The energy and momentum selectivity of time- and angle-resolved photoemission
spectroscopy is exploited to address the ultrafast dynamics of the
antiferromagnetic spin density wave (SDW) transition photoexcited in epitaxial
thin films of chromium. We are able to quantitatively extract the evolution of
the SDW order parameter Δ through the ultrafast phase transition and show that
Δ is governed by the transient temperature of the thermalized electron gas, in
a mean field description. The complete destruction of SDW order on a sub-100
fs time scale is observed, much faster than for conventional charge density
wave materials. Our results reveal that equilibrium concepts for phase
transitions such as the order parameter may be utilized even in the strongly
nonadiabatic regime of ultrafast photoexcitation