  The interfacial Dzyaloshinskii-Moriya interaction (DMI) is important for
chiral domain walls (DWs) and for stabilizing magnetic skyrmions. We study the
effects of introducing increasing thicknesses of Ir, from zero to 2 nm, into a
Pt/Co/Ta multilayer between the Co and Ta. We observe a marked increase in
magnetic moment, due to the suppression of the dead layer at the interface with
Ta, but the perpendicular anisotropy is hardly affected. All samples show a
universal scaling of the field-driven domain wall velocity across the creep and
depinning regimes. Asymmetric bubble expansion shows that DWs in all of the
samples have the left-handed N\'{e}el form. The value of in-plane field at
which the creep velocity shows a minimum drops markedly on the introduction of
Ir, as does the frequency shift of the Stokes and anti-Stokes peaks in
Brillouin light scattering measurements. Despite this qualitative similarity,
there are quantitative differences in the DMI strength given by the two
measurements, with BLS often returning higher values. Many features in bubble
expansion velocity curves do not fit simple models commonly used to date,
namely a lack of symmetry about the velocity minimum and no difference in
velocities at high in-plane field. These features are explained by the use of a
model in which the depinning field is allowed to vary with in-plane field in a
way determined from micromagnetic simulations. This theory shows that velocity
minimum underestimates the DMI field, consistent with BLS returning higher
values. Our results suggest that the DMI at an Ir/Co interface has the same
sign as the DMI at a Pt/Co interface.
