Tailoring optical excitation to control magnetic skyrmion nucleation

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Tailoring optical excitation to control magnetic skyrmion nucleation

L.-M. Kern, B. Pfau, M. Schneider, K. Gerlinger, V. Deinhart, S. Wittrock, T. Sidiropoulos, D. Engel, I. Will, C. M. Günther, K. Litzius, S. Wintz, M. Weigand, F. Büttner, and S. Eisebitt

Phys. Rev. B 106, 054435 – Published 30 August 2022

Abstract

In ferromagnetic multilayers, a single laser pulse with a fluence above an optical nucleation threshold can create magnetic skyrmions, which are randomly distributed over the area of the laser spot. However, in order to study the dynamics of skyrmions and for their application in future data technology, a controllable localization of the skyrmion nucleation sites is crucial. Here, it is demonstrated that patterned reflective masks behind a thin magnetic film can be designed to locally tailor the optical excitation amplitudes reached, leading to spatially controlled skyrmion nucleation on the nanometer scale. Using x-ray microscopy, the influence of nanopatterned backside aluminum masks on the optical excitation is studied in two sample geometries with varying layer sequence of substrate and magnetic Co/Pt multilayer. Surprisingly, the masks' effect on suppressing or enhancing skyrmion nucleation reverses when changing this sequence. Moreover, optical near-field enhancements additionally affect the spatial arrangement of the nucleated skyrmions. Simulations of the spatial modulation of the laser excitation and the following heat transfer across the interfaces in the two sample geometries are employed to explain these observations. The results demonstrate a reliable approach to add nanometer-scale spatial control to optically induced magnetization processes on ultrafast timescales.

Received 17 June 2022
Revised 11 August 2022
Accepted 11 August 2022

DOI:https://doi.org/10.1103/PhysRevB.106.054435

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Magnetism Skyrmions

Magnetic multilayers

X-ray holography X-ray imaging X-ray magnetic circular dichroism

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

L.-M. Kern ¹, B. Pfau ^1,*, M. Schneider¹, K. Gerlinger ¹, V. Deinhart ¹, S. Wittrock¹, T. Sidiropoulos ¹, D. Engel ¹, I. Will¹, C. M. Günther², K. Litzius ³, S. Wintz³, M. Weigand^3,4, F. Büttner ⁴, and S. Eisebitt ^1,5

¹Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, 12489 Berlin, Germany
²Technische Universität Berlin, Zentraleinrichtung Elektronenmikroskopie (ZELMI), 10623 Berlin, Germany
³Max Planck Institute for Intelligent Systems, 70569 Stuttgart, Germany
⁴Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 14109 Berlin, Germany
⁵Technische Universität Berlin, Institut für Optik und Atomare Physik, 10623 Berlin, Germany

^*bastian.pfau@mbi-berlin.de

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Issue

Vol. 106, Iss. 5 — 1 August 2022

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Images

Figure 1
Scheme of the direct and indirect coupling geometry. (a) Direct coupling geometry with direct contact of the magnetic film with the backside reflective mask; and (b) corresponding SEM images of circular apertures, Al lines, and a schematic of hole apertures in Al. (c) Indirect coupling geometry with the substrate sandwiched between the magnetic film and the backside reflective mask; and (d) corresponding SEM images of circular and elliptic apertures and a schematic of Al dots. Scale bars correspond to 1 $μ m$ .
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Figure 2
Geometry-dependent skyrmion configurations in Co/Pt nucleated by a exposure with a single laser pulse. (a), (b) Direct coupling geometry with (a) phase diagram, and (b) representative STXM images for the different regimes identified in (a). (c), (d) Indirect coupling geometry with (c) phase diagram, and (d) representative STXM images for the different regimes identified in (c). Scale bars correspond to 1 $μ m$ .
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Figure 3
Localized optical skyrmion nucleation at the nanometer scale. (a)–(c) Direct coupling: Optical nucleation in laminar gratings with variable spaces [(a) 500 nm, (b) 100 nm], and (c) array of holes with 150 nm diameter. Skyrmions (white dots) form in regions without Al mask (array of gray holes). (d)–(f) Indirect coupling: (d) Optical nucleation in array of dots with 150 nm diameter. Skyrmions (white dots) form in regions with Al mask (array of black dots). Additional near-field effects cause (e) circular or (f) elliptic skyrmion rings in regions without mask. Scale bars correspond to 1 $μ m$ .
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Figure 4
Simulated depth dependence of IR absorption and lattice temperature profiles. Solid curves reflect a central position in the magnetic film without mask, i.e., above the open aperture within the backside reflective mask, while dashed curves denote a central position in the magnetic film under the mask. (a) Depth profile of absorption and (b) maximum lattice temperature for direct coupling at $t = 3.9$ ps. Note that an additional 4 nm oxide layer between magnetic film and aluminum mask is added in the case of the direct coupling geometry to account for exposing the sample to air during the transfer between DC magnetron sputtering and electron-beam evaporation in the sample's fabrication process. (c) Depth profile of absorption and (d) maximum lattice temperature for indirect coupling at $t = 3.2$ ps. Note that the maximum lattice temperatures are reached at the opposite interfaces of the magnetic film for the two situations with and without Al mask.
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Figure 5
Simulated lateral temperature profile. Lateral temperature profile in the center of the magnetic film for (a) direct coupling and (b) indirect coupling geometry. The orange-shaded horizontal lines in (a) and (b) represent relative, qualitative estimates of the material-dependent nucleation threshold that has been overcome in the corresponding regimes in the phase diagram in Figs. 2 and 2, respectively, and which is supported by experimental observations as indicated. Scale bars correspond to 1 $μ m$ . If the magnetic film remains saturated after laser excitation, the nucleation threshold has not been overcome, denoted as the regime “no nucleation.”
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