the domain expansion images in the creep regime. In order to clarify the interpretation of the field-induced measurements, we measured the DW speeds as a function of $H_{x}$ field for the bubble domains shown in Figure \[fig:images\].

The velocity curves are shown in Figure \[fig:speed\_Hcreep\] for the two domain walls propagating along the $x$-axis and having their magnetisation either parallel or antiparallel to the $H_{x}$ field. The up/down and the down/up DWs exhibit the same behaviour for opposite $H_{x}$ fields, as expected for chiral Néel walls. The curves for sample (D) - corresponding to the thicker Gd layer - present the main features found by other authors for DDWs in Pt/Co/Pt films [@Je2013; @Hrabec2014]. The speed of the down/up DW increases for a positive in-plane field, and for negative fields it decreases down to a minimum value between -100 mT and -200 mT, where the velocity starts increasing again. On the other hand, the curves measured for samples (A-C) strongly deviate from the expected behaviour, showing in particular a maximum rather than a minimum in the DW speed.

In the thermally activated regime, the DW velocity is given by: $$\label{speed}
v(H_{z)}=v_{o}\exp(-\eta H_{z}^{-\mu})$$ where $v_{0}$ is the characteristic speed, $\mu$=1/4 is 