  A magnetic Weyl semimetal is a recent focus of extensive research as it may
exhibit large and robust transport phenomena associated with topologically
protected Weyl points in momentum space. Since a magnetic texture provides a
handle for the configuration of the Weyl points and its transport response,
understanding of magnetic dynamics should form a basis of future control of a
topological magnet. Mn3Sn is an example of an antiferromagnetic Weyl semimetal
that exhibits a large response comparable to the one observed in ferromagnets
despite a vanishingly small magnetization. The non-collinear spin order in
Mn3Sn can be viewed as a ferroic order of cluster magnetic octupole and breaks
the time-reversal symmetry, stabilizing Weyl points and the significantly
enhanced Berry curvature near the Fermi energy. Here we report our first
observation of time-resolved octupole oscillation in Mn3Sn. In particular, we
find the giant effective damping of the octupole dynamics, and it is feasible
to conduct an ultrafast switching at < 10 ps, a hundred times faster than the
case of spin-magnetization in a ferromagnet. Moreover, high domain wall
velocity over 10 km/s is theoretically predicted. Our work paves the path
towards realizing ultrafast electronic devices using the topological
antiferromagnet.
