  We have investigated the ultrafast spin dynamics in EuO thin films by
time-resolved Faraday rotation spectroscopy. The photoinduced magnetization is
found to be increased in a transient manner, accompanied with subsequent
demagnetization. The dynamical magnetization enhancement showed a maximum
slightly below the Curie temperature with prolonged tails toward both lower and
higher temperatures and dominates the demagnetization counterpart at 55 K. The
magnetization enhancement component decays in ~1 ns. The realization of the
transient collective ordering is attributable to the enhancement of the f-d
exchange interaction.
