Ultra-fast magnetism: Heating magnets, freezing time

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The image shows the glowing filament which keeps the sample at constant temperatures during measurements. Credit: © HZB

Magnetic solids can be demagnetized quickly with a short laser pulse, and there are already so-called HAMR (Heat Assisted Magnetic Recording) memories on the market that work according to this principle. However, the microscopic mechanisms of ultrafast demagnetization remain unclear. Today, a team from HZB developed a new method at BESSY II to quantify one of these mechanisms and applied it to the rare earth element Gadolinium, whose magnetic properties are caused by electrons on the layers 4f and 5d. This study completes a series of experiments carried out by the team on nickel and iron-nickel alloys. Understanding these mechanisms is useful for developing ultra-fast data storage devices.

New materials are expected to make information processing more efficient, for example, thanks to ultra-fast spintronics devices that store data with less input of energy. 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, thus heating it, and then analyzing the evolution of the system in the first picoseconds that follow.

Snapshot of network status

“Our approach is different,” explains Dr. Régis Decker, lead author of the study. “We keep the sample at a certain temperature while acquiring spectra. And we do this for many temperatures, from -120 ° C to 450 ° C for Gd — and much higher (1000 ° C) for experiments. 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 some point after the ultrashort laser pulse and measure there. “

Gadolinium examined

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 phonon population increases, the more likely spin shifts are to occur due to the scattering of electrons with the phonons in the crystal lattice.

Distinguished dissemination rates

Using the inelastic x-ray scattering method (RIXS), physicists were able not only to determine the number of phonons at a given temperature, but also to 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, known to be one of the main triggers of ultrafast demagnetization, only applies to 5d electrons. Interestingly, it also shows the presence of a threshold of temperature, 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.


Ultrafast Magnetism: Electron-Phonon Interactions Examined at BESSY II


More information:
Régis Decker et al, Transfer of angular momentum spin-lattice of localized electrons and valence in the transient state of demagnetization of gadolinium, Letters of Applied Physics (2021). DOI: 10.1063 / 5.0063404

Régis Decker et al, Measurement of the atomic spin-flip diffusion rate by X-ray emission spectroscopy, Scientific reports (2019). DOI: 10.1038 / s41598-019-45242-8

Artur Born et al, Thresholding of the Elliott-Yafet spin-flip scattering in multi-sub-lattice magnets by the respective exchange energies, Scientific reports (2021). DOI: 10.1038 / s41598-021-81177-9

Provided by the Helmholtz Association of German Research Centers


Quote: Ultrafast magnetism: Heating magnets, freezing time (2021, October 18) retrieved October 18, 2021 from https://phys.org/news/2021-10-ultrafast-magnetism-magnets.html

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