Creation of magnetic skyrmions by surface acoustic waves

Abstract

Non-collinear and non-coplanar spin textures, such as chiral domain walls1 and helical or triangular spin structures2,3, bring about diverse functionalities. Among them, magnetic skyrmions, particle-like non-coplanar topological spin structures characterized by a non-zero integer topological charge called the skyrmion number (Nsk), have great potential for various spintronic applications, such as energy-saving, non-volatile memory and non-von Neumann devices4,5,6,7. Current pulses can initiate skyrmion creation in thin-film samples8,9,10 but require relatively large current densities, which probably causes Joule heating. Moreover, skyrmion creation is localized at a specific position in the film depending on the sample design. Here, we experimentally demonstrate an approach to skyrmion creation employing surface acoustic waves (SAWs); in asymmetric multilayers of Pt/Co/Ir, propagating SAWs induce skyrmions in a wide area of the magnetic film. Micromagnetic simulations reveal that inhomogeneous torque arising from both SAWs and thermal fluctuations creates magnetic textures, with pair structures consisting of a Néel skyrmion-like and an antiskyrmion-like structure. Subsequently, such pairs transform to a Néel skyrmion due to the instability of the antiskyrmion-like structure in a system with interfacial Dzyaloshinskii–Moriya interaction. Our findings provide a tool for efficient manipulation of topological spin objects without heat dissipation and over large areas, given that the propagation length of SAWs is of the order of millimetres.

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Fig. 1: Schematics of Néel skyrmion, experimental setup and basic properties of devices.
Fig. 2: Skyrmion creation by surface acoustic waves.
Fig. 3: Wavelength dependence of the number of created skyrmions and skyrmion size.
Fig. 4: Micromagnetic simulation.

Data availability

The datasets generated and analysed during this study are available from the corresponding authors on reasonable request.

Code availability

The corresponding computer codes are available from the corresponding authors on reasonable request.

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Acknowledgements

We thank J. Puebla, K. Kondou and M. Xu for useful discussions. This work was supported by Scientific Research on Innovative Area ‘Nano Spin Conversion Science’ (grant no. 26103002), by the JSPS Grants-in-Aid for Scientific Research (A) (grant no. 18H03685) and for Young Scientists (19K14667) and by JST PRESTO (grant nos. JPMJPR18L3, JPMJPR18L5 and JPMJPR17I3).

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Authors

Contributions

Y.O., S.K. and T.Y. conceived the project. S. Sugimoto and S.K. deposited the thin films. T.Y. fabricated the devices and conducted the measurements with assistance from B.R., S. Seki and N.O. T.Y. analysed the data and performed the simulation. T.Y. and Y.O. wrote the draft. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Tomoyuki Yokouchi.

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The authors declare no competing interests.

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Extended data

Extended Data Fig. 1 Zoomed-out polar Kerr magnetic images.

Zoomed-out polar Kerr magnetic images before, during, and after exiting a surface acoustic wave (SAW) with the wavelength of 16 μm by applying RF signal with the power of 251 mW at 0.24 mT. The light blue dashed lines and the orange arrow represent the boundary of the Pt/Co/Ir film and propagating direction of the SAW, respectively. The orange dashed lines represents the area presented in Fig. 2a–c in the main text.

Extended Data Fig. 2 The effect of the wavelength on creation of skyrmions.

a, Magnetic field dependence of nsk before and after exciting a SAW with various wavelengths of SAW. b, The ratio of skyrmion density before and after exciting SAW with various wavelengths. The ratio for λSAW=16 μm is the largest among the four wavelengths as in the case of the difference of skyrmion density (Δnsk) (Fig. 3a in the main text), which further supports that the creation efficiency of skyrmions for the SAW with λSAW=16 μm is higher than that with λSAW=8 and 32 μm. The error bars correspond to the standard deviation.

Extended Data Fig. 3 Effect of the uniaxial strain.

a, A top view of the device. b-d, Polar Kerr magnetic images before and during applying constant voltage (± 18.8 V) at various magnetic fields.

Extended Data Fig. 4 Appearance process for inhomogeneous effective torque.

a-p Color maps of strain (εxx) (a-d), z-component of magnetization (e-h), x-component of magnetization (i-l), and effective torques resulting from the magnetoelastic coupling (m-p). The black circles exemplify the expansion of the area where effective torque is nonzero (see also the main text).

Extended Data Fig. 5 Micromagnetic simulations for skyrmion creations by surface acoustic waves.

a-f, Color maps of strain (εxx) (a), magnetization (b), topological charge density (ρtopo) (c), the sum of the exchange energy and Dzyaloshinskii-Moriya energy (Eex.+DM) (d), and effective torques resulting from the magnetoelastic coupling (e). The yellow squares denote the area presented in Fig. 4 in the main text.

Extended Data Fig. 6 Simulated wavelength dependences of the number and the size of skyrmions.

a, b, The wavelength dependences of the number of skyrmions (a) and the size of skyrmions (b). The broad blue line in (a) is guide to the eyes. c, A schematic for explaining the relationship between the wavelength and the length-scale of the effective torque.

Supplementary information

Supplementary Information

Supplementary Figs. 1–7 and discussion.

Supplementary Video 1

A movie representing the time evolution of the strain εxx, spin configurations, topological charge density, the sum of exchange and DMI energy density and effective torques arising from the magnetoelastic coupling in the whole process of the micromagnetic simulation.

Supplementary Video 2

Magnified version of movie S1, which represents the time evolution of the strain εxx, spin configurations, topological charge density, the sum of exchange and DMI energy density and effective torques arising from the magnetoelastic coupling to explicitly show the creation process of skyrmion.

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Yokouchi, T., Sugimoto, S., Rana, B. et al. Creation of magnetic skyrmions by surface acoustic waves. Nat. Nanotechnol. 15, 361–366 (2020). https://doi.org/10.1038/s41565-020-0661-1

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