  Exciting a ferromagnetic sample with an ultrashort laser pulse leads to a
quenching of the magne- tization on a subpicosecond timescale. On the basis of
the equilibration of intensive thermodynamic variables we establish a powerful
model to describe the demagnetization dynamics. We demonstrate that the
magnetization dynamics is mainly driven by the equilibration of chemical
potentials. The minimum of magnetization is revealed as a transient electronic
equilibrium state. Our method iden- tifies the slowing down of ultrafast
magnetization dynamics by a critical region within a magnetic phase diagram.
