  Although all-optical magnetization switching with fs laser pulses has
garnered much technological interest, the ultimate data rates achievable have
scarcely been investigated. Recently it has been shown that after a switching
event in a GdCo alloy, a second laser pulse arriving 7 ps later can
consistently switch the magnetization. However, it is as of yet unknown whether
the same holds in layered ferrimagnetic systems, which hold much promise for
applications. In this work we investigate the minimum time delay required
between two subsequent switching events in synthetic ferrimagnetic Co/Gd
bilayers using two fs laser pulses. We experimentally demonstrate that the
minimum time delay needed for consistent switching can be as low as 10 ps.
Moreover, we demonstrate the importance of engineering heat diffusion away from
the magnetic material, as well as control over the laser pulse power. This
behavior is reproduced using modelling, where we find that the second switch
can occur even when the magnetization is not fully recovered. We further
confirm that heat diffusion is a critical factor in reducing the time delay for
the second switch, while also confirming a critical dependence on laser power.
