The response of ferromagnets to laser excitation is governed by the interplay of electronic, magnetic and lattice degrees of freedom. In the case of 3d ferromagnets, strong coupling between electrons and spins leads to ultrafast demagnetization on femtosecond time scales. The lattice plays an important role in the magnetization dynamics, since it drains energy from the electrons on similar timescales and absorbs angular momentum from the spin system. Here, we study the lattice response of the 3d ferromagnets nickel, iron and cobalt directly using femtosecond electron diffraction (FED). To learn more about the energy flow between electrons, spins and the lattice, we compare the experimental results to spin-resolved DFT calculations combined with energy flow models. We incrementally increase the complexity of these models in 3 steps: While the commonly adopted two-temperature model (TTM) cannot describe our experimental results, we find excellent agreement using a modified TTM that assumesstrong coupling between electrons and spins. In the next step, we discuss how atomistic spin dynamics (ASD) simulations can be employed for a more accurate description of the spin system in out-of-equilibrium conditions. The ASD simulation results for nickel maintain the excellent agreement to the lattice dynamics while yielding a much more consistent description of the dynamics of the system. Our results suggest that the energy cost of ultrafast demagnetization has a strong effect on the lattice dynamics.
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