  Ultrafast dynamics are generally investigated using stroboscopic pump-probe
measurements, which characterize the sample properties for a single, specific
time delay. These measurements are then repeated for a series of discrete time
delays to reconstruct the overall time trace of the process. As a consequence,
this approach is limited to the investigation of fully reversible phenomena. We
recently introduced an off-axis zone plate based X-ray streaking technique,
which overcomes this limitation by sampling the relaxation dynamics with a
single femtosecond X-ray pulse streaked over a picosecond long time window. In
this article we show that the X-ray absorption cross section can be employed as
the contrast mechanism in this novel technique. We show that changes of the
absorption cross section on the percent level can be resolved with this method.
To this end we measure the ultrafast magnetization dynamics in CoDy alloy
films. Investigating different chemical compositions and infrared pump
fluences, we demonstrate the routine applicability of this technique. Probing
in transmission the average magnetization dynamics of the entire film, our
experimental findings indicate that the demagnetization time is independent of
the specific infrared laser pump fluence. These results pave the way for the
investigation of irreversible phenomena in a wide variety of scientific areas.
