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) offers a distinct possibility for 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 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 (Bi 1'x Sb x) 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 E F 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.