  We investigate the ultrafast magnetization dynamics of FePt in the L10 phase
after an optical heating pulse, as used in heat assisted magnetic recording. We
compare continuous and nanogranular thin films and emphasize the impact of the
finite size on the remagnetization dynamics. The remagnetization speeds up
significantly with increasing external magnetic field only for the continuous
film, where domain wall motion governs the dynamics. The ultrafast
remagnetization dynamics in the continuous film are only dominated by heat
transport in the regime of high magnetic fields, whereas the timescale required
for cooling is prevalent in the granular film for all magnetic field strengths.
These findings highlight the necessary conditions for studying the intrinsic
heat transport properties in magnetic materials.
