  Spin-orbit torques (SOTs) act as efficient drivers for nanoscale magnetic
systems, such as in magnetic tunnel junctions, nano-oscillators and racetrack
geometries. In particular, in combination with materials exhibiting high
Dzyaloshinskii--Moriya interaction, SOTs are considered to result in
well-controlled deterministic magnetisation dynamics and are, therefore, used
as robust drives to move and create magnetic skyrmions. In contrast to these
expectations, we here find unpredictable, transiently chaotic dynamics induced
by SOT at an artificial anisotropy-engineered defect in a magnetic racetrack.
Based on these controlled conditions, we directly observe the nanoscale
dynamics with holography-based, time-resolved x-ray imaging. In concert with
micromagnetic simulations, we disclose a regime of violent picosecond
fluctuations, including topological instabilities that, remarkably, result in
deterministic final configurations. In addition, our images expose previously
unseen skyrmion shedding and highlight the potential of transiently chaotic
pathways for topological switching. Our approach offers new perspectives for
the investigation and application of highly non-linear SOT dynamics in
spintronics materials.
