  Ultrafast magnetization dynamics are governed by energy flow between
electronic, magnetic, and lattice degrees of freedom. A quantitative
understanding of these dynamics must be based on a model that agrees with
experimental results for all three subsystems. However, ultrafast dynamics of
the lattice remain largely unexplored experimentally. Here, we combine
femtosecond electron diffraction experiments of the lattice dynamics with
energy-conserving atomistic spin dynamics (ASD) simulations and ab-initio
calculations to study the intrinsic energy flow in the 3d ferromagnets cobalt
(Co) and iron (Fe). The simulations yield a good description of experimental
data, in particular an excellent description of our experimental results for
the lattice dynamics. We find that the lattice dynamics are influenced
significantly by the magnetization dynamics due to the energy cost of
demagnetization. Our results highlight the role of the spin system as the
dominant heat sink in the first hundreds of femtoseconds. Together with
previous findings for nickel [Zahn et al., Phys. Rev. Research 3, 023032
(2021)], our work demonstrates that energy-conserving ASD simulations provide a
general and consistent description of the laser-induced dynamics in all three
elemental 3d ferromagnets.
