Picosecond all-optical switching of Co/Gd–based synthetic ferrimagnets

Single-pulse all-optical switching of magnetization (AOS) in Co/Gd--based synthetic ferrimagnets carries promises for hybrid spintronic-photonic integration. A crucial next step progressing toward this vision is to gain insight into AOS and multidomain state (MDS) behavior using longer pulses, which is compatible with state-of-the-art integrated photonics. In this work, we present our studies on the AOS and MDS of [Co/Gd] ${}_{n} (n=1,2)$ using ps optical pulses across a large composition range. We theoretically and experimentally show that a large Gd layer thickness can enhance the AOS energy efficiency and maximum pulse duration. We have identified two augmenting roles of Gd in extending the maximum pulse duration. On the interatomic level, we found that more Gd offers a prolonged angular momentum supply to Co. On the micromagnetic level, a higher Gd content brings the system to be closer to magnetic compensation in the equilibrized hot state, thereby reducing the driving force for thermally assisted nucleation of domain walls, combating the formation of a MDS. Our study presents a composition overview of AOS in $[\mathrm{Co}/{\mathrm{Gd]}}_{n}$ and offers useful physical insights regarding AOS fundamentals as well as the projected photonic integration.