Topological insulator based spin valve devices: Evidence for spin polarized transport of spin-momentum-locked topological surface states

Abstract

Spin-momentum helical locking is one of the most important properties of the nontrivial topological surface states (TSS) in 3D topological insulators (TI). It underlies the iconic topological protection (suppressing elastic backscattering) of TSS and is foundational to many exotic physics (eg., majorana fermions) and device applications (eg., spintronics) predicted for TIs. Based on this spin-momentum locking, a current flowing on the surface of a TI would be spin-polarized in a characteristic in-plane direction perpendicular to the current, and the spin-polarization would reverse when the current direction reverses. Observing such a spin-helical current in transport measurements is a major goal in TI research and applications. We report spin-dependent transport measurements in spin valve devices fabricated from exfoliated thin flakes of Bi 2 Se 3 (a prototype 3D TI) with ferromagnetic (FM) Ni contacts. Applying an inplane magnetic (B) field to polarize the Ni contacts along their easy axis, we observe an asymmetry in the hysteretic magnetoresistance (MR) between opposite B field directions. The "polarity" of the asymmetry in MR can be reversed by reversing the direction of the DC current. The observed asymmetric MR can be understood as a spin-valve effect between the current-induced spin polarization on the TI surface (due to spin-momentum-locking of TSS) and the spin-polarized ferromagnetic contacts. Our results provide a direct transport evidence for the spin helical current in TSS.2

Detection of current induced spin polarization in epitaxial Bi2Te3 thin film

“…This current induced spin polarization of the SS, controllable by the magnitude and the direction of the current, can be used to torque a ferromagnet (FM) [4,5]. In recent experiments [7][8][9][10][11][12][13][14][15][16][17][18][19], the spin accumulation on the surface of 3D TIs Bi 2 Se 3 , (Bi x Sb 1−x ) 2 Te 3 , Bi 1.5 Sb 0.5 Te 1.7 Se 1.3 , BiSbTeSe 2 , Bi 2 Te 2 Se and Sb 2 Te 3 , mostly grown by molecular beam epitaxy (MBE) or exfoliated, were electrically measured by the voltage probed with FM contact, where the voltage depends on the projection of SS spin polarization onto the FM magnetization direction.In this work, we detect the current induced spin polarization on the surface of an MBE grown Bi 2 Te 3 thin film using Fe contact deposited on the surface and separated by a thin MgO barrier. We also provide a theoretical estimate of the detected spin signal, i.e., the voltage probed with the FM contact.…”

Large Magnetoresistance at Room Temperature in Ferromagnet/Topological Insulator Contacts

“…An essential feature of these TI surfacestates is helical spin-momentum locking due to strong spinorbit interactions, which provides near-ideal transduction between spin and charge transport, making Tis promising material for spintronic applications [1][2][3]. An applied charge current induces a net spin polarization/accumulation at the TI surface, which can be controlled by the amplitude and the polarity of the current [4][5][6][7][8][9][10][11][12][13][14][15]. For example, choosing the x and y directions along the TI surface and the z direction pointing away from the TI, electron flow in the negative x direction within either conduction or valence band surface states results in a net electron spin in the positive y direction.…”

“…Low temperature magneto-transport measurements have been performed by several groups using stack(s) of molecular beam epitaxy (MBE) grown ferromagnetic metal (FM) layers either on TI flakes or on MBE-grown TIs [11][12][13][14]. These groups have used Bi2Se3 or BiSbTe TIs and Fe, Co or Py FMs.…”

Magnetic properties of Ni films deposited on MBE grown Bi2Se3 layers

“…Indeed, the spin momentumlocking phenomenon, which is known as the unique property of TI surface state, is detected in the Ni/Bi 2 Se 3 hybrid structure as shown in Ref. [20], implying that the surface state of TI is survived at Ni/Bi 2 Se 3 interface. Furthermore, it was reported that the spin direction for the magnetic atom on the TI surface tend to orient out-of-plane direction due to the strong spin-orbit interaction.…”

“…Meanwhile, various types of structures containing the ferromagnet/TI interface have been fabricated and used to identify unique spin phenomena of the TI surface states. 11,[19][20][21][22] The ferromagnetic layer interfaced with TI was simply used to detect the spin direction of the carriers in those structures. The precise information of the magnetic properties of ferromagnetic layer that interfaced with the TI surface is, however, prerequisite for the investigation of the spin phenomena in the ferromagnet/TI system.…”