Indirect excitation of ultrafast demagnetization

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.

Ultrafast demagnetization of Pt magnetic moment in L10-FePt probed by magnetic circular dichroism at a hard x-ray free electron laser

“…During this initial nonequilibrium state the electrons scatter with one another and start to thermalize. The hot electrons will furthermore propagate through superdiffusive transport [50] from the laser-excited region deeper into the FePt film, similar to recent observations of superdiffusive spin transport in metallic heterostructures [12,17,51,52]. The hot spin-polarized electrons transported into the deeper region are initially the more mobile majority-spin electrons [53].…”

Ultrafast demagnetization by hot electrons: Diffusion or super-diffusion?

“…For temporal alignment, we use the point, where $\Delta N(t)/\Delta N_{\text{max}}=0.5$, as the shapes of the rising edges are independent of $d_{\text{Al}}$. The demagnetization times strongly depend on the sample thickness; however, we cannot see a significant delay in the onset of $\Delta M$ as described by Vodungbo et al 17 …”

Photon Beam Transport and Scientific Instruments at the European XFEL

“…Scientific objectives include, but are not limited to the understanding and control of complex materials [81][82][83], the investigation of ultrafast magnetization processes on the nanoscale [84,85], the real-time observation of chemical reactions at surfaces and in liquids [86,87], and the exploration of nonlinear X-ray spectroscopic techniques that are cornerstones at optical wavelengths [17,88].…”