Indirect excitation of ultrafast demagnetization

International audienceDoes the excitation of ultrafast magnetization require direct interaction between the photons of the optical pump pulse and the magnetic layer? Here, we demonstrate unambiguously that this is not the case. For this we have studied the magnetization dynamics of a ferromagnetic cobalt/palladium multilayer capped by an IR-opaque aluminum layer. Upon excitation with an intense femtosecond-short IR laser pulse, the film exhibits the classical ultrafast demagnetization phenomenon although only a negligible number of IR photons penetrate the aluminum layer. In comparison with an uncapped cobalt/ palladium reference film, the initial demagnetization of the capped film occurs with a delayed onset and at a slower rate. Both observations are qualitatively in line with energy transport from the aluminum layer into the underlying magnetic film by the excited, hot electrons of the aluminum film. Our data thus confirm recent theoretical predictions. Ultrafast demagnetization is a very intriguing phenomenon. The existence and origin of this rapid loss of a thin film's magnetization on the femtosecond timescale has been controversially debated ever since its discovery in 1996 1. Recently, it has been proposed that the origin of this magnetization loss could be due to spin-dependent motion of the optically excited hot valence electrons causing a spatial redistribution of the magnetization, either to an adjacent metallic layer 2,3 or within the magnetic layer itself 4. Strong experimental evidence for this so-called superdiffusive spin transport has been found lately 5-8. However, the experiment of Eschenlohr and co-workers 8 , similar to our study reported here, has been challenged recently. Khorsand and co-workers 9 showed that the Au cap layer employed in that experiment does not yield the postulated, and for the interpretation of the data crucial, attenuation of the incident IR pump pulse. Implementing a truly IR-opaque capping layer, we evade the limitation affecting the interpretation of the results of Eschenlohr and co-workers 8. Proper characterization of the number of photons reaching the magnetic layer demonstrates that the transmitted intensity is by orders of magnitude too small to excite the ultrafast demagnet-ization process