  Time-resolved ultrafast EUV magnetic scattering was used to test a recent
prediction of >10 km/s domain wall speeds by optically exciting a magnetic
sample with a nanoscale labyrinthine domain pattern. Ultrafast distortion of
the diffraction pattern was observed at markedly different timescales compared
to the magnetization quenching. The diffraction pattern distortion shows a
threshold-dependence with laser fluence, not seen for magnetization quenching,
consistent with a picture of domain wall motion with pinning sites. Supported
by simulations, we show that a speed of $\approx$ 66 km/s for highly curved
domain walls can explain the experimental data. While our data agree with the
prediction of extreme, non-equilibrium wall speeds locally, it differs from the
details of the theory, suggesting that additional mechanisms are required to
fully understand these effects.
