Spin torque and persistent currents caused by percolation of topological surface states

TL;DR

This paper investigates how topological surface states in TI/FMM bilayers percolate into the ferromagnetic metal, affecting persistent currents and spin torque, with implications for spintronic device efficiency.

Contribution

It reveals the hybridization of surface states with FMM bulk bands and their influence on persistent currents and spin torque, highlighting the role of percolation and impurities.

Findings

Surface states merge with FMM bulk bands and hybridize with quantum well states.

Persistent charge and spin currents are induced by band structure distortions.

Spin torque is mainly field-like and depends on surface state percolation degree.

Abstract

The topological insulator/ferromagnetic metal (TI/FMM) bilayer thin films emerged as promising topological surface state-based spintronic devices, most notably in their efficiency of current-induced spin torque. Using a cubic lattice model, we reveal that the surface state Dirac cone of the TI can gradually merge into or be highly intertwined with the FMM bulk bands, and the surface states percolate into the FMM and eventually hybridize with the quantum well states therein. The magnetization can distort the spin-momentum locking of the surface states and yield an asymmetric band structure, which causes a laminar flow of room temperature persistent charge current. Moreover, the proximity to the FMM also promotes a persistent laminar spin current. Through a linear response theory, we elaborate that both the surface state and the FMM bulk bands contribute to the current-induced spin torque, and their real wave functions render the spin torque predominantly field-like, with a magnitude highly influenced by the degree of the percolation of the surface states. On the other hand, impurities can change the spin polarization expected from the Edelstein effect and generate a damping-like torque, and produce a torque even when the magnetization points in-plane and orthogonal to the current direction.