We compared ultrafast spin dynamics in laser-excited Ni85Co15 alloy and pure Ni thin films exhibiting magnetic weak stripe domains using x-ray resonant magnetic scattering. By relying on extreme ultraviolet pulses obtained via high-harmonic generation, we simultaneously probed our samples using four energies around the Co and Ni M2,3 edges. Our results show a significant laser-induced reduction of the intensity of the magnetic scattering peaks on fs timescales and clearly highlight that this effect depends sensitively on the probing wavelength: different quenching amplitudes and characteristic timescales are found when changing the probing energy. More particularly, in our alloyed thin film, we observe a delay of about 17 fs in the scattered intensity dynamics between probing energies close to the Ni and Co edges, respectively. This delay is not observed in the pure Ni sample and is therefore most likely induced by the presence of Co in the thin film, reflecting the different ultrafast magnetic responses of the two 3d metals in these alloys.
Dubois et al. (Mon,) studied this question.