Dzyaloshinskii-Moriya interaction and current-induced magnetization switching in ferrimagnetic [Co/Gd]_N multilayers

The Dzyaloshinskii-Moriya interactions (DMI) naturally breaking spatial inversion symmetry are considered a key factor in realizing current-induced magnetization switching by spin-orbit torque (SOT) without an in-plane assistance field. Ferrimagnets (FIM) composed of rare earth and transition metals show peculiar properties near the magnetic compensation point, including an exceptionally high SOT efficiency and a significantly enhanced DMI effective field. Here, we investigate FIM [Co/Gd]_N multilayers with a magnetic compensation by varying the number of stacking layers (N). The net magnetization is dominated by Co and Gd at small and large N, respectively, with the magnetic compensation point located at N = 9. The DMI effective field, obtained by using a magnetic droplet model, increases substantially when the multilayer approaches its magnetic compensation. Correspondingly, we found the emergence of current-induced magnetization switching in the Gd-dominated region, among which the case of N = 10 was realized without an assistance field. We attribute such magnetization switching behavior to the symmetry breaking provided by DMI, together with its large value and the huge SOT associated with it near the magnetic compensation. Our work establishes the direct connection between the DMI and the field-free SOT switching in rare-earth/transition-metal FIM systems.