  We characterize asymmetric growth of magnetic bubble domains in
perpendicularly magnetized Co/Ni multi-layers grown on Pt$_x$Ir$_{1-x}$
seedlayers by application of perpendicular and in-plane magnetic fields. Using
a refined model of domain wall creep that incorporates contributions from the
anisotropic elastic energy, $\varepsilon$, and a chirality-dependent prefactor,
$v_0$, we elucidate factors that govern the mobility of Dzyaloshinskii domain
walls as a function of seedlayer composition. The interfacial
Dzyaloshinskii-Moriya Interaction magnitude is found to decrease monotonically
with $x_{Ir}$, which is independently confirmed by Brillouin light scattering
(BLS). Moreover, the persistence of significant asymmetry in velocity curves
across the full composition range supports previous assertions that a
chirality-dependent attempt frequency akin to chiral damping could play a
critical role in the observed trends. This work helps resolve fundamental
questions about the factors governing Dzyaloshinskii DW creep and demonstrates
varying Pt-Ir seedlayer composition as a method to tune DMI.
