Skyrmions and antiskyrmions are topologically protected spin structures with opposite vorticities. Particularly in coexisting phases, these two types of magnetic quasiparticles may show fascinating physics and potential for spintronic devices. While skyrmions are observed in a wide range of materials until now antiskyrmions were exclusive to materials with D<sub>2d</sub> symmetry [1].<br/>In our recent work, we have shown first and second-order antiskyrmions stabilized by magnetic dipole-dipole interaction in Fe/Gd-based multilayers [2]. Using Lorentz transmission electron microscopy imaging, we observed coexisting first and second-order antiskyrmions, Bloch skyrmions, and type-2 bubbles (Fig. 1) and determine the range of material properties and magnetic fields where the different spin objects form and dissipate. Phase diagrams of the spin objects were created in dependence on the magnetic out-of-plane field, temperature, saturation magnetization, and uniaxial magnetic anisotropy. We performed micromagnetic simulations to obtain more insight into the studied system and conclude that low values of saturation magnetization and uniaxial magnetic anisotropy lead to the existence of this zoo of different spin objects and that they are primarily stabilized by dipole-dipole interaction. Further, we investigated the nucleation process of antiskyrmions experimentally and theoretically revealing the necessity of a crossing point of three magnetic stripe domains to form an isolated antiskyrmion with an out-of-plane magnetic field.<br/>The previous unobserved second-order antiskyrmions and the disclosed coexistence of antiskyrmions and skyrmions potentially reachable for a whole range of different materials provide great potential for further studies on quasi-particle interactions, spin dynamics as well as for possible future applications in spintronics.

![](https://assets.underline.io/uploads/markdown_image/1/image/50c16dd6974021e442524cdd7ed4e0a3.jpg)

Fig. 1. Underfocused LTEM image of [Fe/Ir/Gd]2/[Fe/Gd]76/[Fe/Ir/Gd]2 showing five different coexisting spin objects (a). Bloch skyrmion with counter- (b) and with clockwise rotation (c), type-2 bubble (d), first-order antiskyrmion (e), and second-order antiskyrmion (f). Zoomed-in spin objects (b-f, I) and simulated LTEM contrast (b-f, II) caused by the theoretical spin textures (b-f, III).

Dipolar-Stabilized First and Second-Order Antiskyrmions in Ferrimagnetic Multilayers

Exchange-coupled systems employing ferrimagnets (FI) provide high tunability, interfacial exchange interaction, and zero magnetic moment at the compensation temperature [1], which is highly beneficial for many applications such as spin valves [2] and magnetic tunnel junction devices [3]. Micromagnetic simulations showed that a partial domain wall is formed at the FI/ferromagnet(FM) interface layer during the reversal of the FM. The minor loop becomes fully reversible if this domain wall generates a hard-axis field that overcomes the anisotropy field of the FM [4]. In this study, the magnetic reversal process of exchange-coupled bilayer systems, consisting of a FI TbFeCo alloy layer and a FM [Co/Ni/Pt]N multilayer (ML), was investigated [5]. In particular, minor loop studies, probing solely the reversal characteristics of the softer FM layer, reveal two distinct reversal mechanisms, which depend strongly on the thickness of the FM layer. For thick layers (N=9, 10.8 nm), irreversible switching of the macroscopic minor loop is observed. The underlying microscopic origin of this reversal process was studied in detail by high-resolution magnetic force microscopy, showing that the reversal is triggered by in-plane domain walls propagating through the FM layer. In contrast, thin FM layers (N=5, 6.0 nm) show a hysteresis-free reversal, which was already reported in previous work [6] and is nucleation-dominated due to grain-to-grain variations in magnetic anisotropy of the Co/Ni/Pt ML and an inhomogeneous exchange coupling with the magnetically hard TbFeCo layer. The results were confirmed by micromagnetic simulations.References: [1] B. Hebler, S. Böttger, M. Albrecht et al., Phys. Rev. B93, 184423 (2016) [2] F. Radu, R. Abrudan, H. Zabel et al., Nat. Commun. 3, 715 (2012) [3] J. S. Moodera, L. R. Kinder, R. Meservey et al., Phys. Rev. Lett. 74, 3273 (1995) [4] C. Vogler, M. Heigl, D. Suess et al., arXiv:2004.04419 (2020) [5] M. Heigl, C. Vogler, M. Albrecht et al., arXiv:2007.00333 (2020) [6] X. Zhao, A.-O. Mandru, H. J. Hug et al., ACS Appl. Nano Mater. 2, 12, 7478 (2019)

Microscopic Origin of Magnetization Reversal in Exchange-coupled Ferro-/Ferrimagnetic Bilayers

**Session chair:** Max Hirschberger, The University of Tokyo



GOF - Skyrmions, Antiskyrmions and Topology

technical paper

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D1: Exchange Bias and Other Novel Exchange in Thin Films

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Michael Heigl

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Dipolar-Stabilized First and Second-Order Antiskyrmions in Ferrimagnetic Multilayers

Microscopic Origin of Magnetization Reversal in Exchange-coupled Ferro-/Ferrimagnetic Bilayers

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