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.
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