Spin Transfer Torque Generated by the Topological Insulator Bi_2Se_3

Abstract: Magnetic devices are a leading contender for implementing memory and logic technologies that are nonvolatile, that can scale to high density and high speed, and that do not suffer wear-out. However, widespread applications of magnetic memory and logic devices will require the development of efficient mechanisms for reorienting their magnetization using the least possible current and power. There has been considerable recent progress in this effort, in particular discoveries that spin-orbit interactions in heavy metal/ferromagnet bilayers can yield strong current-driven torques on the magnetic layer, via the spin Hall effect in the heavy metal or the Rashba-Edelstein effect in the ferromagnet. As part of the search for materials to provide even more efficient spin-orbit-induced torques, some proposals have suggested topological insulators (TIs), which possess a surface state in which the effects of spin-orbit coupling are maximal in the sense that an electron's spin orientation is locked relative to its propagation direction. Here we report experiments showing that charge current flowing in-plane in a thin film of the TI Bi_2Se_3 at room temperature can indeed apply a strong spin-transfer torque to an adjacent ferromagnetic permalloy (Py = Ni81Fe19) thin film, with a direction consistent with that expected from the topological surface state. We find that the strength of the torque per unit charge current density in the Bi_2Se_3 is greater than for any other spin-torque source material measured to date, even for non-ideal TI films wherein the surface states coexist with bulk conduction. Our data suggest that TIs have potential to enable very efficient electrical manipulation of magnetic materials at room temperature for memory and logic applications.

• The paper demonstrates that Bi₂Se₃ produces higher spin-transfer torque per unit charge current density than conventional heavy metals.
• It employs spin torque ferromagnetic resonance (STFMR) to quantify both in-plane and perpendicular torque components at room temperature.
• The findings indicate that refining TI/magnet interfaces could lead to low-power, nonvolatile magnetic memory and logic devices.

Spin-Transfer Torque Generated by the Topological Insulator Bi$_2$Se$_3$

This paper investigates the capacity of the topological insulator bismuth selenide (Bi$_2$Se$_3$) to generate spin-transfer torque (STT) when an in-plane charge current is applied at room temperature. The study is built on recent revelations that spin-orbit interactions in heavy metal/ferromagnet bilayers can produce robust current-driven torques on magnetic layers via mechanisms such as the spin Hall effect and Rashba-Edelstein effect. Given the prowess of topological insulators (TIs) to exhibit maximal spin-orbit coupling due to their unique surface states, the authors hypothesize that TIs can serve as promising candidates for generating efficient spin-orbit torques.

A central finding of this research is that the spin torque generated per unit charge current density in Bi$_2$Se$_3$ surpasses all previously known materials, even in sub-ideal conditions where the topological surface states coexist with bulk conduction. Specifically, the spin torque ferromagnetic resonance (STFMR) technique reveals in-plane and perpendicular torque components for Bi$_2$Se$_3$ to exceed those measured from conventional heavy metals like Pt and Ta, often used in similar applications. The computed spin current conductivities, $\sigma_{S,\parallel}$ and $\sigma_{S,\perp}$, for Bi$_2$Se$_3$ were reported as significantly higher than those of traditional spin current generating materials.

The implications of this research are substantial for the design of nonvolatile magnetic memory and logic devices, offering prospects for lower power consumption and enhanced efficiency. The Spin Torque Angle $\theta$, which quantifies the torque induced per unit applied charge current density, is notably larger for Bi$_2$Se$_3$ when compared to other materials. However, the study acknowledges that current device structures, comprising TI/metallic magnet bilayers, shunt most current through the metallic layer, limiting the full exploitation of Bi$_2$Se$_3$'s potential. Moving forward, interfacing TIs with insulating or high resistivity magnets could further enhance operational efficiencies by preventing current bypass through more conductive layers and overcoming additional challenges such as Gilbert damping.

Future research directions may involve optimizing the interface quality between Bi$_2$Se$_3$ and magnetic layers and exploring electrostatic gating to manage chemical potential and minimize bulk conduction. Theoretical modeling should further refine the interaction dynamics at the TI-magnet interface, aiming to solidify understanding of the underpinning spin accumulation and transport phenomena. As technological applications advance, deploying TIs as efficient room-temperature spin-torque sources holds promise for pioneering developments in nonvolatile magnetic technology.