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Control of spin–orbit torques through crystal symmetry in WTe2/ferromagnet bilayers

Nature Physics volume 13, pages 300305 (2017) | Download Citation

Abstract

Recent discoveries regarding current-induced spin–orbit torques produced by heavy-metal/ferromagnet and topological-insulator/ferromagnet bilayers provide the potential for dramatically improved efficiency in the manipulation of magnetic devices. However, in experiments performed to date, spin–orbit torques have an important limitation—the component of torque that can compensate magnetic damping is required by symmetry to lie within the device plane. This means that spin–orbit torques can drive the most current-efficient type of magnetic reversal (antidamping switching) only for magnetic devices with in-plane anisotropy, not the devices with perpendicular magnetic anisotropy that are needed for high-density applications. Here we show experimentally that this state of affairs is not fundamental, but rather one can change the allowed symmetries of spin–orbit torques in spin-source/ferromagnet bilayer devices by using a spin-source material with low crystalline symmetry. We use WTe2, a transition-metal dichalcogenide whose surface crystal structure has only one mirror plane and no two-fold rotational invariance. Consistent with these symmetries, we generate an out-of-plane antidamping torque when current is applied along a low-symmetry axis of WTe2/Permalloy bilayers, but not when current is applied along a high-symmetry axis. Controlling spin–orbit torques by crystal symmetries in multilayer samples provides a new strategy for optimizing future magnetic technologies.

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Acknowledgements

We thank N. D. Reynolds for experimental assistance, R. De Alba for help with the graphics, and G. D. Fuchs and P. G. Gowtham for comments on the manuscript. This work was supported by the National Science Foundation (DMR-1406333) and the Army Research Office (W911NF-15-1-0447). G.M.S. acknowledges support by a National Science Foundation Graduate Research Fellowship under Grant No. DGE-1144153. M.H.D.G. acknowledges support by the Netherlands Organization for Scientific Research (NWO 680-50-1311) and the Kavli Institute at Cornell for Nanoscale Science. This work made use of the NSF-supported Cornell Nanoscale Facility (ECCS-1542081), the Cornell Center for Materials Research Shared Facilities, which are supported through the NSF MRSEC Program (DMR-1120296), and the NSF-supported Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM) (DMR-1539918).

Author information

Author notes

    • D. MacNeill
    •  & G. M. Stiehl

    These authors contributed equally to this work.

Affiliations

  1. Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York 14853, USA

    • D. MacNeill
    • , G. M. Stiehl
    • , M. H. D. Guimaraes
    •  & D. C. Ralph
  2. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA

    • M. H. D. Guimaraes
    • , J. Park
    •  & D. C. Ralph
  3. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA

    • R. A. Buhrman
  4. Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, USA

    • J. Park

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Contributions

D.M., G.M.S., M.H.D.G. and D.C.R. conceived the idea for the experiment. D.M. performed the sample fabrication. G.M.S. made the measurements. D.M. and G.M.S. performed the analysis with help from M.H.D.G., R.A.B., J.P. and D.C.R. D.M., G.M.S. and D.C.R. wrote the manuscript and all authors contributed to the final version.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to D. C. Ralph.

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DOI

https://doi.org/10.1038/nphys3933

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