New materials are expected to make information processing more efficient, for example, thanks to ultra-fast spintronics devices that store data with less energy input. But to date, the microscopic mechanisms of ultrafast demagnetization are not fully understood. Typically, the demagnetization process is studied by sending an ultrashort laser pulse to the sample, thereby heating it, and then analyzing the evolution of the system in the first picoseconds that follow.
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âOur approach is different,â explains Dr. RÃ©gis Decker, lead author of the study. “We keep the sample at a certain temperature while acquiring the spectra. And we do this for many temperatures, from -120 Â° C to 450 Â° C for Gd – and much higher (1000 Â° C) for the experiments. previous ones with Ni and FeNi This allows us to quantify the effect of the phonons for each temperature on ultrafast demagnetization, where the temperatures of the lattice, electron and spin subsystems change over time. In other words, by placing the system at a certain temperature, we capture the state of the network at a given moment after the ultrashort laser pulse and we measure there. “
The element gadolinium has 4f and 5d electronic orbitals, both of which contribute to its ferromagnetic properties. The higher the temperature, the more the crystalline sample vibrates – and as physicists say: the more the population of phonons increases, the more spin shifts are likely to occur due to the scattering of electrons with the phonons in the lattice. crystalline.
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Using the inelastic x-ray scattering method (RIXS), physicists were not only able to determine the number of phonons at a given temperature, but also distinguish the interactions between phonons and 4f and 5d electrons. Using the strict selection rules of X-ray spectroscopic symmetry, the evaluation was able to distinguish between 4f and 5d electron scattering rates.
5d electrons interact with phonons
The data shows that there is virtually no scattering between 4f electrons and localized phonons, but most of the scattering process takes place between 5d electrons and phonons, so a spin shift does not occur. occurs only at this location. “Our approach shows that electron-phonon scattering, which is known to be one of the main triggers of ultrafast demagnetization, only applies to 5d electrons. Interestingly, it also shows the presence of a temperature threshold, which depends on the material, below which this mechanism does not occur. This indicates the existence of another microscopic mechanism at lower temperature, as the theory predicts, “explains Decker.
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