Controlled coexcitation of direct and indirect ultrafast demagnetization in Co/Pd multilayers with large perpendicular magnetic anisotropy

Ever since its discovery in 1996, ultrafast demagnetization has ignited immense research interest due to its scientific rigor and technological potential. A flurry of recent theoretical and experimental investigations has proposed direct and indirect excitation processes in separate systems. However, it still lacks a unified mechanism and remains highly debatable. Here, we demonstrate that instead of either direct or indirect interaction, simultaneous and controlled excitation of both direct and indirect mechanisms of demagnetization is possible in multilayers composed of repeated Co/Pd bilayers. Moreover, we are able to modulate demagnetization time (from ∼350 to ∼750 fs) by fluence and thickness-dependent indirect excitation due to heat current flowing vertically downward from top layers, which is combined with an altogether different scenario of direct irradiation. Finally, by regulating the pump wavelength, we can effectively control the contribution of indirect process, which gives a confirmation to our understanding of the ultrafast demagnetization process.