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Kern, L. -M.; Kuchkin, V. M.; Deinhart, V.; Klose, C.; Sidiropoulos, T.; Auer, M.; Gaebel, S.; Gerlinger, K.; Battistelli, R.; Wittrock, S.; Karaman, T.; Schneider, M.; Günther, C. M.; Engel, D.; Will, I.; Wintz, S.; Weigand, M.; Büttner, F.; Höflich, K.; Eisebitt, S.; Pfau, B. Controlled Formation of Skyrmion Bags Journal Article Adv. Mater. 2501250 (2025), (contributed). Abstract | Links | BibTeX | Project(s): C2 @article{kern_controlled_2025,Abstract Topologically non-trivial magnetic solitons are complex spin textures with a distinct single-particle nature. Although magnetic skyrmions, especially those with unity topological charge, have attracted substantial interest due to their potential applications, more complex topological textures remain largely theoretical. In this work, the stabilization of isolated higher-order skyrmion bags beyond the prototypical π-skyrmion in ferromagnetic thin films is experimentally demonstrate, which has posed considerable challenges to date. Specifically, controlled generation of skyrmionium (2π-skyrmion), target skyrmion (3π-skyrmion), and skyrmion bags (with variable topological charge) are achieved through the introduction of artificially engineered anisotropy defects via local ion irradiation. They act as preferential sites for the field- or laser-induced nucleation of skyrmion bags. Remarkably, ultrafast laser pulses achieve a substantially higher conversion rate transforming skyrmions into higher-order skyrmion bags compared to their formation driven by magnetic fields. High-resolution x-ray imaging enables direct observation of the resulting skyrmion bags. Complementary micromagnetic simulations reveal the pivotal role of defect geometry–particularly diameter–in stabilizing closed-loop domain textures. The findings not only broaden the experimental horizon for skyrmion research, but also suggest strategies for exploiting complex topological spin textures within a unified material platform for practical applications. |  |
Metternich, D.; Litzius, K.; Wintz, S.; Gerlinger, K.; Petz, S.; Engel, D.; Sidiropoulos, T.; Battistelli, R.; Steinbach, F.; Weigand, M.; Wittrock, S.; Schmising, C. Korff; Büttner, F. Defects in magnetic domain walls after single-shot all-optical switching Journal Article Struct. Dyn. 12, 024504 (2025). Abstract | Links | BibTeX | Project(s): C2 @article{metternich_defects_2025,Helicity-independent all-optical switching (HI-AOS) is the fastest known way to switch the magnetic order parameter. While the switching process of extended areas is well understood, the formation of domain walls enclosing switched areas remains less explored. Here, we study domain walls around all-optically nucleated magnetic domains using x-ray vector spin imaging and observe a high density of vertical Bloch line defects. Surprisingly, the defect density appears to be independent of optical pulse parameters, significantly varies between materials, and is only slightly higher than in domain walls generated by field cycling. A possible explanation is given by time-resolved Kerr microscopy, which reveals that magnetic domains considerably expand after the initial AOS process. During this expansion, and likewise during field cycling, domain walls propagate at speeds above the Walker breakdown. Micromagnetic simulations suggest that at such speeds, domain walls accumulate defects when moving over magnetic pinning sites, explaining similar defect densities after two very different switching processes. The slightly larger defect density after AOS compared to field-induced switching indicates that some defects are created already when the domain wall comes into existence. Our work shows that engineered low-pinning materials are a key ingredient to uncover the intrinsic dynamics of domain wall formation during ultrafast all-optical switching. | |