  We review and discuss the process of single-shot helicity-independent
all-optical switching of magnetization by which a single suitably-ultrafast
excitation, under the right conditions, toggles magnetization from one stable
state to another. For almost a decade, this phenomenon was only consistently
observed in specific rare-earth-transition-metal ferrimagnetic alloys of
GdFeCo, but breakthrough experiments in recent years have revealed that the
same behavior can be achieved in a wide range of multi-sublattice magnets
including TbCo alloys doped with minute amounts of Gd, Gd/Co and Tb/Co
synthetic ferrimagnets, and the rare-earth-free Heusler alloy Mn$_2$Ru$_x$Ga.
Aiming to resolve the conditions that allow switching, a series of experiments
have shown that the process in the ferrimagnetic alloys GdFeCo and
Mn$_2$Ru$_x$Ga is highly sensitive to the pulse duration, starting temperature
and the alloy composition. We argue here that the switching displayed by these
two very different ferrimagnetic alloys can be generally understood within a
single phenomenological framework describing the flow of angular momentum
between the constituent sublattices and from the sublattices to the
environment. The conditions that facilitate switching stem from the properties
of these channels of angular momentum flow in combination with the size of the
angular momentum reservoirs. We conclude with providing an outlook in this
vibrant research field, with emphasis on the outstanding open questions
pertaining to the underlying physics along with noting the advances in
exploiting this switching process in technological applications.
