  In the MnSi bulk chiral magnet, magnetic skyrmion strings of 17 nm in
diameter appear in the form of a lattice, penetrating the sample thickness,
10-1,000 um. Although such a bundle of skyrmion strings may exhibit complex
soft-matter-like dynamics when starting to move under the influence of a random
pinning potential, the details remain highly elusive. Here, we show that a
metastable skyrmion-string lattice is subject to topological unwinding under
the application of pulsed currents of 3-5x106 A m-2 rather than being
transported, as evidenced by measurements of the topological Hall effect. The
critical current density above which the topological unwinding occurs is larger
for a shorter pulse width, reminiscent of the viscoelastic characteristics
accompanying the pinning-creep transition observed in domain-wall motion.
Numerical simulations reveal that current-induced depinning of already
segmented skyrmion strings initiates the topological unwinding. Thus, the
skyrmion-string length is an element to consider when studying current-induced
motion.
