Role of the topological singularity in the dynamics of chiral spin textures*

Topology is one of the key concepts in modern condensed matter physics. Since the early discovery of the Kosterlitz-Thouless transition in vortices, the importance of topological protection in understanding novel physical phenomena has been increasingly widely recognized, as evidenced in the quantum Hall effect, topological insulators, and Weyl semimetals. In the case of chiral spin textures such as magnetic skyrmions bubbles, and magnetic vortices, the concept of topological protection is essential in not only understanding their fundamentals but also harnessing them to future spintronic devices. Recently, it has been known that topological protection in chiral spin textures can be broken and the topology can be switched through the atomical discontinuity of magnetic materials or confinement of magnetic medium [1]. In this mechanism, a unique spin texture, a Bloch point (BP) which is a topological singularity, is mainly related. Unlike other spin textures, a BP has a unique feature – the local magnetization at a BP completely vanishes [2]. While it has also been theoretically proposed to bear critical roles in dynamic phenomena of chiral spin textures such as a magnetic vortex and skyrmion switching dynamics, it has not yet been experimentally well explored.

In this work [3], we have successfully established stable BPs embedded within nontrivially distorted magnetic vortex cores in asymmetric Ni₈₀Fe₂₀ nanostructures. Our MTXM measurements, combined with micromagnetic simulations, clearly demonstrate that BPs remain topologically stable against large field-driven lateral motions and BPs play a critical role in vortex-core dynamics. The intrinsic atomic-scale nature of BPs is experimentally proven through direct observation of BP dynamics.

[1] S. Rohart et al., Phys. Rev. B 93, 214412 (2016).

[2] C. Donnelly et al., Nature 547, 328-321 (2017).

[3] M.-Y Im et al., Nature Commun. 10, 593 (2019).