Verification of ultrafast spin transfer effects in iron-nickel alloys

Optical intersite spin transfer (OISTR), which is driven by an ultrafast optical excitation, was recently found in several materials, but there is some disagreement over how this phenomenon can be observed experimentally. Here, the authors investigate the mechanism of intersite spin transfer in a set of FeNi alloys and make a comparison with pure Ni, demonstrating and discussing the challenges of observing OISTR using magneto-optical measurements. The optical intersite spin transfer (OISTR) effect was recently verified in Fe_50Ni_50 using extreme ultraviolet magneto-optical Kerr measurements. 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_50Ni_50, Fe_19Ni_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.