Robust formation of nanoscale magnetic skyrmions in easy-plane anisotropy thin film multilayers with low damping

Letter

Robust formation of nanoscale magnetic skyrmions in easy-plane anisotropy thin film multilayers with low damping

Luis Flacke, Valentin Ahrens, Simon Mendisch, Lukas Körber, Tobias Böttcher, Elisabeth Meidinger, Misbah Yaqoob, Manuel Müller, Lukas Liensberger, Attila Kákay, Markus Becherer, Philipp Pirro, Matthias Althammer, Stephan Geprägs, Hans Huebl, Rudolf Gross, and Mathias Weiler

Phys. Rev. B 104, L100417 – Published 27 September 2021

Abstract

We experimentally demonstrate the formation of room-temperature skyrmions with radii of about 25 nm in easy-plane anisotropy multilayers with an interfacial Dzyaloshinskii-Moriya interaction (DMI). We detect the formation of individual magnetic skyrmions by magnetic force microscopy and find that the skyrmions are stable in out-of-plane fields up to about 200 mT. We determine the interlayer exchange coupling as well as the strength of the interfacial DMI. Additionally, we investigate the dynamic microwave spin excitations by broadband magnetic resonance spectroscopy. From the uniform Kittel mode we determine the magnetic anisotropy and low damping $α_{G} < 0.04$ . We also find clear magnetic resonance signatures in the nonuniform (skyrmion) state. Our findings demonstrate that skyrmions in easy-plane multilayers are promising for spin-dynamical applications.

Received 23 February 2021
Revised 26 May 2021
Accepted 2 September 2021

DOI:https://doi.org/10.1103/PhysRevB.104.L100417

Physics Subject Headings (PhySH)

Skyrmions Spintronics

Chiral magnets Magnetic multilayers

Ferromagnetic resonance Magnetic force microscopy Micromagnetic modeling

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Luis Flacke ^1,2,*, Valentin Ahrens ³, Simon Mendisch ³, Lukas Körber^4,5, Tobias Böttcher ⁶, Elisabeth Meidinger^1,2, Misbah Yaqoob ^1,2,6, Manuel Müller ^1,2, Lukas Liensberger^1,2, Attila Kákay⁴, Markus Becherer ³, Philipp Pirro⁶, Matthias Althammer ^1,2, Stephan Geprägs ¹, Hans Huebl ^1,2,7, Rudolf Gross ^1,2,7, and Mathias Weiler ^1,2,6,†

¹Walther-Meißner Institut, Bayerische Akademie der Wissenschaften, 85748 Garching, Germany
²Physics Department, Technical University of Munich, 85748 Garching, Germany
³Department of Electrical and Computer Engineering, Technical University of Munich, 80333 Munich, Germany
⁴Helmholtz-Zentrum Dresden - Rossendorf, Institute of Ion Beam Physics and Materials Research, 01328 Dresden, Germany
⁵Fakultät Physik, Technische Universität Dresden, 01062 Dresden, Germany
⁶Fachbereich Physik and Landesforschungszentrum OPTIMAS, Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany
⁷Munich Center for Quantum Science and Technology (MCQST), 80799 Munich, Germany

^*luis.flacke@wmi.badw.de
^†weiler@physik.uni-kl.de

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Issue

Vol. 104, Iss. 10 — 1 September 2021

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Images

Figure 1
(a) Sketch of the multilayer system, with a sixfold repetition of the Pt/CoFe/Ir trilayer (numbers give the film thickness in nm). (b) MFM images recorded with OOP magnetic fields (field values are indicated at the top of each frame). With decreasing field, the magnetic texture shows transitions from a saturated ferromagnetic state (1), to isolated skyrmions [(2), (3)], to a dense, skyrmion arrangement [(4)–(6)], and eventually forms a maze state at zero field (7). Individual skyrmions between the spin spirals can still be found at zero field. The three highlighted skyrmions [marked with three colored circles in (7)] are analyzed further in Fig. 2. The scale bar in all images corresponds to 500 nm.
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Figure 2
(a) Azimuthally averaged MFM phase contrast of the skyrmions highlighted in Fig. 1 (offset is used for clarity). The radial profile is fitted with a Gaussian function, from which the radius $r = (25.4 \pm 1.6) nm$ is found at $H_{ext} = 0$ . (b) Apparent radius vs applied magnetic field in the OOP direction. The black squares indicate the average value taken from 28–115 skyrmions from images (2)–(6) of Fig. 1 for a field down sweep starting at saturation, whereas the blue circles indicate skyrmion sizes determined from an up sweep starting at $(66 \pm 6)$ mT. The $y$ error represents the standard deviation of the size distribution. The $x$ error bar indicates the uncertainty of the field calibration. The red dotted line is a guide to the eye.
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Figure 3
(a) Saturation fields $μ_{0} H_{S}$ of samples with varying Ir thickness $t_{Ir}$ are determined by anomalous Hall effect measurements. In the AFM RKKY coupling regime, $H_{S}$ correlates with the strength of exchange coupling. Within the FM regime, $H_{S}$ is independent of coupling strength, and small pockets in the hysteresis curves allow for a qualitative separation of the regimes with AFM and FM coupling. Corresponding samples show a characteristic field $μ_{0} H_{B} < μ_{0} H_{S}$ below which saturation starts to break down. (b) The hysteresis curves from the sample discussed in Fig. 1 are recorded by SQUID magnetometry in the IP and OOP direction. The insets depict MFM images at the indicated fields (vertical dotted lines) along the descending OOP branch. The IP graph exhibits an easy-plane switching loop with a coercivity of less than 4 mT. The decrease of magnetic moment for $| μ_{0} H_{ext} | < 200 mT$ is attributed to domain formation due to iDMI. The scale bar in all images corresponds to 500 nm.
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Figure 4
The color plot shows the real part of the background corrected FMR signal. For $μ_{0} H_{ext} > 240 mT$ applied along the OOP direction we see the characteristic FM response of the CoFe multilayer. The black squares indicate the micromagnetic simulation results (see SM [24]). Below the critical field we see an additional dynamic response, which is attributed to the interskyrmion region. Dashed lines and the gray area indicate the field and its uncertainty, respectively, at which the correspondingly numbered MFM graphs in Fig. 1 were recorded. The Gilbert damping parameter is $α_{G} = (37 \pm 1) \times 10^{- 3}$ (see SM [24]).
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