  Photoinduced magnetization dynamics is investigated in chemically ordered
$(\mathrm{LaMnO}_3)_{2n}/(\mathrm{SrMnO}_3)_n$ superlattices using the
time-resolved magneto-optic Kerr effect. A monotonic frequency-field dependence
is observed for the $n=1$ superlattice, indicating a single spin population
consistent with a homogeneous hole distribution. In contrast, for $n\geq2$
superlattices, a large precession frequency is observed at low fields
indicating the presence of an exchange torque in the dynamic regime. We propose
a model that ascribes the emergence of exchange torque to the coupling between
two spin populations -- viscous and fast spins.
