  We present an experimental and theoretical investigation of all-optical
switching by single femtosecond laser pulses. Our experimental results
demonstrate that, unlike rare earth-transition metal ferrimagnetic alloys,
Pt/Co/[Ni/Co]$_N$/Gd can be switched in the absence of a magnetization
compensation temperature, indicative for strikingly different switching
conditions. In order to understand the underlying mechanism, we model the
laser-induced magnetization dynamics in Co/Gd bilayers and GdCo alloys on an
equal footing, using an extension of the microscopic three-temperature model to
multiple magnetic sublattices and including exchange scattering. In agreement
with our experimental observations, the model shows that Co/Gd bilayers can be
switched for an arbitrary thickness of the Co layer, i.e, even far away from
compensating the total Co and Gd magnetic moment. We identify the switching
mechanism in Co/Gd bilayers as a front of reversed Co magnetization that
nucleates at the Co/Gd interface and propagates through the Co layer driven by
exchange scattering.
