Driving skyrmions in flow regime in synthetic ferrimagnets.

The last decade has seen significant improvements in our understanding of skyrmions current induced dynamics, along with their room temperature stabilization, however, the impact of local material inhomogeneities still remains an issue that impedes reaching the regime of steady state motion of these spin textures. Here, we study the spin-torque driven motion of skyrmions in synthetic ferrimagnetic multilayers with the aim of achieving high mobility and reduced skyrmion Hall effect. We consider Pt|Co|Tb multilayers of various thicknesses with antiferromagnetic coupling between the Co and Tb magnetization. The increase of Tb thickness in the multilayers reduces the total magnetic moment and increases the spin-orbit torques allowing to reach velocities up to 400 ms for skyrmions with diameters of about 160 nm. We demonstrate that due to reduced skyrmion Hall effect combined with the edge repulsion of the magnetic track, the skyrmions move along the track without any transverse deflection. Further, by comparing the field-induced domain wall motion and current-induced skyrmion motion, we demonstrate that the skyrmions at the largest current densities present all the characteristics of a dynamical flow regime.