Electrical Detection of Spin-Polarized Surface States Conduction in (Bi 0.53 Sb 0.47 ) 2 Te 3 Topological Insulator Jianshi Tang,* ,† Li-Te Chang, † Xufeng Kou, † Koichi Murata, † Eun Sang Choi, ‡ Murong Lang, † Yabin Fan, † Ying Jiang, § Mohammad Montazeri, † Wanjun Jiang, † Yong Wang, § Liang He,* ,† and Kang L. Wang* ,† † Device Research Laboratory, Department of Electrical Engineering, University of California, Los Angeles, California 90095, United States ‡ National High Magnetic Field Laboratory, Tallahassee, Florida 32310, United States § Center for Electron Microscopy and State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China * S Supporting Information ABSTRACT: Strong spin-orbit interaction and time-reversal symmetry in topological insulators enable the spin-momentum locking for the helical surface states. To date, however, there has been little report of direct electrical spin injection/ detection in topological insulator. In this Letter, we report the electrical detection of spin-polarized surface states conduction using a Co/Al 2 O 3 ferromagnetic tunneling contact in which the compound topological insulator (Bi 0.53 Sb 0.47 ) 2 Te 3 was used to achieve low bulk carrier density. Resistance (voltage) hysteresis with the amplitude up to about 10 Ω was observed when sweeping the magnetic field to change the relative orientation between the Co electrode magnetization and the spin polarization of surface states. The two resistance states were reversible by changing the electric current direction, affirming the spin-momentum locking in the topological surface states. Angle-dependent measurement was also performed to further confirm that the abrupt change in the voltage (resistance) was associated with the magnetization switching of the Co electrode. The spin voltage amplitude was quantitatively analyzed to yield an effective spin polarization of 1.02% for the surface states conduction in (Bi 0.53 Sb 0.47 ) 2 Te 3 . Our results show a direct evidence of spin polarization in the topological surface states conduction. It might open up great opportunities to explore energy-efficient spintronic devices based on topological insulators. KEYWORDS: Topological insulator, spin polarization, surface states, spin-momentum locking, spin detection S ince the discovery of two-dimensional (2D) and three- dimensional (3D) topological insulators (TIs), 1-5 they have attracted extensive research interest for their exotic physical properties that could lead to dissipationless transport in the quantum spin Hall state. 6-9 Recent studies have shown a giant spin-orbit torque in TI originating from the strong spin- orbit interaction, 10,11 which enabled the current-induced magnetization switching through spin-transfer torque with a low current density. The unique feature of 3D TI, for instance, is that it has both insulating bulk and gapless Dirac surface states. 8,9 Ternary TI compounds, such as (Bi x Sb 1-x ) 2 Te 3 , have been widely investigated for their tunability to achieve low bulk carrier density and manifest topological surface states conduction. 12,13 The presence of surface states is supported by extensive angle-resolved photoemission spectroscopy (ARPES) measurements and transport studies, 14-20 such as Shubnikov-de Haas (SdH) and Aharonov Bohm (AB) quantum oscillations. Because of the strong spin-orbital interaction in TI, direct back scatterings from nonmagnetic impurities are prohibited by the time-reversal symmetry. 8,9 More importantly, the spin-momentum locking naturally leads to a current- induced spin polarization in surface states; 21 the surface states conduction is spin-polarized once an electric current is passed through a TI film, and this spin polarization can be accordingly reversed by simply flipping the electric current direction. 22,23 As a result, it has been proposed to use TI as a promising spin injection source to inject spin-polarized carriers into non- magnetic materials, such as metal and graphene. 24-26 The presence of spin-polarized surface states has been mainly examined using optical methods. For example, spin-resolved ARPES has been widely used to resolve the helical spin texture at different energy levels, 14-16 and the spin texture is found to be opposite for above and below the Dirac point. 15 Another approach is to use circularly polarized light to excite spin- Received: July 10, 2014 Revised: August 24, 2014 Published: August 26, 2014 Letter pubs.acs.org/NanoLett © 2014 American Chemical Society 5423 dx.doi.org/10.1021/nl5026198 | Nano Lett. 2014, 14, 5423-5429