Generating and controlling spin currents at magnetic/nonmagnetic layer interfaces using ultrashort laser pulses has triggered the development of new high- operational frequency spintronic devices. Recent studies showed that laser-driven spin currents and opto-magnetic torques acting on spins are most effective when an interface is created between ferromagnetic Co and nonmagnetic Pt thin films. Our study focuses on the role of the Co-Pt interface on laser-induced optical torques in the strongly spin-orbit coupled Co/Pt model system. We varied the average roughness at the interface, in the range of 0.1-1.0 nm, by tuning the deposition pressure conditions during fabrication. With the aid of time-resolved THz-emission spectroscopy we detected both the laser-induced helicity-independent(HI) and helicity-dependent(HD) THz-emission due to spin-Hall and spin-orbit torque effects, respectively. We reveal a dramatic change in the detected THz-signals when the interface roughness is varied. For example, the HD-THz emission is observed only when the roughness is 0.3 nm or above. To study the role of intermixing a CoPt spacer layer, with varying compositions, is introduced at the interface. However, the detected THz-emission signals rule out the intermixing effects in determining the helicity-dependence. Moreover, static spin-hall conductivity measurements provide with new insights in understanding the role of spin-orbit coupling, at the Co/Pt interfaces, in laser-induced optical torques on net magnetization.
This research is funded by DOE:DE-SC001823