Abstract
We investigate the thickness optimization for maximum current-induced spin-orbit torque (SOT) generated by topological surface states (TSS’s) in a bilayer system comprising of a ferromagnetic layer coupled to a thin topological insulator (TI) film. We show that by reducing the TI thickness, two competing effects on the SOT are induced: (i) the torque strength is stronger as the bulk contribution is decreased; (ii) on the other hand, the torque strength becomes suppressed due to increasing hybridization of the surface states. The latter is attributed to the opposite helicities of the coupled TSS’s. We theoretically model the interplay of these two effects and derive the optimal TI thickness to maximize the spin torque, which is estimated to be about 3–5 nm for typical Bi2Se3 films.
Highlights
Spin torque is one of the most actively researched topics in spintronics
In a thick topological insulator (TI) film, only the topological surface state coupled to the magnetic layer is active, whereas the other has no effect and can be ignored
It is easy to see that the coupling between the bottom surface and the ferromagnet leads to a reduction in the total spin torque, and it even vanishes for λ = 1
Summary
Spin torque is one of the most actively researched topics in spintronics. In spin-orbit coupling (SOC) systems, the spin-orbit torque (SOT) comprises of two types: field-like torque[1,2,3,4,5,6] and damping-like torque[7, 8]. The spin-locked topological surface states of TI are robust under time-reversal symmetric impurity scattering. These factors enable TI to be one of the most promising candidates for SOT devices. When a current is passed onto the TI surface (interface), a non-equilibrium spin density will be induced on that surface[10], which is directed in-plane and perpendicular to the applied current as a result of the spin-momentum locking of the surface states[10], similar to the Rashba-Edelstein effect[13]. Since the bulk states do not exhibit any spin-momentum locking, TI with a large bulk conductance will induce a smaller torque on an adjacent ferromagnetic (FM) layer.
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