  We use femtosecond time-resolved hard x-ray scattering to detect coherent
acoustic phonons excited during ultrafast laser demagnetization of bcc Fe
films. We determine the lattice strain propagating through the film through
analysis of the oscillations in the x-ray scattering signal as a function of
momentum transfer. The width of the strain wavefront is ~100 fs, similar to
demagnetization timescales. First-principles calculations show that the
high-frequency Fourier components of the strain, which give rise to the sharp
wavefront, could in part originate from non-thermal dynamics of the lattice not
considered in the two-temperature model.
