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Toward ultrafast magnetic depth profiling using time-resolved x-ray resonant magnetic reflectivity.

Valentin ChardonnetMarcel HennesRomain JarrierRenaud DelaunayNicolas JaouenMarion KuhlmannNagitha EkanayakeCyril LéveilléClemens von Korff SchmisingDaniel SchickKelvin YaoXuan LiuGheorghe Sorin ChiuzbaianJan LüningBoris VondungboEmmanuelle Jal
Published in: Structural dynamics (Melville, N.Y.) (2021)
During the last two decades, a variety of models have been developed to explain the ultrafast quenching of magnetization following femtosecond optical excitation. These models can be classified into two broad categories, relying either on a local or a non-local transfer of angular momentum. The acquisition of the magnetic depth profiles with femtosecond resolution, using time-resolved x-ray resonant magnetic reflectivity, can distinguish local and non-local effects. Here, we demonstrate the feasibility of this technique in a pump-probe geometry using a custom-built reflectometer at the FLASH2 free-electron laser (FEL). Although FLASH2 is limited to the production of photons with a fundamental wavelength of 4 nm ( ≃ 310   eV ), we were able to probe close to the Fe L 3 edge ( 706.8   eV ) of a magnetic thin film employing the third harmonic of the FEL. Our approach allows us to extract structural and magnetic asymmetry signals revealing two dynamics on different time scales which underpin a non-homogeneous loss of magnetization and a significant dilation of 2 Å of the layer thickness followed by oscillations. Future analysis of the data will pave the way to a full quantitative description of the transient magnetic depth profile combining femtosecond with nanometer resolution, which will provide further insight into the microscopic mechanisms underlying ultrafast demagnetization.
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