  We carried out measurements of domain wall (DW) velocities driven by magnetic
field pulses in symmetric Pt/Co/Pt and asymmetric Pt/Co/AlOx, Pt/Co/GdOx and
Pt/Co/Gd trilayers with ultrathin Co layers and perpendicular magnetic
anisotropy. In agreement with theoretical models, the maximum observed velocity
is much larger in the asymmetric samples, where the interfacial
Dzyaloshinskii-Moriya interaction (DMI) stabilises chiral N\'eel walls, than in
the symmetric stack. In addition, in Pt/Co/Gd very large DW speeds (up to 600
m/s) are obtained, 2.5 times larger than in samples with oxidised Gd. Magnetic
measurements reveal that this may be explained by the anti-parallel coupling
between the magnetic moments of Gd and Co at the Gd/Co interface, leading to a
decrease of the total magnetisation. In quantitative agreement with analytical
models, in all samples the maximum observed DW speed scales as D=Ms, where D is
the strength of the DMI and Ms the spontaneous magnetisation.
