Abstract
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 TSS has been lacking till very recently. Many of the results are from samples with significant bulk conduction, such as Bi2Se3, where it can be challenging to separate the surface and bulk contribution to the spin signal. Here, we report spin potentiometric measurements in flakes exfoliated from bulk insulating Bi2Te2Se crystals, using two outside nonmagnetic contacts for driving a DC spin helical current and a middle ferromagnetic (FM)-Al2O3 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. Importantly, the direction of the voltage change can be reversed by reversing the direction of current, and the amplitude of the change as measured by the difference in the detector voltage between opposite FM magnetization increases linearly with increasing 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 TSS for applications in nanoelectronics and spintronics.
Highlights
We first discuss the theoretical principles behind the spin potentiometric measurements to probe the current induced spin polarization arising from the TSS on the top surface (Fig. 1c)
One can expect a low voltage or high potential when M of the FM detector is antiparallel to the channel spin polarization S, while a high voltage or low potential when M is parallel to S
Note that the above expression contains a ballistic conductance (1/RB), used to count the number of propagating modes in the channel, the formula is valid for both ballistic and diffusive regimes as discussed in ref. 36), PFM is the effective spin polarization[26,36] of the FM detector (Py), Mu = − ey is the unit vector along the positive detector magnetization direction (+ M), and p = pey with the p being the portion of the current I that is spin polarized, reflecting the degree of current-induced channel spin polarization, and the sign of p determined by the spin helicity
Summary
Spin-pumping[25] in TIs. The direct all-electrical detection of spin helical current or current induced spin polarization from spin-momentum-locked TSS in 3D TIs using spin-sensitive transport measurements is a key step for potential applications in spintronics. Bi2Te2Se (BTS221) is one of the first and best-studied bulk insulating TIs to allow better access of the TSS transport[34,35] It has not been explored in spin transport experiments to measure the current-induced spin polarization from TSS. The “polarity” of this step-like voltage change reverses when the direction of the DC current is reversed The amplitude of this voltage change (voltage difference between opposite FM magnetizations) is found to increase with increasing DC current but decrease with increasing temperature, and offers a clear electrical signal for the current-induced spin polarization from the TSS detected by our spin potentiometric measurement
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