  Based on state-of-the-art density-functional-theory methods we calculate the
stacking-fault energy of the paramagnetic random Fe-22.5at.%Mn alloy between
300-800 K. We estimate magnetic thermal excitations by considering longitudinal
spin-fluctuations. Our results demonstrate that the interplay between the
magnetic excitations and the thermal lattice expansion is the main factor
determining the anti-Invar effect, the hcp-fcc transformation temperature, and
the stacking-fault energy, which is in excellent agreement with measurements.
