The role of interface roughness on laser-induced helicity-dependent THz-emission in thin film Co/Pt bilayers

Generating and controlling spin currents at magnetic/nonmagnetic layer interfaces using ultrashort laser pulses has triggered the development of 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 interface on laser-induced optical torques in strongly spin-orbit coupled Co/Pt model system. We varied 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. Furthermore, the 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.