  All-optical re-switching has been investigated in the half-metallic Heusler
ferrimagnet Mn2Ru0.9Ga, where Mn atoms occupy two inequivalent sites in the
XA-type structure. The effect of a second 200 fs 800 nm pump pulse that follows
a first pulse, when both are above the threshold for switching, is studied as a
function of t12, the time between them. The aims are to identify the physical
mechanisms involved and to determine the minimum time needed for re-switching.
The time trajectory of the switching process on a plot of sublattice angular
momentum, S4a vs S4c, is in three stages; When t < 0.1 ps, the sublattice
moments are rapidly disordered, but not destroyed, while conserving net angular
momentum via optical spin-wave excitations. This leads to transient parallel
alignment of the residual Mn spins in the first quadrant. The net angular
momentum associated with the majority sublattice then flips in about 2 ps, and
a fully-reversed ferrimagnetic state is then established via the spin-lattice
interaction, which allows re-switching provided t12 > 10 ps.
