Magnetic Chirality Controlled by the Interlayer Exchange Interaction

Mariëlle J. Meijer, Juriaan Lucassen, Fabian Kloodt-Twesten, Robert Frömter, Oleg Kurnosikov, Rembert A. Duine, Henk J. M. Swagten, Bert Koopmans, and Reinoud Lavrijsen

Phys. Rev. Lett. 124, 207203 – Published 22 May 2020

Abstract

Chiral magnetism, wherein there is a preferred sense of rotation of the magnetization, determines the chiral nature of magnetic textures such as skyrmions, domain walls, or spin spirals. Current research focuses on identifying and controlling the interactions that define the magnetic chirality in thin film multilayers. The influence of the interfacial Dzyaloshinskii-Moriya interaction (IDMI) and, recently, the dipolar interactions have been reported. Here, we experimentally demonstrate that an indirect interlayer exchange interaction can be used as an additional tool to effectively manipulate the magnetic chirality. We image the chirality of magnetic domain walls in a coupled bilayer system using scanning electron microscopy with polarization analysis. Upon increasing the interlayer exchange coupling, we induce a transition of the magnetic chirality from clockwise rotating Néel walls to degenerate Bloch-Néel domain walls and we confirm our findings with micromagnetic simulations. In multilayered systems relevant for skyrmion research, a uniform magnetic chirality across the magnetic layers is often desired. Additional simulations show that this can be achieved for reduced IDMI values (up to 30%) when exploiting the interlayer exchange interaction. This work opens up new ways to control and tailor the magnetic chirality by the interlayer exchange interaction.

Received 30 October 2019
Revised 3 April 2020
Accepted 27 April 2020
Corrected 23 July 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.207203

Physics Subject Headings (PhySH)

Dipolar interaction Magnetic interactions Magnetic texture RKKY interaction

Condensed Matter, Materials & Applied Physics

Corrections

23 July 2020

Correction: The use of the grouped byline format resulted in an incorrect author order, which has been rectified.

Authors & Affiliations

Mariëlle J. Meijer ^1,*, Juriaan Lucassen ¹, Fabian Kloodt-Twesten ², Robert Frömter ², Oleg Kurnosikov ¹, Rembert A. Duine^1,3, Henk J. M. Swagten¹, Bert Koopmans¹, and Reinoud Lavrijsen ¹

¹Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands
²Universität Hamburg, Center for Hybrid Nanostructures, Luruper Chaussee 149, 22761 Hamburg, Germany
³Institute for Theoretical Physics, Utrecht University, Leuvenlaan 4, 3584 CE Utrecht, Netherlands

^*m.j.meijer@tue.nl

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Vol. 124, Iss. 20 — 22 May 2020

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Images

Figure 1
(a) Elementary model of two magnetic CoNi layers separated by an Ir spacer layer in the absence of an IDMI (side view). The up and down domains are indicated by the white and black areas, respectively, and they generate dipolar fields (gray dashed lines). The in-plane magnetization of the domain wall aligns with the dipolar field (along the arrow), resulting in CW (CCW) Néel walls in the top (bottom) magnetic layer. A ferromagnetic RKKY interaction (blue dashed line) rotates the in-plane magnetization toward Bloch walls (blue arrows). (b) Micromagnetic simulation results for $J = 0 mJ m^{- 2}$ and the two degenerate cases for $J = 1 mJ m^{- 2}$ . The in-plane magnetization is indicated by the arrows. (c) Angle $α_{top}$ as a function of RKKY coupling strength $J$ obtained from micromagnetic simulations. $α_{top}$ defines the angle between the in-plane magnetization and the horizontal of the top magnetic layer (see inset). The insets show a top view of the magnetization direction for $J = 0 mJ m^{- 2}$ (left-hand inset) and $J = 0.4 mJ m^{- 2}$ (right-hand insets for the degenerate case).
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Figure 2
(a),(b) SEMPA images of the top magnetic layer at an Ir thickness of $t = 0.68 nm$ . Panel (a) shows $m_{y} + out$ -of-plane contrast ( $m_{z}$ ) and panel (b) $m_{x} + out$ -of-plane contrast ( $m_{z}$ ) for the same area. The in-plane magnetic contrast direction is indicated by the arrow in the bottom right-hand corner. (c) Composite image constructed from (a) and (b), with the in-plane magnetization indicated by the color wheel and the out-of-plane contrast by the white and black areas (up and down magnetization, respectively). (d) Composite image at an Ir thickness of $t = 0.85 nm$ . The same scale bar is used for all images. (e),(f) Histograms of the angle $α$ for all pixels in the domain walls of panels (c) and (d), respectively. $α$ is defined as the difference between the domain wall normal $n$ and magnetization in the domain wall $m$ [see inset of (e)]. The solid line is a (double) Gaussian fit with the maximum(s) at $α^{*}$ . (g) Maximum(s) $α^{*}$ of the Gaussian fits of the histograms as a function of Ir thickness $t$ . In the purple-shaded area two maximums are found and the insets schematically show the corresponding magnetization texture in this region. The color indications from (c) are used. (h) RKKY coupling strength $J$ as a function of Ir spacer layer thickness $t$ . The MOKE data are extracted from hysteresis loops (see Supplemental Material SIV for details [44]) and the SEMPA data in (g) are translated to a value of $J$ with the help of Fig. 1. Both datasets are fitted with the theoretical RKKY function from Ref. [30] (solid black curve).
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Figure 3
(a) Micromagnetic simulations of a multilayer stack containing 6 magnetic layers with $D = 0.5 mJ m^{- 2}$ . The up and down domains are indicated by the white and black areas, respectively, and the colors in the domain walls show the in-plane magnetization direction according to the color wheel of Fig. 2. The arrows in the gray spacer layer indicate the magnetization inside the magnetic layer above. In the left-hand image $J = 0 mJ m^{- 2}$ , and in the right-hand image $J = 1 mJ m^{- 2}$ . (b) Phase diagram of the angle $Δ α$ as a function of $J$ and $D$ . $Δ α$ is defined as the difference between $α$ in the bottom and top magnetic layer. For $Δ α = 0 °$ there is a uniform chirality throughout the multilayer stack. The black line indicates from which $D$ onward $Δ α < 0.1 °$ , marking the transition between a nonuniform and a uniform magnetization.
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