  The optical intersite spin transfer (OISTR) effect was recently verified in
Fe$_{50}$Ni$_{50}$ using magneto-optical Kerr measurements in the extreme
ultraviolet range. However, one of the main experimental signatures analyzed in
this work, namely a magnetic moment increase at a specific energy in Ni, was
subsequently found also in pure Ni, where no transfer from one element to
another is possible. Hence, it is a much-discussed issue whether OISTR in FeNi
alloys is real and whether it can be verified experimentally or not. Here, we
present a comparative study of spin transfer in Fe$_{50}$Ni$_{50}$,
Fe$_{19}$Ni$_{81}$ and pure Ni. We conclusively show that an increase in the
magneto-optical signal is indeed insufficient to verify OISTR. However, we also
show how an extended data analysis overcomes this problem and allows to
unambiguously identify spin transfer effects. Concomitantly, our work solves
the long-standing riddle about the origin of delayed demagnetization behavior
of Ni in FeNi alloys.
