  Optically driven intersite and interlayer spin transfer are individually
known as the fastest processes for manipulating the spin order of magnetic
materials on the sub 100 fs time scale. However, their competing influence on
the ultrafast magnetization dynamics remains unexplored. In our work, we show
that optically induced intersite spin transfer (also known as OISTR) dominates
the ultrafast magnetization dynamics of ferromagnetic alloys such as Permalloy
(Ni80Fe20) only in the absence of interlayer spin transfer into a substrate.
Once interlayer spin transfer is possible, the influence of OISTR is
significantly reduced and interlayer spin transfer dominates the ultrafast
magnetization dynamics. This provides a new approach to control the
magnetization dynamics of alloys on extremely short time scales by fine-tuning
the interlayer spin transfer.
