  The original observation of the Einstein-de Haas effect was a landmark
experiment in the early history of modern physics that illustrates the
relationship between magnetism and angular momentum. Today the effect is still
discussed in elementary physics courses to demonstrate that the angular
momentum associated with the aligned electron spins in a ferromagnet can be
converted to mechanical angular momentum by reversing the direction of
magnetisation using an external magnetic field. In recent times, a related
problem in magnetism concerns the time-scale over which this angular momentum
transfer can occur. It is known experimentally for several metallic
ferromagnets that intense photoexcitation leads to a drop in the magnetisation
on a time scale shorter than 100 fs, a phenomenon called ultrafast
demagnetisation. The microscopic mechanism for this process has been hotly
debated, with one key question still unanswered: where does the angular
momentum go on these sub-picosecond time scales? Here we show using femtosecond
time-resolved x-ray diffraction that a large fraction of the angular momentum
lost from the spin system on the laserinduced demagnetisation of ferromagnetic
iron is transferred to the lattice on sub-picosecond timescales, manifesting as
a transverse strain wave that propagates from the surface into the bulk. By
fitting a simple model of the x-ray data to simulations and optical data, we
roughly estimate that the angular momentum occurs on a time scale of 200 fs and
corresponds to 80% of the angular momentum lost from the spin system. Our
results show that interaction with the lattice plays an essential role in the
process of ultrafast demagnetisation in this system.
