Self-induced ultrafast demagnetization limits the amount of light diffracted from magnetic samples at soft x-ray energies.
Free electron X-ray lasers deliver intense ultrashort pulses of x-rays, which can be used to image nanometer-scale objects in a single shot. When the x-ray wavelength is tuned to an electronic resonance, magnetization patterns can be made visible. However, using increasingly intense pulses, the magnetization image fades away. The mechanism responsible for this loss in resonant magnetic scattering intensity has now been clarified.
A team of researchers from Max Born Institute Berlin (Germany), Helmholtz-Zentrum Berlin (Germany), Elettra Sincrotrone Trieste (Italy) and Sorbonne Université (France), has now precisely recorded the dependence of the resonant magnetic scattering intensity as a function of the x-ray intensity incident per unit area (the “fluence”) on a ferromagnetic domain sample. Via integration of a device to detect the intensity of every single shot hitting the actual sample area, they were able record the scattering intensity over three orders of magnitude in fluence with unprecedented precision, in spite of the intrinsic shot-to-shot variations of the x-ray beam hitting the tiny samples. The experiments with soft x-rays were carried out at the FERMI free-electron x-ray laser in Trieste, Italy.
In the results presented in the journal Physical Review Letters, the researchers show that while the loss in magnetic scattering in resonance with the Co 2p core levels has been attributed to stimulated emission in the past, for scattering in resonance with the shallower Co 3p core levels this process is not significant. The experimental data over the entire fluence range are well described by simply considering the actual demagnetization occurring within each magnetic domain, which the experimental team had previously characterized with laser-based experiments. Given the short lifetime of the Co 3p core, dominated by Auger decay, it is likely that the hot electrons generated by the Auger cascade, in concert with subsequent electron scattering events, lead to a reshuffling of spin up and spin down electrons transiently quenching the magnetization.
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Image: Schematic sketch of the scattering experiment with two competing processes. The soft x-ray beam (blue line) hits the magnetic sample where it scatters from the microscopic, labyrinth-like magnetization pattern. In this process, an x-ray photon is first absorbed by a Co 3p core level (1). The resulting excited state can then relax either spontaneously (2), emitting a photon in a new direction (purple arrow), or by means the interaction with a second photon via stimulated emission (3). In this last case, the photons are emitted in the direction of the incident beam (blue arrow towards right).