  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.
