  Directly modifying spin exchange energies in a magnetic material with light
can enable ultrafast control of its magnetic states. Current approaches rely on
tuning charge hopping amplitudes that mediate exchange by optically exciting
either virtual or real charge-transfer transitions (CT) between magnetic sites.
Here we show that when exchange is mediated by a non-magnetic ligand, it can be
substantially enhanced by optically exciting a real CT transition from the
ligand to magnetic site, introducing lower order virtual hopping contributions.
We demonstrate sub-picosecond enhancement in a superexchange dominated
ferromagnet CrSiTe3 through this mechanism using phase-resolved coherent phonon
spectroscopy. This technique can also be applied in the paramagnetic phase to
disentangle light induced exchange modification from other ultrafast effects
that alter the magnetization. This protocol can potentially be broadly applied
to engineer thermally inaccessible spin Hamiltonians in superexchange dominated
magnets.
