  Synthetic ferrimagnets based on Co and Gd bear promise for directly bridging
the gap between volatile information in the photonic domain and non-volatile
information in the magnetic domain, without the need for any intermediary
electronic conversion. Specifically, these systems exhibit strong spin-orbit
torque effects, fast domain wall motion and single-pulse all-optical switching
of the magnetization. An important open challenge to bring these materials to
the brink of applications is to achieve long-term stability of their magnetic
properties. In this work, we address the time-evolution of the magnetic moment
and compensation temperature of magnetron sputter grown Pt/Co/Gd trilayers with
various capping layers. Over the course of three months, the net magnetic
moment and compensation temperature change significantly, which we attribute to
quenching of the Gd magnetization. We identify that intermixing of the capping
layer and Gd is primarily responsible for this effect, which can be alleviated
by choosing nitrides for capping as long as reduction of nitride to oxide is
properly addressed. In short, this work provides an overview of the relevant
aging effects that should be taken into account when designing synthetic
ferrimagnets based on Co and Gd for spintronic applications.
