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Spin-transfer torque generated by a topological insulator

Abstract

Magnetic devices are a leading contender for the implementation of memory and logic technologies that are non-volatile, that can scale to high density and high speed, and that do not wear out. However, widespread application of magnetic memory and logic devices will require the development of efficient mechanisms for reorienting their magnetization using the least possible current and power1. There has been considerable recent progress in this effort; in particular, it has been discovered that spin–orbit interactions in heavy-metal/ferromagnet bilayers can produce strong current-driven torques on the magnetic layer2,3,4,5,6,7,8,9,10,11, via the spin Hall effect12,13 in the heavy metal or the Rashba–Edelstein effect14,15 in the ferromagnet. In the search for materials to provide even more efficient spin–orbit-induced torques, some proposals16,17,18,19 have suggested topological insulators20,21, 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 fixed relative to its propagation direction. Here we report experiments showing that charge current flowing in-plane in a thin film of the topological insulator bismuth selenide (Bi2Se3) at room temperature can indeed exert a strong spin-transfer torque on an adjacent ferromagnetic permalloy (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 Bi2Se3 is greater than for any source of spin-transfer torque measured so far, even for non-ideal topological insulator films in which the surface states coexist with bulk conduction. Our data suggest that topological insulators could enable very efficient electrical manipulation of magnetic materials at room temperature, for memory and logic applications.

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Figure 1: The mechanism of current-induced spin accumulation in topological insulators and the sample geometry used in the measurement.
Figure 2: ST-FMR measurements of the current-induced torque, with fits.

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Acknowledgements

We thank R. Buhrman, C.-F. Pai, N. Reynolds, J. Gibbons, A. Alemi and M. Bierbaum for discussions and assistance with experiments. Work at Cornell and Penn State was supported by DARPA (N66001-11-1-4110). We acknowledge additional funding for work at Cornell from the NSF/MRSEC-funded Cornell Center for Materials Research (DMR-1120296), the Army Research Office (W911NF-08-2-0032) and the NSF (DMR-1010768), and for work at Penn State from the Office of Naval Research (N00014-12-1-0117). A.R.M. acknowledges a DOE Office of Science graduate fellowship and J.L.G. acknowledges an NSF graduate fellowship. J.S.L. and N.S. acknowledge partial support through C-SPIN, one of six centres of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA. This work was performed in part at the Cornell NanoScale Facility and the Penn State Nanofabrication Facility, both nodes of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the NSF (ECS-0335765), and in the facilities of the Cornell Center for Materials Research.

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Authors

Contributions

A.R.M., J.S.L., A.R., N.S. and D.C.R. had the idea for and designed the experiments. A.R.M., J.L.G. and P.J.M. performed the sample fabrication, measurements and analysis. J.S.L., A.R. and N.S. developed the growth process for the Bi2Se3 layers. M.H.F., A.V., A.M. and E.-A.K. performed theoretical modelling. N.S. and D.C.R. provided oversight and advice. A.R.M. and D.C.R. wrote the manuscript and all authors contributed to its final version.

Corresponding author

Correspondence to D. C. Ralph.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Change in resonant field as a function of direct current.

The data correspond to an 8 nm Bi2Se3/16 nm permalloy device with dimensions 50 μm × 15 μm, averaged over frequencies between 6 and 10 GHz. Error bars, 1 s.d.

Extended Data Figure 2 Measured linewidth versus frequency.

The data correspond to an 8 nm Bi2Se3/16 nm permalloy device with dimensions 50 μm × 10 μm at different direct currents. Error bars, 1 s.d.

Extended Data Figure 3 Control experiments.

a, A comparison between the ST-FMR signals measured for two 50 μm × 15 μm devices at 8 GHz, one with 8 nm Bi2Se3/16 nm permalloy and the other a single layer of 16 nm permalloy. The absorbed radio-frequency power was 5 dBm and ϕ = 45° in both cases. b, ST-FMR measurement for a 6 nm Pt/16 nm permalloy device with dimensions 80 μm × 24 μm. The absorbed power was 5 dBm and ϕ = 45°.

Extended Data Figure 4 Schematic band structures.

a, A schematic band structure with a Dirac surface state and a Rashba-split 2DEG as observed in refs 26, 32. b, The corresponding spin angular momentum structure at the Fermi energy.

Extended Data Figure 5 The predicted value of the out-of-plane spin torque ratio relative to the in-plane spin torque ratio.

as a function of , for = 5 nm.

Extended Data Figure 6 Anisotropic magnetoresistance calibration.

The data correspond to an 8 nm Bi2Se3/16 nm permalloy device with dimensions 50 μm × 15 μm, and were measured at room temperature.

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Mellnik, A., Lee, J., Richardella, A. et al. Spin-transfer torque generated by a topological insulator. Nature 511, 449–451 (2014). https://doi.org/10.1038/nature13534

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