  The mechanism underlying femtosecond laser pulse induced ultrafast
magnetization dynamics remains elusive despite two decades of intense research
on this phenomenon. Most experiments focused so far on characterizing
magnetization and charge carrier dynamics, while first direct measurements of
structural dynamics during ultrafast demagnetization were reported only very
recently. We here present our investigation of the infrared laser pulse induced
ultrafast demagnetization process in a thin Ni film, which characterizes
simultaneously magnetization and structural dynamics. This is achieved by
employing femtosecond time resolved X-ray resonant magnetic reflectivity
(tr-XRMR) as probe technique. The experimental results reveal unambiguously
that the sub-picosecond magnetization quenching is accompanied by strong
changes in non-magnetic X-ray reflectivity. These changes vary with reflection
angle and changes up to 30$\%$ have been observed. Modeling the X-ray
reflectivity of the investigated thin film, we can reproduce these changes by a
variation of the apparent Ni layer thickness of up to 1$\%$. Extending these
simulations to larger incidence angles we show that tr-XRMR can be employed to
discriminate experimentally between currently discussed models describing the
ultrafast demagnetization phenomenon.
