We demonstrate ultrafast magnetization dynamics in a 5d transition metal
using circularly-polarized x-ray free electron laser in the hard x-ray region.
A decay time of light-induced demagnetization of L1${}_0$-FePt was determined
to be $\tau_\textrm{Pt} = 0.6\ \textrm{ps}$ using time-resolved x-ray magnetic
circular dichroism at the Pt L${}_3$ edge, whereas magneto-optical Kerr
measurements indicated the decay time for total magnetization as
$\tau_\textrm{total} = 0.1\ \textrm{ps}$. A transient magnetic state with the
photo-modulated magnetic coupling between the 3d and 5d elements is firstly
demonstrated.