  We study field-driven domain wall (DW) velocities in asymmetric multilayer
stacks with perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya
interaction (DMI), both experimentally and by micromagnetic simulations. Using
magneto-optical Kerr microscopy under intense and nanoseconds-long fields, we
show that DWs in these films propagate at velocities up to hundreds of m/s and
that, instead of the expected decrease of velocity after the Walker field, a
long plateau with constant velocity is observed, before breakdown. Both the
maximum speed and the field extent of the velocity plateau strongly depend on
the values of the spontaneous magnetization and the DMI strength, as well as on
the magnetic anisotropy. Micromagnetic simulations reproduce these features in
sufficiently wide strips, even for perfect samples. A physical model explaining
the microscopic origin of the velocity plateau is proposed.
