Discovery of a Bloch point quadrupole constituting hybrid topological strings

Abstract: Topological magnetic (anti)skrymions are robust string-like objects heralded as potential components in next-generation topological spintronics devices due to their manipulability via low-energy stimuli such as magnetic fields, heat, and electric/thermal current. While these two-dimensional (2D) topological objects are widely studied, intrinsically three-dimensional (3D) electron-spin real-space topology remains less explored despite its prevalence in bulky magnets. Here, we capture the 3D structure of antiskyrmions in a single-crystal, precision-doped (Fe_{0.63}Ni_{0.3}Pd_{0.07})_{3}P lamellae using holographic vector field electron tomography at room temperature and zero field. Our measurements reveal hybrid string-like solitons composed of skyrmions with topological number W = -1 on the lamellae's surfaces and an antiskyrmion (W = +1) connecting them. High resolution images uncover a Bloch point (BP) quadrupole (four magnetic (anti)monopoles) positioned along the rectangular antiskyrmion's four corners (Bloch lines), which enable the observed lengthwise topological transitions. Furthermore, we calculate and compare the energy densities of hybrid strings with ideal (anti)skyrmion strings using micromagnetic simulations, which suggest that this composite (anti)BP structure stabilizes via the subtle interplay between the magnetostatic interaction and anisotropic Dzyaloshinskii-Moriya interaction. The discovery of these hybrid spin textures enables topological tunabilty, a tunable topological Hall effect, and the suppression of skyrmion Hall motion, disrupting existing paradigms within spintronics.