Ultra-fast magnetism: heating magnets, freezin



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

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.

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“Our approach is different,” explains Dr. Régis 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 ° C to 450 ° C for Gd – and much higher (1000 ° C) for the 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 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.

Distinguished diffusion 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.

To note:

This study completes a series of experiments carried out by the HZB team at BESSY II on Nickel, Iron-Nickel Alloys and now Gadolinium.

New publication on Gadolinium: 10.1063 / 5.0063404

About nickel: 10.1038 / s41598-019-45242-8

About Iron-Nickel-Alloy: 10.1038 / s41598-021-81177-9

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