  Light-matter interaction at the nanoscale in magnetic alloys and
heterostructures is a topic of intense research in view of potential
applications in high-density magnetic recording. While the element-specific
dynamics of electron spins is directly accessible to resonant x-ray pulses with
femtosecond time structure, the possible element-specific atomic motion remains
largely unexplored. We use ultrafast electron diffraction to probe the temporal
evolution of lattice Bragg peaks of FePt nanoparticles embedded in a carbon
matrix following excitation by an optical femtosecond laser pulse. The
diffraction interference between Fe and Pt sublattices enables us to
demonstrate that the Fe mean-square vibration amplitudes are significantly
larger that those of Pt as expected from their different atomic mass. Both are
found to increase as energy is transferred from the laser-excited electrons to
the lattice. Contrary to this intuitive behavior, we observe a laser-induced
lattice expansion that is larger for Pt than for Fe atoms during the first
picosecond after laser excitation. This effect points to the strain-wave driven
lattice expansion with the longitudinal acoustic Pt motion dominating that of
Fe.
