  We investigate both the ferromagnetic (FM) and antiferromagnetic (AFM)
ultrafast dynamics of a strongly correlated oxide system, GdBaCo$_2$O$_{5.5}$
thin film, by time-resolved x-ray magnetic circular dichroism in reflectivity
(XMCDR) and resonant magnetic x-ray diffraction (RMXD). A photo-induced AFM-FM
transition characterized by an increase of the transient XMCDR beyond the
unpumped value and a decay of RMXD was observed. The photon-energy dependence
of the transient XMCDR and reflectivity can likely be interpreted as a
concomitant photo-induced spinstate transition.
