Engineering the Spin-Orbit Torques and Interfacial Dzyaloshinkii-Moriya interaction by stacking order in Tb/Co ferrimagnetic multilayers

Abstract

We characterized the spin-orbit torques (SOTs), current-induced switching, domain wall (DW) motion and interfacial Dzyaloshinkii-Moriya interaction (DMI) in synthetic ferrimagnets consisting of Co/Tb layers with differing stacking order grown on a Pt underlayer. We find that the magnetotransport properties and magnetization switching of these systems are highly sensitive to the stacking order of the Co and Tb layers and to the element in contact with Pt. Our study shows that Tb is an efficient SOT generator when in contact with Co, and its position in the stack can be adjusted to generate torques additive to those generated by the Pt layer and the Pt/Co interface. With optimal stacking and layer thickness, the damping-like SOT efficiency (the effective spin Hall angle) reaches up to 0.3, well beyond that expected from the Pt/Co bilayer. Moreover, the magnetization can be easily switched by the injection of pulses with current density of about 107 A/cm2 despite the extremely high perpendicular magnetic anisotropy barrier (up to 3.5 T). Efficient switching is due to the combination of large SOTs and low saturation magnetization owing to the ferrimagnetic character of the multilayers. We observed current-driven DW motion in the absence of an external field, which is indicative of homochiral Néel-type DWs stabilized by the interfacial DMI. Characterization of the DMI in separate multilayers by Brillouin light scattering reveals that the position of the Tb layer plays a critical role also for the amplitude of the DMI. The largest DMI is obtained in layers where Co is sandwiched between Pt and Tb, similarly to the stack where large SOTs are observed. These results shows that the stacking order in transition metal/rare-earth synthetic ferrimagnets plays a major role in determining the magnetotransport properties relevant for spintronic applications. **