  The current-driven domain wall motion along two exchange-coupled
ferromagnetic layers with perpendicular anisotropy is studied by means of
micromagnetic simulations and compared to the conventional case of a single
ferromagnetic layer. Our results, where only the lower ferromagnetic layer is
subjected to the interfacial Dzyaloshinskii-Moriya interaction and to the spin
Hall effect, indicate that the domain walls can be synchronously driven in the
presence of a strong interlayer exchange coupling, and that the velocity is
significantly enhanced due to the antiferromagnetic exchange coupling as
compared with the single-layer case. On the contrary, when the coupling is of
ferromagnetic nature, the velocity is reduced. We provide a full micromagnetic
characterization of the current-driven motion in these multilayers, both in the
absence and in the presence of longitudinal fields, and the results are
explained based on a one-dimensional model. The interfacial
Dzyaloshinskii-Moriya interaction, only necessary in this lower layer, gives
the required chirality to the magnetization textures, while the interlayer
exchange coupling favors the synchronous movement of the coupled walls by a
dragging mechanism, without significant tilting of the domain wall plane.
Finally, the domain wall dynamics along curved strips is also evaluated. These
results indicate that the antiferromagnetic coupling between the ferromagnetic
layers mitigates the tilting of the walls, which suggest these systems to
achieve efficient and highly packed displacement of trains of walls for
spintronics devices. A study, taking into account defects and thermal
fluctuations, allows to analyze the validity range of these claims.
