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  • Spintronics Based on Topological Insulators

    Yabin Fan* and Kang L. Wang

    Department of Electrical EngineeringUniversity of California, Los Angeles

    California 90095, USA*yabin@seas.ucla.eduwang@ee.ucla.edu

    Received 6 September 2016Accepted 14 October 2016Published 31 October 2016

    Spintronics using topological insulators (TIs) as strong spinorbit coupling (SOC) materials haveemerged and shown rapid progress in the past few years. Dierent from traditional heavy metals,TIs exhibit very strong SOC and nontrivial topological surface states that originate in the bulkband topology order, which can provide very ecient means to manipulate adjacent magneticmaterials when passing a charge current through them. In this paper, we review the recentprogress in the TI-based magnetic spintronics research eld. In particular, we focus on the spinorbit torque (SOT)-induced magnetization switching in the magnetic TI structures, spintorqueferromagnetic resonance (ST-FMR) measurements in the TI/ferromagnet structures, spinpumping and spin injection eects in the TI/magnet structures, as well as the electrical detectionof the surface spin-polarized current in TIs. Finally, we discuss the challenges and opportunities inthe TI-based spintronics eld and its potential applications in ultralow power dissipation spin-tronic memory and logic devices.

    Keywords: Topological insulators; spinorbit torques; magnetization switching; topologicalspintronics.

    1. Introduction

    One goal in the eld of spintronics is to controland manipulate magnetic moment using the leastpossible power though electrical means.13 Begin-ning with the discoveries of giant magnetoresis-tance (GMR)4,5 and tunneling magnetoresistance(TMR),612 which allowed electrical readout ofthe relative orientation of magnetic moments inspin valves13,14 and magnetic tunnel junctions(MTJs), the spintronics eld has further developedthrough theoretical prediction and experimentalobservation of the spin-transfer torque (STT)

    eect,1520 which allows manipulation and switchingof magnetic moment by the electrical current.STT-based two-terminal MTJ structures haveserved as ecient magnetoresistive random accessmemory (MRAM)2123 elements, where the `read'and `write' processes use the same current path, andthereby have the intrinsic electrical wear-out prob-lem, that is, the dielectric breakdown of the tunnelbarrier due to the large current in the `write' pro-cess.24 One of the methods to overcome this issue isto use high spin-orbit coupling (SOC) materials,such as heavy metals and topological insulators

    Corresponding author.

    SPINVol. 6, No. 2 (2016) 1640001 (13 pages) World Scientic Publishing CompanyDOI: 10.1142/S2010324716400014

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    http://dx.doi.org/10.1142/S2010324716400014

  • (TIs), where the spin Hall eect (SHE) or the to-pological surface states can be employed to generatelarge spin torques by passing a lateral current in thethree-terminal MTJ structures.25,26 In this case, the`read' current path and the `write' current path canbe well separated and the spin torque is generatedwithout charge current traversing the tunnel barri-er. This separation signicantly improves the MTJreliability while also giving better output signal.Since these materials all have strong SOC we call thegenerated torques spinorbit torques (SOTs).27,28 Inthe following two paragraphs, we briey introducethe recent SOT research in the heavy metal and TI-based magnetic structures.

    In heavy metals (e.g., Pt and Ta), due to thestrong relativistic SOC, the SHE2933 is resultedwhich can generate a transverse spin current andconsequently, a steady-state spin polarization/ac-cumulation at the edges or surfaces of the heavymetals when passing a charge current through them.The SHE arises from both the intrinsic34,35 and ex-trinsic mechanisms.3639 The SHE was rst detectedby optical method in semiconductors with largeSOC (e.g., GaAs and InGaAs)40,41 and later, it wasdiscovered in heavy metals and has been studiedextensively, due to the fact that SHE in heavymetals is very ecient to produce spin current thatcan apply very strong SOTs on an adjacent mag-netic layer and signicantly inuence its dynamics,for example, the SHE-induced tuning of magneticdamping4245 and spin wave attenuation,46,47 exci-tation of spin wave oscillations,48 magnetic preces-sion,43 SHE-induced switching25,4951 and magneticdomain wall motion,5254 etc., have been reported inthe past several years. In particular, the SHE-in-duced magnetization switching25,49,50 represents thelandmark achievements in the SHE research eld,which suggest potential applications in heavy metal-based MRAM devices.26 The spin Hall angle,25,29,50

    dened as the generated spin current density versusthe incident charge current density, is used toquantify the SHE strength in heavy metals and thenumber is usually smaller than 1. Therefore, mate-rials that can generate SOT more eciently thanheavy metals await explorations.

    Then it comes to TIs.5557 TIs are such materialsthat the SOC is large enough to invert the bandstructure in the bulk,58 and consequently they areexpected to be the most promising candidates toexploit the SOTs when coupled to magnetic mate-rials.5961 Compared with the research on SHE in

    heavy metals, TI-based spintronics is a relativelynew emerging research eld. However, it has shownrapid progress62 and promising applications on SOTdevices.6365 Distinct from heavy metals, TIs notonly have very strong SOC, but also possess theunique spin-momentum-locked Dirac fermions onthe surface,5557 as demonstrated by the spin-resolved ARPES66,67 and circularly-polarized light-induced photocurrent experiments,6870 which makethem very ecient for generating spin current/ac-cumulation when passing a charge current throughthem.71 Starting with the electrical detection ofsurface spin-polarized current in TIs,7277 SOT andspin current-related research has been carried out inTI-based magnetic structures. For example, SOT-induced magnetization switching in magnetic TImaterials,63,64 spintorque ferromagnetic resonance(ST-FMR) measurements65,78 and spin pumping/spin injection experiments7983 in TI/ferromagnetmetal structures, and spin-polarized tunnelingspectroscopy study in TI/oxide/ferromagnet struc-tures84 have been carried out recently. The TI-basedmagnetic spintronics research is currently a fastdeveloping eld.

    In this paper, we review the recent progresses inthis newly emerged eld of TI-based spintronics.The paper is organized as follows. First, Sec. 2 oersan introduction to the giant SOT-induced magne-tization switching in magnetically doped TI struc-tures. The SOT eciency in TI is revealed to bealmost three orders of magnitude larger than thosereported in heavy metals. Section 3 discusses theST-FMR measurements in TI/ferromagnet metalstructures which show very large SOT eciencyat room temperature. Section 4 reviews the spinpumping and spin injection experiments in variousTI/magnet structures which suggest promising spin-to-charge conversion eect. Electrical detection ofsurface spin-polarized current in TIs is discussed inSec. 5, emphasizing the interplay between magneticelectrode and accumulated spins on the topologicalsurface. This is followed by summary and conclu-sions in Sec. 6, discussing the potential and oppor-tunities in the TI-based spintronics eld.

    2. SOT-Induced MagnetizationSwitching in Magnetic TI Materials

    TIs are a new class of materials which have invertedband structure in the bulk due to the strong SOC58

    and nontrivial metallic Dirac fermions on the surface

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  • which are protected by the bulk topology order.5557

    The surface Dirac fermions are described by theHamiltonian55,56,58,85,86: HDirackx; ky }vDxkyykx, where } is the reduced Planck constant, vD isthe Dirac electron velocity, kx and ky are the Diracelectron momentum, x and y are the Pauli ma-trices denoting the electron spin. The eigenstates ofthis Hamiltonian naturally give rise to the spin-momentum locking feature, namely, the Dirac elec-tron's spin is locked perpendicular to its momentumin the kxky-plane and the spin texture is clockwiseabove the Dirac point while anticlockwise below theDirac point, as depicted in Fig. 1(a). In the TI ma-terial when a lateral electric eld is applied, due tothe spin-momentum locking feature of the surfacestates and the shift of the Fermi surface in the k-space, a certain spin polarization is induced withthe surface charge current,71 and the polarization isgoverned by the charge current direction. For in-stance, when the surface charge current is owingalong the x direction (i:e., Ix < 0), as shown inFig. 1(b), the Dirac electrons' spin is polarized alongthe y direction; the reversed current case is dis-played in Fig. 1(c). Because of the strong SOC andthe topology protection nature of the surface states,these spin-polarized surface currents are expected tobe able to apply very ecient SOTs to adjacentmagnetic materials.5961

    In light of the above arguments, dierent TI/magnet structures6365,78 have been studied t

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