Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H19: Focus Session: Spin Transport & Magnetization Dynamics in Metals III |
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Sponsoring Units: GMAG DMP Chair: Goran Mihajlovic, Hitachi GST Room: D170 |
Tuesday, March 22, 2011 8:00AM - 8:12AM |
H19.00001: Large and inverted spin signals in nonlocal spin valves Yi Ji, Han Zou, Siu-Tat Chui, Xiaojun Wang For a metallic nonlocal spin valve (NLSV), usually the nonlocal resistance value is high for the parallel (P) state of spin injector and detector and the value is low for the antiparallel (AP) state. The difference between two states is known as the spin signal. We show that a 6 miliohms spin signal was observed in a typical NLSV device. However, in another NLSV device with apparently similar structure and dimensions as the previous one, we found a very large spin signal of 90 milliohms with an inverted sign, meaning that the P state yields a low value and the AP state yields a high value. The resistance between the spin detector and the Cu channel is extremely large, exceeding mega-ohms. We conclude that a break-junction is formed at the detector/Cu interface due to static discharge. The large magnitude of the spin signal is due to the spin-charge coupling at the low-conductance break-junction interface. The inverted sign is due to a very different spin-dependent density of states near the break-junction. Work supported by DOE grant No. DE-FG02-07ER46374. [Preview Abstract] |
Tuesday, March 22, 2011 8:12AM - 8:24AM |
H19.00002: Spin transfer effects in non-local spin valves with sustained d.c. currents Han Zou, Yi Ji We utilized pure spin current in a nonlocal spin valve (NLSV) for spin-transfer. The submicron lateral device consists of a Py spin injector (80 nm wide), a Py spin detector (60 nm wide), and a Cu nonmagnetic channel (100 nm wide). The thickness of the spin detector is 3.5 nm, and a nanoscale magnetic domain (60 nm by 100 nm) in the detector underneath the Cu channel can be switched by spin-transfer. We explore reversible spin-transfer switching over a wide temperature range and using both d.c. current pulses and sustained dc currents. Since a d.c. current changes the baseline of the nonlocal resistance, spin-transfer in NLSV has only been explored by d.c. current pulses. In this work, we achieved NLSV spin-transfer with sustained d.c. currents. The hysteresis of nonlocal resistance as a function of the sustained current is clearly observed, despite the baseline variations. High field and polarity-dependent features in the nonlocal MR curves indicate evidence of spin-transfer induced magnetization dynamics. Work supported by US DOE grant No. DE-FG02-07ER46374. [Preview Abstract] |
Tuesday, March 22, 2011 8:24AM - 8:36AM |
H19.00003: Spin injection into ferromagnetic insulators Saburo Takahashi, Jun-ichiro Ohe, Hiroto Adachi, Sadamichi Maekawa Spin current in a junction of a normal metal and a ferromagnetic insulator is theoretically studied. At the interface, spins of conduction electrons in a normal metal interact with localized spins in ferromagnetic insulator through the exchange interaction. When a spin accumulation is present in the normal metal, accumulated spins decay into magnons via spin-flip scattering of conduction electrons at the interface, thereby creating a magnon spin current in the ferromagnet. Using the linear response theory, we calculate the spin current across the interface and find that spin accumulation plays a role of spin voltage for generating the spin current thorough the junction. Using the spin Hall effect, the spin current injection from a normal metal into a ferromagnetic insulator is demonstrated. We also discuss the spin current in the presence of temperature difference between the normal metal and the ferromagnetic insulator. [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 9:12AM |
H19.00004: Quantifying Spin Hall Effects in Nonmagnetic Metals Invited Speaker: Spin Hall effects intermix spin and charge currents even in nonmagnetic materials and, therefore, offer the possibility to generate and detect spin currents without the need for using ferromagnetic materials. In order to gain insight into the underlying physical mechanism and to identify technologically relevant materials, it is important to quantify the spin Hall angle $\gamma$, which is a direct measure of the charge-to-spin (and vice versa) conversion efficiency. Towards this end we utilized non-local transport measurements with double Hall bars fabricated from gold and copper.\footnote{G.~Mihajlovi\'{c}, J.~E.~Pearson, M.~A.~Garcia, S.~D.~Bader, and A.~Hoffmann, Phys.Rev.\ Lett.\ {\bf 103}, 166601 (2009).} In principle, this geometry permits the study of spin currents both generated and detected via spin Hall effects. We observe an unusual non-local resistivity that changes sign as a function of temperature. However, this results is quantitatively similar in gold and cooper, indicating that the non-local signals are not due to spin transport. An analysis of the data based on a combination of diffusive and quasi-ballistic transport leads to an upper limit of $\gamma< 0.027$ for gold at room temperature. Therefore we developed an approach based on spin pumping, which enables us to quantify even small spin Hall angles with high accuracy. Spin pumping utilizes microwave excitation of a ferromagnetic layer adjacent to a normal metal to generate over a macroscopic area a homogeneous {\em dc} spin current, which can be quantified from the line-width of the ferromagnetic resonance. In this geometry voltages from spin Hall effects scale with the device dimension and therefore good signal-to-noise can be obtained even for materials with small spin Hall angles. We integrated ferromagnet/normal metal bilayers into a co-planar waveguide and determined the spin Hall angle for a variety of non-magnetic materials (Pt, Pd, Au, and Mo) at room temperature. Of these materials Pt shows the largest spin Hall angle with $\gamma = 0.013\pm0.002$.\footnote{O.~Mosendz, V.~Vlaminck, J.~E.~Pearson, F.~Y.~Fradin, G.~E.~W.~Bauer, S.~D.~Bader, and A.~Hoffmann, arXiv:1009.5089; O.~Mosendz, J.~E.~Pearson, F.~Y.~Fradin, G.~E.~W.~Bauer, S.~D.~Bader, and A.~Hoffmann, Phys.\ Rev.Lett.{\bf 104}, 046601 (2010).} [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:24AM |
H19.00005: Giant spin Hall effect of Au films with Pt impurities: Surface-assisted skew scattering Bo Gu, Timothy Ziman, Guang-Yu Guo, Naoto Nagaosa, Sadamichi Maekawa We show theoretically a novel route to obtain giant room temperature spin Hall effect (SHE) using surface-assisted skew scattering. By a combined approach of density functional theory and the quantum Monte Carlo (QMC) method, we have studied the SHE due to a Pt impurity in different Au hosts. We show that the spin Hall angle (SHA) could become larger than 0.1 on the Au (111) surface, and decreases by about a half on the Au (001) surface, while it is small in bulk Au. The QMC results show that the spin-orbit interaction (SOI) of the Pt impurity on the Au (001) and Au (111) surfaces is enhanced, because the Pt 5 levels are lifted to the Fermi level due to the valence fluctuations. In addition, there are two SOI channels on the Au (111) surface, while only one for Pt either on the Au (001) surface or in bulk Au. [Preview Abstract] |
Tuesday, March 22, 2011 9:24AM - 9:36AM |
H19.00006: Spin Hall angle in Pd below the spin diffusion length V. Vlaminck, H. Schultheiss, J. Pearson, F. Fradin, S. Bader, A. Hoffmann, O. Mosendz The spin-orbit coupling gives rise to an inter-conversion of spin and charge currents. A pure spin current is accompanied by a charge accumulation perpendicular to both the spin polarization and spin current, so-called inverse spin Hall effect (ISHE). We report measurements of the ISHE in a permalloy/palladium (Py/Pd) bilayer integrated with a coplanar wave-guide by pumping a pure spin current via ferromagnetic resonance (FMR) [1]. The magnetization precession creates a spin accumulation at the Py/Pd interface that diffuses into the normal metal and partially scatters back into the permalloy when the Pd thickness is smaller than the spin diffusion length. We observe an increasing broadening of the FMR linewidth with increasing thickness of Pd from which we extract the spin diffusion length in Pd and an average spin mixing conductance. The resultant pure spin current induces, in turn, a spin Hall voltage that is measured across the metallic layer. The spin Hall angle obtained from fitting the dc voltage [1] remains fairly constant even for thickness smaller than the spin diffusion length.\\[4pt] [1] O. Mosendz et al., Phys. Rev. B (in press). arXiv: 1009.5089 [Preview Abstract] |
Tuesday, March 22, 2011 9:36AM - 9:48AM |
H19.00007: Spin Torque Ferromagnetic Resonance Induced by the Spin Hall Effect Luqiao Liu, Takahiro Moriyama, Dan Ralph, Robert Buhrman We demonstrate that the spin Hall effect in a thin film with strong spin-orbit scattering can excite magnetic precession in an adjacent ferromagnetic film. The flow of alternating current through a Pt/NiFe bilayer generates an oscillating transverse spin current in the Pt, and the resultant transfer of spin angular momentum to the NiFe induces ferromagnetic resonance (FMR) dynamics. The Oersted field from the current also generates an FMR signal but with a different symmetry. The ratio of these two signals allows a quantitative determination of the spin current. As an independent check, we also apply a DC charge current to the Pt/NiFe bilayer while measuring the FMR signal. The effective damping of the NiFe layer can be increased or decreased depending on the relative angle between the magnetic moment and the injected spin. The amplitude of spin current extracted from this measurement agrees quite well with that obtained from the FMR lineshape. The self-calibration nature of this new technique makes it an excellent solution for a quantitative measurement of the SHE in a ferromagnetic/non-magnetic metal bilayer. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:00AM |
H19.00008: Spin-orbit dichroism in SX-ARPES of Pt(111) Jun Miyawaki, Ashish Chainani, Yasutaka Takata, Masaki Oura, Yasunori Senba, Haruhiko Ohashi, Shik Shin We study the bulk electronic structure of Pt(111) using polarization dependent soft x-ray (SX)-ARPES ($h\nu$=450--610 eV). Pt is known to exhibit the largest spin Hall conductivity of all metals, which is derived from its large spin orbit coupling [1,2]. We have measured band dispersions along $\Gamma$-K-X ($h\nu$=466 eV) with clockwise and counterclockwise circularly polarized x-rays and obtained circular dichroism (CD) in the valence band of Pt. A comparison with calculated band dispersions including spin-orbit coupling gives a very good match with the experimental results [3,4], thus establishing the role of spin-orbit coupling in the electronic structure of Pt. Our results also identify (i) a hybridization gap with symmetry switching dichroism and (ii) strong CD of bands at the Fermi level, which provide the carriers responsible for SHE.\\[0pt] [1] T.~Kimura, {\it et al.}, Phys.~Rev.~Lett.~{\bf 98}, 156601 (2007). [2] M.~Morota, {\it et al.}, arXiv:1008.0158v1. [3] G.~Y.~Guo, {\it et al.}, Phys.~Rev.~Lett.~{\bf 100}, 096401 (2008). [4] A.~D.~Corso and A.~M.~Conte, Phys.~Rev.~B {\bf 71}, 115106 (2005). [Preview Abstract] |
Tuesday, March 22, 2011 10:00AM - 10:12AM |
H19.00009: Anisotropic Spin Hall Effect from First Principles Frank Freimuth, Stefan Bl\"ugel, Yuriy Mokrousov We present first principles calculations [1] of the intrinsic non-dissipative spin Hall conductivity (SHC) for 3$d$, 4$d$ and 5$d$ transition metals focusing in particular on the anisotropy of the SHC in nonmagnetic hcp metals and in antiferromagnetic Cr. For the metals of this study we generally find large anisotropies. We derive the general relation between the SHC vector and the direction of spin-polarization and discuss its consequences for hcp metals. Especially, it is predicted that for systems where the SHC changes sign due to the anisotropy the spin Hall effect may be tuned such that the spin polarization is parallel either to the electric field or to the spin current. Additionally, we describe our computational method [2,3] emphasizing the Wannier interpolation technique and the definition of the conserved spin current.\\[4pt] [1] e-print: http://arxiv.org/abs/1011.2714\\[0pt] [2] F. Freimuth et al. Phys. Rev. B \textbf{78}, 035120 (2008)\\[0pt] [3] www.flapw.de [Preview Abstract] |
Tuesday, March 22, 2011 10:12AM - 10:24AM |
H19.00010: Detection of the transverse voltage associated with the spin Seebeck effect in ferromagnetic thin films Azure D. Avery, Rubina Sultan, Dain Bassett, Matthew R. Pufall, Barry L. Zink The spin Seebeck effect, the generation of spin current in response to an applied thermal bias across a sample, is a novel effect involving spin current that is being researched in nanostructures for advances in spin caloritronics. Understanding the fundamental physics governing heat transport at the nanoscale is challenging because thermal properties of nanostructures are often difficult measurements to make. We present a novel technique for detecting the presence of a thermally generated spin current based on a micromachined thermal isolation platform. Our technique offers advantages including the ability to measure this effect in a reduced dimension sample, to reverse the thermal gradient, and to generate a large $\triangle T$ across the sample. We present results for a range of thin films and compare to previously reported similar larger scale structures. We discuss future experiments to probe the local nature of the spin Seebeck effect, additional thermal properties including the traditional Seebeck effect and thermal conductivity, and the application of our technique to an array of nanowires. [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 10:36AM |
H19.00011: Gigantic enhancement of spin Seebeck effect by phonon drag Hiroto Adachi, Ken-ichi Uchida, Eiji Saitoh, Jun-ichiro Ohe, Saburo Takahashi, Sadamichi Maekawa We investigate both theoretically and experimentally a gigantic enhancement of the spin Seebeck effect [K. Uchida et al., Nature 455, 778 (2008); C. M. Jaworski et al., Nature Mater. 9, 898 (2010); K. Uchida et al., Nature Mater. 9, 894 (2010)] in a prototypical magnet LaY$_2$Fe$_5$O$_{12}$ at low temperatures. Our theoretical analysis sheds light on the important role of phonons; the spin Seebeck effect is enormously enhanced by nonequilibrium phonons that drag the low-lying spin excitations. We further argue that this scenario gives a clue to understand the observation of the spin Seebeck effect that is unaccompanied by a global spin current, and predict that the substrate condition affects the observed signal. [Preview Abstract] |
Tuesday, March 22, 2011 10:36AM - 10:48AM |
H19.00012: Two Exponentials Associated with Temperature in Spin-Seebeck Effect Geometry Wayne Saslow, Matthew Sears Recent experiments report the observation of a Spin-Seebeck effect, where an applied thermal gradient along (x) a very thin (z), narrow (y) ferromagnetic sample F is associated with a spin current.\footnote{K. Uchida et al, Nature 455, 778 (2008).} In present geometries this spin current is measured indirectly via a Pt bar above (z) the sample; a voltage difference $V$ along y is measured and interpreted as being due to a spin current $j_s$ into (z) the Pt, which then causes an inverse Spin Hall effect ($j_s$ causes transverse $V$). Measured voltages often show a $\sinh(x/s)$ dependence, where $s$ is long compared to any relevant spin-diffusion length.\footnote{C. M. Jaworski et al, Nature Materials 9, 898 (2010).} The spin current has been interpreted as accompanying a temperature disequilibrium between the phonons and magnons in F.\footnote{J. Xiao et al, Phys. Rev. B 81, 214418 (2010).} The present work uses irreversible thermodynamics to include magnon-phonon equilibration in F and the thermal properties of the (non-magnetic) substrate S. We find two exponentials describing the overall thermal response along x, the second one associated with equilibration between F and S. If the thermal coupling between F and S is poor, then the second length can be rather long. [Preview Abstract] |
Tuesday, March 22, 2011 10:48AM - 11:00AM |
H19.00013: Dissipationless mechanism of skyrmion Hall effect in two-dimensional double-exchange ferromagnets Shigeki Onoda, Ki-Seok Kim We revisit a theory of nonequilibrium single-skyrmion transport in two-dimensional double-exchange ferromagnets with the Rashba spin-orbit interaction. Combining the collective-coordinate approach with the Keldysh formalism and an effective U(1) gauge theory, the velocity of a skyrmion core is calculated under the electric field. Then, it is found that the emergent Chern-Simons term and the associated intrinsic anomalous Hall can produce a dissipationless skyrmion Hall current through the coupling between electrons and localized spins, which takes the form of the spin transfer torque. In metals, this is additive to the conventional dissipative motion of magnetic vortices, which relies on phenomenological damping terms in the Landau-Lifshitz-Gilbert equation. [Preview Abstract] |
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