  Magnetic skyrmions are nano-sized topological spin textures stabilized by a
delicate balance of magnetic energy terms. The chemical substitution of the
underlying crystal structure of skyrmion-hosting materials offers a route to
manipulate these energy contributions, but also introduces additional effects
such as disorder and pinning. While the effects of doping and disorder have
been well studied in B20 metallic materials such as Fe$_{1-x}$Co$_x$Si and
Mn$_{1-x}$Fe$_x$Si, the consequences of chemical substitution in the
magnetoelectric insulator Cu$_2$OSeO$_3$ have not been fully explored. In this
work, we utilize a combination of AC magnetometry and small angle neutron
scattering to investigate the magnetic phase transition dynamics in pristine
and Zn-substituted Cu$_2$OSeO$_3$. The results demonstrate that the first order
helical-conical phase transition exhibits two thermally separated behavioural
regimes: at high temperatures, the helimagnetic domains transform by
large-scale, continuous rotations, while at low temperatures, the two phases
coexist. Remarkably, the effects of pinning in the substituted sample are less
prevalent at low temperatures, compared to high temperatures, despite the
reduction of available thermal activation energy. We attribute this behaviour
to the large, temperature-dependent, cubic anisotropy unique to Cu$_2$OSeO$_3$,
which becomes strong enough to overcome the pinning energy at low temperatures.
Consideration and further exploration of these effects will be crucial when
engineering skyrmion materials towards future applications.
