Fermi level dependent charge-to-spin current conversion by Dirac surface state of topological insulators

TL;DR

This study investigates how the Fermi level position in topological insulators affects charge-to-spin current conversion efficiency via Dirac surface states, revealing optimal conditions for maximizing spin-momentum locking effects.

Contribution

It provides a quantitative analysis of the Fermi level dependence of charge-to-spin conversion efficiency in topological insulators using spin torque ferromagnetic resonance.

Findings

Charge-to-spin conversion efficiency is nearly constant in bulk insulating conditions.

Efficiency is suppressed when the Fermi level crosses the Dirac point.

Fine tuning of the Fermi level is essential for optimal device performance.

Abstract

The spin-momentum locking at the Dirac surface state of a topological insulator (TI) offers a distinct possibility of a highly efficient charge-to-spin current (C-S) conversion compared with spin Hall effects in conventional paramagnetic metals. For the development of TI-based spin current devices, it is essential to evaluate its conversion efficiency quantitatively as a function of the Fermi level EF position. Here we exemplify a coefficient of qICS to characterize the interface C-S conversion effect by using spin torque ferromagnetic resonance (ST-FMR) for (Bi1-xSbx)2Te3 thin films whose EF is tuned across the band gap. In bulk insulating conditions, interface C-S conversion effect via Dirac surface state is evaluated as nearly constant large values of qICS, reflecting that the qICS is inversely proportional to the Fermi velocity vF that is almost constant. However, when EF traverses through the Dirac point, the qICS is remarkably suppressed possibly due to the degeneracy of surface spins or instability of helical spin structure. These results demonstrate that the fine tuning of the EF in TI based heterostructures is critical to maximizing the efficiency using the spin-momentum locking mechanism.