Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G52: Invited Session: Spin-Momentum Coupling in Topological Insulator Surface States and Semiconductors |
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Sponsoring Units: DCMP Chair: Daniel Ralph, Cornell Unversity Room: Grand Ballroom C2 |
Tuesday, March 3, 2015 11:15AM - 11:51AM |
G52.00001: Spin Circuit Model for Spin Orbit Torques in 2D Channels Invited Speaker: Seokmin Hong Recently, the unique coupling between charge and spin in topological insulators has been explored through various types of electrical measurements, which could have interesting applications. In this talk, we present a spin circuit model [1] for spin orbit torques in topological insulator surface states and other 2D channels. We show with a simple example that results from the circuit model agree well with those obtained from nonequilibrium Green's function (NEGF) based quantum transport simulation. Some predictions [2] of our model have already received experimental support and we hope this model can provide a unifying framework that can be used to critically evaluate experimental results, to explore new types of devices as well as to answer fundamental questions regarding these materials. The model for spin-orbit torques described here can be incorporated into a broader spin-circuit approach [3] which, we believe, provides a natural platform for multi-physics, multi-component spintronic devices. \\[4pt] [1] S. Hong, S. Sayed, and S. Datta, ``Spin circuit model for spin orbit torques in 2D channels." (in preparation). \\[0pt] [2] S. Hong, V. Diep, S. Datta, and Y. P. Chen, ``Modeling potentiometric measurements in topological insulators including parallel channels," \textit{ Phys. Rev. B.}, 86, 085131 (2012). \\[0pt] [3] https://nanohub.org/groups/spintronics [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:27PM |
G52.00002: Magnetization switching through giant spin-orbit torque in the magnetically doped topological insulators Invited Speaker: Yabin Fan Recent demonstrations of magnetization switching induced by in-plane current in heavy metal/ferromagnetic heterostructures (HMFHs) have drawn great interest to spin torques arising from the large spin-orbit coupling (SOC)...[1-3] in heavy metals. Considering the intrinsic strong SOC, topological insulators (TIs) are expected to be promising candidates for exploring spin-orbit torque (SOT)-related physics...[4, 5]. In this talk, we report the magnetization switching through giant SOT in the magnetically doped TI structures. In particular, we demonstrate the magnetization switching in a chromium-doped TI bilayer heterostructure, and the current induced SOT possibly has contribution from the spin-momentum locked surface states of TI. The critical current density for switching is below 8.9 $\times$ 10$^{4}$A/cm$^{2}$ at 1.9 K. Moreover, we use second-harmonic methods to measure the spin torque efficiencies which are more than three orders of magnitude larger than those reported in heavy metals. The giant SOT and efficient current-induced magnetization switching exhibited by the bilayer heterostructure may lead to innovative spintronics applications such as ultralow power dissipation memory and logic devices.\\[4pt] [1] L. Liu\textit{ et al.}, Science \textbf{336}, 555 (2012).\\[0pt] [2] L. Liu\textit{ et al.}, Phys. Rev. Lett. \textbf{109}, 096602 (2012).\\[0pt] [3] I. M. Miron\textit{ et al.}, Nature \textbf{476}, 189 (2011).\\[0pt] [4] Y. Fan\textit{ et al.}, Nature Mater. \textbf{13}, 699 (2014).\\[0pt] [5] A. R. Mellnik\textit{ et al.}, Nature \textbf{511}, 449 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 1:03PM |
G52.00003: Spin-transfer torque generated by a topological insulator Invited Speaker: Alex Mellnik Magnetic devices are a leading contender for the implementation of memory and logic technologies that are non-volatile, that can scale to high density and high speed, and that do not wear out. However, widespread application of magnetic memory and logic devices will require the development of efficient mechanisms for reorienting their magnetization using the least possible current and power. We report experiments showing that charge current flowing in-plane in a thin film of the topological insulator Bi$_{2}$Se$_{3}$ at room temperature can exert a strong spin-transfer torque on an adjacent metallic ferromagnetic layer, with a direction consistent with that expected from a topological surface state. The spin torque efficiency per unit charge current density in the Bi$_{2}$Se$_{3}$ is larger than any previously measured at room temperature. Our data suggest that topological insulators could enable very efficient electrical manipulation of magnetic materials at room temperature, for memory and logic applications. Related publications: A. R. Mellnik, J. S. Lee, A. Richardella, J. L. Grab, P. J. Mintun, M. H. Fischer, A. Vaezi, A. Manchon, E.-A. Kim, N. Samarth, D. C. Ralph, Nature 511, 449-451 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:39PM |
G52.00004: Observation of chiral currents at the magnetic domain boundary of a topological insulator Invited Speaker: Yihua Wang The broken time-reversal symmetry (TRS) states on the surface of a three-dimensional topological insulator (3D-TI) promise many exotic quantum phenomena. Breaking TRS opens a band gap on the surface Dirac cone and transforms the metallic surface into a Chern insulator. The TRS-broken surface states coupled to a superconductor are predicted to lead to Majorana fermions, which are the fundamental ingredients of topological quantum computation. Just as the surface Dirac cone is a signature of the non-trivial topological bulk band structure of a time-reversal invariant 3D-TI, bulk-boundary correspondence dictates that the TRS-broken surface states with a nonzero Chern number is manifested by a gapless chiral edge state (CES) at the domain boundary. In the special case where the domain boundary is the edge of the sample surface, CES along the edge leads to a quantized anomalous Hall conductance, which was recently measured in a magnetically doped 3D-TI. More generally, a magnetic domain boundary on the surface of TI hosts a CES, which is yet to be directly demonstrated because any local change of conductivity due to the CES does not affect conductance globally. Here we use a scanning superconducting quantum interference device (SQUID) to show that in a uniformly magnetized topological insulator - ferromagnetic insulator (TI-FMI) heterostructure current flows at the edge of the surface of the topological insulator when the Fermi level is gate-tuned to the surface band gap. We further induce micron-scale magnetic structures using the field coil of the SQUID and show that there emerges a chiral edge current at the magnetic domain boundary. In both cases the magnitude of the chiral edge current depends on the chemical potential rather than the applied current. Such magnetic nano-structures, which can be readily created on a TI in an arbitrary geometry, provide a versatile platform for detecting topological magnetoelectric effects and may allow the engineering of magnetically defined quantum bits and spin-based electronics. Hybridization with conventional superconductors may open the door to topological quantum computation. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 2:15PM |
G52.00005: Current-induced spin polarization in anisotropic spin-orbit fields Invited Speaker: Vanessa Sih Current-induced spin polarization is a phenomenon in which carrier spins are oriented when subjected to current flow\footnote{Y. K. Kato, R. C. Myers, A. C. Gossard, and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{93}, 176601 (2004).}. However, the mechanism that produces this spin polarization remains an open question. Existing theory predicts that the spin polarization should be proportional to the spin-orbit splitting yet no clear trend has been observed experimentally. We perform experiments on semiconductor samples designed so that the magnitude and direction of the in-plane current and applied magnetic field can be varied and measure the electrical spin generation efficiency and spin-orbit splitting using optical techniques\footnote{B. M. Norman, C. J. Trowbridge, D. D. Awschalom, and V. Sih, \textit{Phys. Rev. Lett.} \textbf{112}, 056601 (2014).}. Contrary to expectation, the magnitude of the current-induced spin polarization is shown to be larger for momentum directions corresponding to smaller spin-orbit splitting. In addition, angle-dependent measurements demonstrate that the steady-state in-plane spin polarization is not along the direction of the spin-orbit field, which we attribute to anisotropic spin relaxation. Furthermore, we show that this electrically-generated electron spin polarization can produce a nuclear spin hyperpolarization through dynamic nuclear polarization\footnote{C. J. Trowbridge, B. M. Norman, Y. K. Kato, D. D. Awschalom, and V. Sih, \textit{Phys. Rev. B} \textbf{90}, 085122 (2014).}. [Preview Abstract] |
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