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

Spin-momentum locking in the Dirac surface state of a topological insulator (TI)(1-6) offers a distinct possibility for highly efficient charge-to-spin current (C-S) conversion compared with spin Hall effects in conventional paramagnetic metals(7-13). For the development of TI-based spin current devices, it is essential to evaluate this conversion efficiency quantitatively as a function of the Fermi level position E-F. Here we introduce a coefficient q(ICS) to characterize the interface C-S conversion effect by means of the spin torque ferromagnetic resonance (ST-FMR) for (Bi1-xSbx)(2)Te-3 thin films as E-F is tuned across the bandgap. In bulk insulating conditions, the interface C-S conversion effect via the Dirac surface state is evaluated as having large, nearly constant values of q(ICS), reflecting that q(ICS) is inversely proportional to the Fermi velocity v(F), which is almost constant. However, when EF traverses through the Dirac point, the q(ICS) is remarkably reduced, possibly due to inhomogeneity of k(F) and/or instability of the helical spin structure. These results demonstrate that fine tuning of E-F in TI-based heterostructures is critical in maximizing the efficiency using the spin-momentum locking mechanism.