Dissipationless transport by design in ultrathin magnetic topological insulator films

TL;DR

Magnetic impurities in topological insulators typically disrupt dissipationless surface transport by inducing spin-flip scattering. The authors propose a transport-engineering strategy that combines magnetization tilting and electrostatic gating in ultrathin Bi2Se3 films to suppress backscattering and restore spin-polarized conduction. Using a combination of a low-energy surface-state model with Boltzmann transport and real-space tight-binding simulations with Landauer-Büttiker calculations, they show that a single-band regime can yield vanishing longitudinal resistance under suitable gate and magnetization conditions, with spin coherence preserved. These findings provide practical, design-oriented guidelines for ultralow-power magneto-electronic devices based on magnetic TI thin films, with robust applicability across thickness and relevance to multi-band regimes.

Abstract

Magnetic topological insulators (MTIs) are among the prominent platforms for the next generation of high-speed and low-power spintronic devices. However, unlike their non-magnetic counterparts, where the surface spin-momentum locking prevents electrons from being scattered by non-magnetic impurities and results in a dissipationless electronic flow, magnetic impurities in MTIs cause dissipation by exerting magnetic torque on the electron spin. Decreasing this resistance is desired to reduce energy consumption and optimize performance of MTIs in envisaged applications. Here we reveal how electronic backscattering can be suppressed in a MTI thin film by external magnetic and/or electronic stimuli, to yield an entirely dissipationless spin-polarized charge transport. Our findings thus present an effective route to preserve spin coherence and enhance spin-current functionality in magnetic topological materials, suggesting design strategies for magneto-electronic and spintronic devices with strongly reduced energy consumption.

Figures (8)

• Figure 1: (a) Crystal structure of the Bi$_2$Se$_3$ TI family, consisting of quintuple layers arranged in a rhombohedral lattice. (b) Schematic illustration of the system under study, a magnetically doped ultrathin TI film sandwiched between two magnetic layers, and its single-band regime. The proximity-induced exchange field lifts the degeneracy of the conduction subbands. The Fermi level is then tuned into the conduction regime, where only the lower surface conduction band is occupied.
• Figure 2: (a) Scattering probability $T_{\boldsymbol{k} \boldsymbol{k}'}^2$ as a function of magnetization angle $\theta_M$ and outgoing momentum angle $\phi_{\boldsymbol{k}'}$ for a 5QL-thick Bi$_2$Se$_3$ film under an in-plane electric field $\boldsymbol{E} = E \hat{x}$ with incoming electron momentum $\phi_{\boldsymbol{k}} = \pi$. (b) Longitudinal resistivity $\rho_{xx}$ as a function of magnetization tilt angle $\theta_M$, for magnetically doped Bi$_2$Se$_3$ films with thicknesses of 2-5 QL, in absence of gating ($V_{g}=0$). The non-monotonic behavior highlights the tunability of resistive transport via magnetic orientation, with thickness-dependent resistivity minima emerging at intermediate angles.
• Figure 3: Longitudinal resistivity $\rho_{xx}$ as a function of total voltage for magnetically doped Bi$_2$Se$_3$ films with thicknesses of 2-5QL for a fixed magnetization angle $\theta_M = 0$. Increasing the voltage modifies the band structure and reduces spin-dependent scattering, resulting in a monotonic decrease in resistivity.
• Figure 4: Gate-tunable scattering dynamics in a magnetically-doped 5QL-thick Bi$_2$Se$_3$ film. Angular scattering probability as a function of magnetization tilt angle $\theta_M$ and scattering angle $\phi_{\boldsymbol{k'}}$ for different total gate voltages, V$_{total}$ = V$_{SIA}$ + V$_g$ = 0, 20 meV and 40 meV. The scattering profiles reveal significant suppression of backscattering with applying gate voltage and magnetization tilt.
• Figure 5: Longitudinal resistivity $\rho_{xx}$ as a function of $\theta_M$ for different V$_{total}$, demonstrating that combined gating and magnetization control can drastically reduce resistivity in the system.