In any change of the topological charge in a three dimensional (3D) spin configuration such as a vortex core and skyrmion switching, a Bloch point (BP) - a unique topological singularity should be mediated  [1-3]. Consequently, it is crucial to know the intrinsic nature of BP and its role in magnetization dynamics for understanding the various dynamical phenomena of spin structures. Since BP is a virtual point at which the local magnetization vanishes among lattice points as well as BP is rarely stabilized in magnetic nanostructures, the intrinsic natures of a BP and its dynamic behaviours have not been verified experimentally.
In this presentation, we demonstrate how to form stable BP in a ferromagnetic patterned nanodisk which can provide a flatform for experimental observation of  BP and its dynamic behaviour. We successfully formed a steady-state BP in the middle of elongated magnetic vortex core structures in asymmetrically shaped Ni₈₀Fe₂₀ nanodisks [4] . By using the magnetic transmission soft X-ray microscopy (MTXM) at the Advanced Light Source [5], we measured detailed static and dynamic characteristics of BPs. Based on time-resolved nanoscale magnetic X-ray imaging combined with micromagnetic simulations, the role of BP in vortex-core dynamics will be elucidated in this presentation.

[1]   Thiaville, A. & Miltat, J. Europhys. Lett. 26, 57 (1994).
[2]   Wohlhüter, P. et al. Nat. Commun. 6, 7836 (2015).
[3]   Noske, M., Stoll, H., Fähnle, M., Hertel, R. & Schütz, G. Phys. Rev. B 91, 014414 (2015).
[4]   Im, M., Han, H.-S., et al. Nature Commun. 10, 593 (2019)
[5]   Fischer, P. et al. Mater. Today 9, 26–33 (2006).