  Recently it has been shown that not only GdFeCo alloys exhibit single-pulse
helicity-independent all-optical switching (HI-AOS), but that this effect is
also seen in Co/Gd bilayers. However, there have been no reports on the
explicit time dynamics of the switching process in these bilayers as of yet.
Furthermore, time-resolved measurements of switching of other materials are
typically done with a constant applied field to reset the magnetization between
consecutive pulses and thus ensure repeatable behavior. In this paper we
experimentally resolve the explicit dynamics of the switching process in Co/Gd,
and the influence of applied magnetic fields on the switching process. We
observe that after a switch within several picoseconds, the magnetization
switches back at a timescale of hundreds of picoseconds. This backswitch
includes a strong dependence on the magnetic field strength even at sub-tesla
fields, significantly smaller than the exchange fields that govern the
switching dynamics. This surprising behaviour is explained by a combination of
longitudinal switching (on a picosecond timescale), precessional switching (on
a nanosecond timescale) and domain-wall motion (on a timescale of 10 ns and
beyond). We discuss these different switching regimes and their relative
importance using simple model calculations.
