  It is found that subtle changes in the external magnetic field and
temperature result in dramatic changes in the ultrafast response of spins to a
femtosecond laser excitation in a ferrimagnetic Gd/FeCo multilayer. A total of
six distinct types of spin dynamics were observed and explained by considering
the spin-flop transition to the noncollinear phase and the concept of a
tricritical point in the $H$-$T$ phase diagram. A particularly interesting type
of dynamics is the exchange-driven reversal. These exchange-driven dynamics
provide new insights into the tricritical point, which is shown to separate two
thermodynamically distinct noncollinear phases with the transition-metal
magnetization pointing on adjacent sides of the anisotropy plane.
