  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.
