Helicity-Driven Spin Currents and Magnetization Dynamics in Au-Based Structures

Ultrafast helicity-dependent all-optical switching of magnetization by a femtosecond laser pulse has attracted extensive interest, because it holds the promise for future high-rate magnetic storage. Controlling the spin angular momentum transfer from light to magnetic materials is the key to achieve the helicity-dependent switching of a single layer’s magnetization. Researchers have found that a capping layer made of heavy metal with large spin-orbital coupling strength, such as Pt, Ta, and W, could enhance the spin momentum transfer, namely optical spin-transfer torque, compared to the direct interaction between the light and magnetic materials. In this work, we will discuss the potential of Au as a better choice for the capping material, because it supports plasmonic resonances which enable both enhanced spin-momentum transfer and nanoscale plasmonic integration. We have studied the magnetization dynamics of Au/Co structures with a time-resolved magnetic-optical Kerr effect spectroscopy. The existence of large spin-transfer torque is observed in the system. The overall efficiency is as high as that in Pt/Co. The mechanism is revealed by the detailed thickness- and wavelength-dependent measurements of magnetization dynamics. Our findings will stimulate future research using Au plasmonic structures to achieve more efficient and denser magnetic storage.