Viscous spin exchange torque on precessional magnetization in $({\mathrm{L}\mathrm{a}\mathrm{M}\mathrm{n}\mathrm{O}}_3{)}_{2n}\mathrm{/}({\mathrm{S}\mathrm{r}\mathrm{M}\mathrm{n}\mathrm{O}}_3{)}_n$ superlattices

Photoinduced magnetization dynamics is investigated in chemically ordered $({\mathrm{L}\mathrm{a}\mathrm{M}\mathrm{n}\mathrm{O}}_3{)}_{2n}\mathrm{/}({\mathrm{S}\mathrm{r}\mathrm{M}\mathrm{n}\mathrm{O}}_3{)}_n$ 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 $n\ge 2$ 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.