  Topological insulators (TIs) are an unusual phase of quantum matter with
nontrivial spin-momentum locked topological surface states (TSS). The
electrical detection of spin-momentum locking of the TSS in 3D TIs has been
lacking till very recently. Many of the results are measured on samples with
significant bulk conduction, such as metallic Bi2Se3, where it can be
challenging to separate the surface and bulk contribution to the measured spin
signal. Here, we report spin potentiometric measurements in thin flakes
exfoliated from bulk insulating 3D TI Bi2Te2Se (BTS221) crystals, using two
outside nonmagnetic (Au) contacts for driving a DC spin helical current and a
middle ferromagnetic (FM)-Al2O3 tunneling contact for detecting spin
polarization. The voltage measured by the FM electrode exhibits a hysteretic
step-like change when sweeping an in-plane magnetic field between opposite
directions along the easy axis of the FM contact to switch its magnetization.
Importantly, the direction of this step-like voltage change can be reversed by
reversing the direction of the DC current, and the amplitude of the change as
measured by the difference in the detector voltage between opposite FM
magnetization increases linearly with increasing bias current, consistent with
the current-induced spin polarization of spin-momentum-locked TSS. Our work
directly demonstrates the electrical injection and detection of spin
polarization in TI and may enable utilization of spin-helical TSS for future
applications in nanoelectronics and spintronics.
