Photocontrol of magnetic structure in a metallic ferromagnet

We study theoretically photoinduced phenomena in a metallic ferromagnet described by the double-exchange (DE) model, where conduction electrons mediate the ferromagnetic (FM) DE interaction between localized magnetic moments in equilibrium. We found that the DE interaction acts as the antiferromagnetic (AFM) interaction in highly photoexcited states, which is in sharp contrast to the conventional DE mechanism. Numerical computations of real-time dynamics described by the Schrödinger equation combined with the Landau-Lifshitz-Gilbert equation reveal that the FM metallic initial state is transformed into the AFM steady state through a transient state with emergent topological magnetic defects. We also calculated magnetic excitation spectra in the photoirradiated FM metallic state by using the Floquet Green function method and found that magnon is softened by photoirradiation, which indicates that the AFM instability is induced by the Stoner excitation in the Floquet state.