DWs while in sample (B) the asymmetry is practically vanishing, *i.e.* up/down and down/up DWs move at the same speed. In sample (C) the DW speed asymmetry reverses with respect to (A), *i.e.* the up/down DWs move faster. Finally, in (D) the asymmetry found in (A) is recovered.

According to previous work [@Je2013; @Hrabec2014], the cancellation (resp. change of sign) of the DW speed asymmetry may be attributed to a vanishing (resp. reversed) value of the DMI. This result is unexpected and counter-intuitive. As one moves across the sample, from (A) to (D), the decreasing degree of oxygen content modifies the composition of the Co/Gd interface, as shown experimentally by the changing PMA. However the sample presents a considerable PMA for the thinner Gd layers, which is an indication that the oxidation only concerns the top Co interface. Therefore the bottom Pt/Co interface, which is expected to provide the most important contribution to the DMI [@Freimuth2014], should not be affected by the varying Gd thickness. This is confirmed by X-ray reflectivity data.

In order to clarify the interpretation of the DW dynamics in the creep regime and to have an independent measurement of the sign of the DMI, we carried 