Ultrafast spin transport as key to femtosecond demagnetization

Abstract

Irradiating a ferromagnet with a femtosecond laser pulse is known to induce an ultrafast demagnetization within a few hundred femtoseconds. Here we demonstrate that direct laser irradiation is in fact not essential for ultrafast demagnetization, and that electron cascades caused by hot electron currents accomplish it very efficiently. We optically excite a Au/Ni layered structure in which the 30 nm Au capping layer absorbs the incident laser pump pulse and subsequently use the X-ray magnetic circular dichroism technique to probe the femtosecond demagnetization of the adjacent 15 nm Ni layer. A demagnetization effect corresponding to the scenario in which the laser directly excites the Ni film is observed, but with a slight temporal delay. We explain this unexpected observation by means of the demagnetizing effect of a superdiffusive current of non-equilibrium, non-spin-polarized electrons generated in the Au layer.

Spin current generated by thermally driven ultrafast demagnetization

“…Battiato et al 20 proposed that spin-dependent relaxation and fast transport of hot electrons play a crucial role and suggested that ultrafast demagnetization produces spinpolarized hot electrons, which move to an adjacent metallic layer by a so-called 'superdiffusive current'. The findings of several subsequent experiments have been interpreted as supporting this hypothesis [21][22][23][24][25] . However, the superdiffusive model is based on non-thermal electronic motion 20 and the transport of thermal energy is not considered in these experiments [21][22][23][24][25] .…”

“…The findings of several subsequent experiments have been interpreted as supporting this hypothesis [21][22][23][24][25] . However, the superdiffusive model is based on non-thermal electronic motion 20 and the transport of thermal energy is not considered in these experiments [21][22][23][24][25] . Recently, we have shown that the exchange of thermal energy between metal layers is crucial in the interpretation of demagnetization of a ferromagnet (FM) in a metallic multilayer structure 26 .…”

“…When the density of far-from equilibrium hot electrons is negligible in the [Co/Pt] layer, the superdiffusive model 20 does not predict the demagnetization or a demagnetization-induced spin current. Eschenlohr et al 24 reported that Ni has a comparable demagnetization whether it is excited directly by laser pulse or indirectly through a Au layer. This result can be interpreted, as indicating that the superdiffusive model is applicable even when hot electrons transport through the Au layer.…”

Speed and efficiency of femtosecond spin current injection into a nonmagnetic material

“…The microscopic mechanisms responsible for this photoinduced ultrafast loss of magnetic order can be classified as spin-flip scattering phenomena [2,7,8] and spin transport phenomena [9][10][11][12]. Today we know that both processes clearly contribute to the ultrafast demagnetization and that the contributions of both mechanisms depend on the details of the material system and multilayer structure [8,[12][13][14][15]. In particular, optically induced spin transport via superdiffusive spin currents [4] yields a great potential for further applications.…”

Exploring laser-induced interlayer spin transfer by an all-optical method

“…Recently, it was realized that apart from local mechanisms, also the laser-induced transfer of spin polarized carriers over several to tens of nanometers can be a source of ultrafast magnetization dynamics [3]. This notion triggered fascinating new studies [4][5][6][7][8][9][10][11][12][13][14], but also led to intense scientific debate. It was experimentally demonstrated that laser-induced demagnetization can be enhanced by transfer of spins aligned antiparallel to the magnetization direction [4].…”