  Magnetic skyrmion, i.e. a topologically stable swirling spin texture, appears
as a particle-like object in the two-dimensional (2D) systems, and has recently
attracted attention as a candidate of novel information carrier. In the real
three-dimensional (3D) systems, a skyrmion is expected to form a string
structure along an extra dimension, while its experimental identification has
rarely been achieved. Here, we report the direct visualization of 3D shape of
individual skyrmion strings, for the recently discovered room-temperature
skyrmion-hosting noncentrosymmetric compound Mn1.4Pt0.9Pd0.1Sn. For this
purpose, we have newly developed the magnetic X-ray tomography measurement
system that can apply magnetic field, which plays a key role on the present
achievement. Through the tomographic reconstruction of the 3D magnetization
distribution based on the transmission images taken from various angles, a
genuine skyrmion string running through the entire thickness of the sample, as
well as various defect structures such as the interrupted and Y-shaped strings,
are successfully identified. The observed point defect may represent the
emergent magnetic monopole, as recently proposed theoretically. The present
tomographic approach with tunable magnetic field paves the way for the direct
visualization of the structural dynamics of individual skyrmion strings in the
3D space, which will contribute to the better understanding of the creation,
annihilation and transfer process of these topological objects toward the
potential device applications.
