Indirect excitation of ultrafast demagnetization

TLDR

Whether ultrafast magnetization excitation requires direct photon interaction with the magnetic layer is unclear. We demonstrate that direct photon interaction is not necessary. We studied magnetization dynamics of a cobalt/palladium multilayer capped with an IR‑opaque aluminum layer. An intense femtosecond IR pulse induces classical ultrafast demagnetization in the capped film despite negligible IR photon penetration, with delayed onset and slower initial rate compared to an uncapped film, consistent with energy transport via hot electrons from the aluminum layer and confirming recent theoretical predictions.

Abstract

Abstract Does 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.