Engineering antiferromagnetic exchange coupling for single-pulse all-optical switching

Single-pulse all-optical switching (SP-AOS) offers an appealing route to ultrafast magnetic information writing, yet the magnetic parameter that fundamentally determines whether toggle switching occurs has remained unclear. Here we incorporate a Ru spacer into Co/Gd heterostructures to tune the interlayer exchange coupling strength J_(Gd-Co) through the combined contributions of direct Co–Gd coupling and oscillatory RKKY interactions. The SP-AOS switching signal shows an oscillation nature with the period of Ru thickness less than 1 nm in coincide with the RKKY-defined oscillatory coercive field and saturation magnetization. SP-AOS emerges above a critical antiferromagnetic (AFM) exchange coupling strength of J_(Gd-Co)≈0.69mJ/m2 and reaches 100% at J_(Gd-Co)≈1.14 mJ/m2, where the stronger AFM coupling also lowers the threshold fluence and broadens the operational energy window. Atomistic spin-dynamics simulations further show that the strong J_(Gd-Co) accelerates the demagnetization dynamics of both Co and Gd sublattices and promotes the formation of the transient ferromagnetic state to enable SP-AOS, whereas weak J_(Gd-Co) suppresses this transient state and forces the system into a slower, dissipative spin–orbit–driven pathway. These results demonstrate that the key determinant of SP-AOS is not merely associated with the demagnetization times of the two sublattices, but is directly linked to the strength of antiferromagnetic exchange coupling. This study establishes the antiferromagnetic exchange coupling strength as a practical and designable criterion for nanoscale materials engineering of ultrafast photomagnetic switching.