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Engineering of Intrinsic Chiral Torques in Magnetic Thin Films Based on the Dzyaloshinskii-Moriya Interaction

Zhentao Liu, Zhaochu Luo, Stanislas Rohart, Laura J. Heyderman, Pietro Gambardella, and Aleš Hrabec
Phys. Rev. Applied 16, 054049 – Published 29 November 2021
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  • INTRODUCTION
  • EFFECTIVE FIELD DUE TO CHIRAL COUPLING…
  • CURRENT-DRIVEN DOMAIN WALL MOTION
  • FIELD-DRIVEN DOMAIN WALL MOTION
  • CHIRAL COUPLING IN Pt/CoB/AlOx TRILAYER
  • CONCLUSION
  • ACKNOWLEDGMENTS
  • APPENDICES
    • Supplemental Material
    References

    Abstract

    The establishment of chiral coupling in thin magnetic films with inhomogeneous anisotropy has led to the development of artificial systems of fundamental and technological interest. The chiral coupling itself is enabled by the Dzyaloshinskii-Moriya interaction (DMI) enforced by the patterned noncollinear magnetization. Here, we create a domain wall track with out-of-plane magnetization coupled on each side to a narrow parallel strip with in-plane magnetization. With this we show that the chiral torques emerging from the DMI at the boundary between the regions of noncollinear magnetization in a single magnetic layer can be used to bias the domain wall velocity. To tune the chiral torques, the design of the magnetic racetracks can be modified by varying the width of the tracks or the width of the transition region between noncollinear magnetizations, reaching effective chiral magnetic fields of up to 7.8 mT. Furthermore, we show how the magnitude of the chiral torques can be estimated by measuring asymmetric domain wall velocities, and demonstrate spontaneous domain wall motion propelled by intrinsic torques even in the absence of any external driving force.

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    • Received 9 July 2021
    • Revised 9 September 2021
    • Accepted 4 November 2021

    DOI:https://doi.org/10.1103/PhysRevApplied.16.054049

    © 2021 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Domain wallsDzyaloshinskii-Moriya interactionMagnetic textureMagnetismSpintronics
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    Zhentao Liu1,2,*, Zhaochu Luo1,2, Stanislas Rohart3, Laura J. Heyderman1,2, Pietro Gambardella4, and Aleš Hrabec1,2,4,†

    • 1Laboratory for Mesoscopic Systems, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland
    • 2Laboratory for Multiscale Materials Experiments, Paul Scherrer Institute, 5232 Villigen, Switzerland
    • 3Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, 91405 Orsay, France
    • 4Laboratory for Magnetism and Interface Physics, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland

    • *zhentao.liu@psi.ch
    • †ales.hrabec@psi.ch

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    Vol. 16, Iss. 5 — November 2021

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    Images

    • Figure 1
      Figure 1

      (a) Schematic of the energy barrier between the unfavorable ←⊙ and favorable ←⊗ chiral configurations of adjacent IP-OOP magnetic regions. The energy barrier between the favorable and unfavorable configuration is given mainly by the shape and uniaxial anisotropies of the IP and OOP regions, respectively. (b) Schematic of a DW racetrack with OOP magnetization delimited by two strips with IP anisotropy. The width of the OOP track and IP strips are wOOP and wIP, respectively. The energy landscape of the DW racetrack is monotonic, except for local DW pinning sites, with the slope given by the energy difference between E⊙⊙⊙ and E⊙⊗⊙. (c) Sequence of Kerr images of asymmetric DW motion in a racetrack with wOOP= 950 nm with corresponding schematics. The dashed lines correspond to the position of the IP magnetized strip whereas the solid lines correspond to the boundary of the magnetic track. Eighty current pulses are applied between each image; the pulse duration is 50 ns and the current density is3.6×1011A/m2. The initial magnetic configuration of both inner and outer OOP regions is ⊙ and ⊗ in the left and right panel, respectively.

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    • Figure 2
      Figure 2

      (a) DW velocity as function of current density for DW tracks with fixed wIP=55nm and wOOP ranging from 350 to 950 nm with +Hz initialization (left panel) and −Hz initialization (right panel). The lines are guides to the eye. The error bars are standard deviations for ten individual measurements. (b) Current-induced DW velocity for different values of the OOP magnetic field Hz. The red and blue arrows show the trend of suppressing and enhancing the DW velocity with increasing field, respectively.

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    • Figure 3
      Figure 3

      (a) DW velocity induced by an OOP field Hz measured in Pt/Co/AlOx devices with wIP=55nmand for various wOOP with +Hzinitialization (upper panel) and −Hzinitialization (lower panel). Control measurements of a 3-µm-wide DW track without the IP strips are also shown. The lines are fits according to Eq. (3). (b) Effective chiral coupling strength (Heff) derived from the creep model. The red line is a linear fit with respect to 1/wOOP. Error bars represent the standard deviation of Heff measured using both positive and negative magnetic fields.

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    • Figure 4
      Figure 4

      (a) Stopping field versus 1/wOOP for the DWs in Pt/CoB/AlOx racetracks with wIP= 50 nm. (b) Stopping field in racetracks with wOOP= 200 nm and wIP ranging from 30 to 90 nm. (c)–(d) Micromagnetic simulations of the stopping field in a track with a fixed wIP= 50 nm and variable 1/wOOP with different interfacial DMI strengths D = 0.1 and 0.2 mJ/m2 (c) and in a track with fixed wOOP= 200 nm and variable wIP (d) where D = 0.2 mJ/m2 with different uniaxial anisotropy constants (Ku). The inset in (c) shows a snapshot of the simulations. Red (blue) color corresponds to the OOP magnetization pointing along the +z (−z) direction. The lines in (a) and (c) are linear fits of the stopping field versus 1/wOOP, whereas the lines in (b) and (d) are guides to the eye.

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    • Figure 5
      Figure 5

      (a) 1D micromagnetic calculation of Heff versus 1/wOOP in a Pt/CoB/AlOx racetrack with fixed wIP= 20 nm. Heffis extracted from the calculation of the linear energy density of ⊙⊙⊙ and ⊙⊗⊙ regions using Eq. (2) with D = 0.2 mJ/m2. (b) Simulated Heffas a function of wIP for wOOP= 50 to 500 nm. (c) Color map of Heff versus wIP and wOOP.

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    • Figure 6
      Figure 6

      Sketch of the sample hosting a DW in the central track.

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    • Figure 7
      Figure 7

      (a) 1D micromagnetic calculation of energy difference E⊙⊙⊙−E⊙⊗⊙ versus 1/wOOP in a Pt/CoB/Al racetrack with fixed wIP= 20 nm. (b) Simulated energy difference as a function of wIP for wOOP= 50 nm up to 500 nm. (c) Color map of energy difference as a function of wIP and wOOP.

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