  Manipulation of three-dimensional (3D) topological objects is of both
fundamental interest and practical importance in many branches of physics.
Here, we show by spin dynamics simulations that the bifurcation of a 3D
skyrmion string in a layered frustrated system could be induced by the
dampinglike spin-orbit torque. The bifurcation of a skyrmion string happens
when the skyrmion string carries a minimal topological charge of $Q=2$. We
demonstrate that three types of bifurcations could be realized by applying
different current injection geometries, which lead to the transformation from
I-shaped skyrmion strings to Y-, X-, and O-shaped ones. Besides, different
branches of a bifurcated skyrmion string may merge into an isolated skyrmion
string spontaneously. The mechanism of bifurcation should be universal to any
skyrmion strings with $Q\geq 2$ in the layered frustrated system and could
offer a general approach to manipulate 3D stringlike topological objects for
spintronic functions.

Mesoscale and Nanoscale Physics