Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L5: Spin Manipulation in Semiconductors and Metals for Spintronics |
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Sponsoring Units: DCMP Chair: Naoto Nagaosa, University of Tokyo Room: Colorado Convention Center Korbel 1A-1B |
Tuesday, March 6, 2007 2:30PM - 3:06PM |
L5.00001: Coulomb Interaction in the Spin-Hall Effect Invited Speaker: The spin-Hall effect is the generation of a steady spin current perpendicular to an externally imposed d.c. electric field. The effect is driven by spin-orbit interactions but its details are influenced by several processes like electron-impurity scattering, electron-electron scattering, and spin precession. In this talk I describe our recent work on the role of electron- electron scattering in the spin Hall effect in an n-type [110] GaAs quantum well, where spin precession is absent. We have studied the spin Hall conductivity (SHC) by a combination of the Boltzmann equation and the Kubo formula for the spin current [1],[2]. The two main contributions to the SHC -- ``skew scattering'' (SS) and ``side-jump'' (SJ) -- respond very differently to the inclusion of Coulomb interactions. The SS contribution is significantly reduced by the spin Coulomb drag -- the Coulomb friction between electrons of opposite spin orientations. At the same time, the SJ contribution remains completely unaffected by Coulomb scattering. The different behaviors of the SS and SJ contributions result in a Coulomb- induced reduction of the spin accumulation at the edges of a spin Hall bar, even when the spin current is zero. We have also pointed out that the relative size of the SJ and SS contributions depends on mobility and we have proposed an experiment to distinguish between the two [2]. [1] E. M. Hankiewicz and G. Vignale Phys. Rev. B 73, 115339 (2006) [2] E. M. Hankiewicz, G. Vignale and M. E. Flatt\'e cond- mat/0603144 (PRL in press) [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:42PM |
L5.00002: Electrically-Induced Polarization and the Spin Hall Effect in Semiconductors at Room Temperature Invited Speaker: The capability to generate and manipulate spin polarization through the spin-orbit interaction inspires growing interest in all-electrical techniques to exploit electron spins for applications in semiconductor spintronics. Experiments show spin polarization can be electrically generated by current- induced spin polarization from internal magnetic fields in the bulk of a conducting channel, or accumulation of spin polarization near sample edges due to transverse spin currents generated by the spin Hall. These spin currents can drive spin accumulation over micron length scales in semiconductor arms transverse to a conducting channel \footnote{V. Sih, W. H. Lau, R. C. Myers, V. R. Horowitz, A. C. Gossard and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{97}, 096605 (2006).}. More recently, we investigate electrical generation of spin polarization in n-ZnSe epilayers using Kerr rotation spectroscopy\footnote{N. P. Stern, S. Ghosh, G. Xiang, M. Zhu, N. Samarth, and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{97}, 126603 (2006)}. The internal magnetic field is studied and found to only be measurable in strained layers, likely due to the weak spin-orbit interaction in ZnSe. Despite this, unstrained n-ZnSe layers exhibit both in-plane bulk current-induced spin polarization and an out-of-plane spin accumulation of opposite sign on opposite edges of a conducting channel indicative of the spin Hall effect. The spin Hall conductivity is estimated according to a spin accumulation model and is found to be consistent with the extrinsic spin- dependent scattering mechanism. Both the current-induced spin polarization and the spin Hall effect are robust to room temperature in ZnSe. These results suggest the potential for practical utilization of electrically generated spin polarization in room temperature semiconductor devices. [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 4:18PM |
L5.00003: Spin Transport and Scattering in Ferromagnetic Semiconductor Heterostructures Invited Speaker: A fundamental understanding of the transport and scattering of spin-polarized carriers in semiconductors is central to the development of semiconductor spintronics. We describe recent work that probes the spin-dependent transport of holes in heterostructures derived from the ferromagnetic semiconductor (Ga,Mn)As. In tensile-strained (Ga,Mn)As/(In,Ga)As heterostructures with perpendicular magnetic anisotropy, we observe a longitudinal magnetoresistance that is {\it antisymmetric} in magnetic field and attributed to slowly propagating magnetic domain walls [1]. This is confirmed both by a simple calculation and by measuring patterned submicron channels designed to trap single domain walls. In (Ga,Mn)As/p-GaAs/(Ga,Mn)As trilayer heterostructures, we demonstrate an all-semiconductor spin-valve effect, despite short spin-diffusion and elastic scattering lengths in the spacer layer [2]. Magnetoresistance (MR) measurements carried out in the current-in-plane geometry reveal positive MR peaks when the two ferromagnetic layers are magnetized orthogonal to each other. Measurements with different post-growth annealing conditions and spacer layer thickness show that the positive MR originates in a noncollinear spin valve effect due to spin-dependent scattering at interfaces. \newline \newline [1] G. Xiang, A. W. Holleitner, B. L. Sheu, F. M. Mendoza, O. Maksimov, M. B. Stone, P. Schiffer, D. D. Awschalom, N. Samarth, Phys. Rev. B {\bf 71}, 241307(R) (2005).\newline [2] G. Xiang, M. Zhu, B. L. Sheu, P. Schiffer, N. Samarth, cond-mat/0607580. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:54PM |
L5.00004: Electrical detection of spin transport in lateral ferromagnet-semiconductor devices Invited Speaker: A fully electrical scheme of spin injection, transport, and detection in a single ferromagnet-semiconductor structure has been a long-standing goal in the field of spintronics. In this talk, we report on an experimental demonstration of such a scheme. The devices are fabricated from epitaxial Fe/GaAs (100) heterostructures with highly doped GaAs as a Schottky tunnel barrier. A set of closely spaced Fe contacts on the top of an n-GaAs channel are used as spin injectors and detectors. Reference electrodes are placed at the far ends of the channel, allowing for non-local spin detection [1]. The electro-chemical potential of the detector is sensitive to the relative magnetizations of the injector and detector. In spin-valve measurements, a magnetic field is applied along the Fe easy axis to switch the relative magnetizations of injector and detector from parallel to antiparallel, resulting in a voltage jump that is proportional to the non-equilibrium spin polarization in the channel. A more rigorous test of electrical spin detection is the observation of the Hanle effect, in which an out-of-plane magnetic field is used to modulate and dephase the spin polarization in the channel. The magnitudes of the observed Hanle curves agree with the results of the spin-valve measurements. The dependence of the Hanle curves on temperature and contact separation is studied in detail and is consistent with a drift-diffusion model incorporating spin precession and relaxation. The spin polarization generated by spin injection (reverse bias at the injector) or spin accumulation (forward bias at the injector) is measured using the magneto-optical Kerr effect and is found to be in good agreement with the spin-dependent non-local voltage. Both the transport and optical measurements show a non-linear relationship between the bias voltage at the injector and the spin polarization in the channel. [1] M. Johnson and R. H. Silsbee, Phys. Rev. Lett. \textbf{55}, 1790 (1985). [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:30PM |
L5.00005: Current-induced domain wall motion in ferromagnetic semiconductors Invited Speaker: Low magnetization ($\sim $0.05 T) and high spin-polarization in ferromagnetism of transition metal-doped GaAs allow us to explore a number of spin-dependent phenomena not readily accessible in metal ferromagnets. Spin-polarized current induced domain wall (DW) motion in (Ga,Mn)As [1, 2] reveals rich physics resulting from the interaction between spin-polarized electrons and localized spins inside a magnetic DW. By using a 30 nm thick (Ga,Mn)As layer ($x_{Mn}$ = 0.045) with perpendicular magnetic anisotropy, we have measured by magneto-optical Kerr microscopy a wide range of velocity-current density curves in the sample temperature range of 97 -- 107 K. Two regimes are found in the current density dependence of the DW velocity. At high-current densities ($>$ 2 x 10$^{5}$ A/cm$^{2})$, the domain wall velocity is approximately a linear function of the current density above a threshold current density. This result will be compared to the recent theories of DW motion. At low-current densities, the functional form of the velocity-current curves follow an empirical scaling law, obtained by modifying the one for magnetic-field induced creep. This shows that current-induced DW creep is present. We have also determined the intrinsic resistance of the DW in a similar configuration [3]. \begin{enumerate} \item M. Yamanouchi, D. Chiba, F. Matsukura, and H. Ohno, Nature \textbf{428}, 539 (2004). \item M. Yamanouchi, D. Chiba, F. Matsukura, T. Dietl and H. Ohno, Phys. Rev. Lett. \textbf{96}, 096601 (2006). \item D. Chiba, M. Yamanouchi, F. Matsukura, T. Dietl, and H. Ohno, Phys. Rev. Lett. \textbf{96}, 096602 (2006). \end{enumerate} [Preview Abstract] |
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