  The ultrafast dynamics of magnetic order in a ferromagnet are governed by the
interplay between electronic, magnetic and lattice degrees of freedom. In order
to obtain a microscopic understanding of ultrafast demagnetization, information
on the response of all three subsystems is required. A consistent description
of demagnetization and microscopic energy flow, however, is still missing.
Here, we combine a femtosecond electron diffraction study of the ultrafast
lattice response of nickel to laser excitation with ab initio calculations of
the electron-phonon interaction and energy-conserving atomistic spin dynamics
simulations. Our model is in agreement with the observed lattice dynamics and
previously reported electron and magnetization dynamics. Our approach reveals
that the spin system is the dominating heat sink in the initial few hundreds of
femtoseconds and implies a transient non-thermal state of the spins. Our
results provide a clear picture of the microscopic energy flow between
electronic, magnetic and lattice degrees of freedom on ultrafast timescales and
constitute a foundation for theoretical descriptions of demagnetization that
are consistent with the dynamics of all three subsystems.
