Electrical injection and detection of spin - polarized currents in topological insulator

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.