  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 towards this vision is to gain
insight into AOS and multi-domain 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 inter-atomic 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
[Co/Gd]n and offers useful physical insights regarding AOS fundamentals as well
as the projected photonic integration.
