Ballistic electron-driven magnetization dynamics induced by femtosecond laser excitation

• Applicants: Professor Dr. Uwe Bovensiepen; Dr. Alexey Melnikov
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2009 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 137051180

Electronic excitations in a ferromagnet can trigger ultrafast spin dynamics with potential applications in a speed increase in magnetic recording. The project investigates ultrafast magnetization dynamics, which is driven in metallic layers by ballistic hot electrons. In a ferromagnet these electrons induce a change in the absolute value of the magnetization M through spin-dependent scattering. If the electrons are spin-polarized, scattering at the interface of a noble metal and a ferromagnet results in spin-transfer torque and hence modifies the direction of M. To reveal the underlying mechanisms, we plan to study model systems, which at first realized by layers of Au with its large ballistic mean free path and Fe as an itinerant ferromagnet. Later we will compare different systems. The project aims at (i) understanding of ultrafast demagnetization and (ii) femtosecond all-optical generation of spin transfer torque effects. The launched dynamics will be probed by magneto-optics in a time-resolved experiment. We will analyze the elementary processes like electron-electron, electron-magnon, and electron-phonon scattering in the excited state. Our approach avoids a net electrical current through the layer stack and the resulting magnetic Oersted fields, because the propagation of ballistic hot electrons is screened by conduction electrons remaining in equilibrium. The structures investigated in the first funding period consist of a 10 - 100 nm thick stack of Fe/Au and Fe/Au/Fe which is grown epitaxially on MgO(001). Optical excitation of the outer Fe or Au layer and detection at the Fe layer adjacent to the transparent substrate by (non-) linear optics in a back-pump front-probe scheme, facilitates the proposed study. Our approach leads the way to non-equilibrium electron driven spin dynamics, which has the potential to overcome limitations of phonon-mediated angular momentum transfer acting in equilibrium in currently employed schemes and applications.

• DFG Programme: Research Grants