spin modulation in semiconductor lasers [7] improves performance, as compared to the conventional (spin-unpolarized) counterparts. We reveal that the spin-polarized injection can lead to an enhanced bandwidth and desirable switching properties of spin-lasers. Supported by ONR, AFOSR, NSF-ECCS CAREER. \\[4pt] [1] J. Rudolph et al., Appl. Phys. Lett. 82, 4516 (2003). \\[0pt] [2] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007). \\[0pt] [3] S. Hovel et al., Appl. Phys. Lett. 92, 041118 (2008). \\[0pt] [4] C. Gothgen, R. Oszwaldowski, A. Petrou, I. Zutic, Appl. Phys. Lett. 93, 042513 (2008). \\[0pt] [5] I. Vurgaftman et al., Appl. Phys. Lett. 93, 031102 (2008). \\[0pt] [6] I. Zutic, J. Fabian, and S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004). \\[0pt] [7] J. Lee, W. Falls, R. Oszwaldowski, and I. Zutic, preprint. [Preview Abstract] |
Tuesday, March 16, 2010 5:18PM - 5:30PM |
L35.00015: Theory of Quantum-Dot Spin-Polarized Lasers Christian Gothgen, Rafal Oszwaldowski, Jeongsu Lee, Igor Zutic Semiconductor spin-lasers, relying on injection of spin polarized carriers, have demonstrated the potential to go beyond magnetoresistive effects, both in steady state [1-5] and modulated-injection regime [6]. Here, we develop Rate Equations to study differences between spin lasers based on Quantum Wells or Quantum Dots as the active region. In the latter system, the carriers are captured in the active region from the wetting layer. The finite capture rate imposes constraints on the parameters determining the laser action threshold. In spite of this, we find that the maximum reduction of the threshold with respect to an equivalent spin-unpolarized Quantum-Dot laser is still sizable. We also discuss how the spin-laser saturation depends on the parameters appearing in the Rate Equations. Supported by ONR, AFOSR, and NSF-ECCS CAREER. [1] J. Rudolph et al., Appl. Phys. Lett. 82, 4516 (2003). [2] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007). [3] S. Hoevel et al., Appl. Phys. Lett. 92, 041118 (2008). [4] D. Basu et al., Appl. Phys. Lett. 92, 09119 (2008). [5] C. Gothgen, R. Oszwaldowski, A. Petrou, I. Zutic, Appl. Phys. Lett. 93, 042513 (2008). [6] I. Vurgaftman et al., Appl. Phys. Lett. 93, 031102 (2008). [7] J. Lee, W. Falls, R. Oszwaldowski, and I. Zutic, preprint. [Preview Abstract] |
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