Polarized phonons carry angular momentum in ultrafast demagnetization

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Nature. Springer Nature. 2022, 602(7895), pp. 73-77. ISSN 0028-0836. eISSN 1476-4687. Available under: doi: 10.1038/s41586-021-04306-4
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Magnetic phenomena are ubiquitous in nature and indispensable for modern science and technology, but it is notoriously difficult to change the magnetic order of a material in a rapid way. However, if a thin nickel film is subjected to ultrashort laser pulses, it loses its magnetic order almost completely within femtosecond timescales1. This phenomenon is widespread2-7 and offers opportunities for rapid information processing8-11 or ultrafast spintronics at frequencies approaching those of light8,9,12. Consequently, the physics of ultrafast demagnetization is central to modern materials research1-7,13-28, but a crucial question has remained elusive: if a material loses its magnetization within mere femtoseconds, where is the missing angular momentum in such a short time? Here we use ultrafast electron diffraction to reveal in nickel an almost instantaneous, long-lasting, non-equilibrium population of anisotropic high-frequency phonons that appear within 150-750 fs. The anisotropy plane is perpendicular to the direction of the initial magnetization and the atomic oscillation amplitude is 2 pm. We explain these observations by means of circularly polarized phonons that quickly absorb the angular momentum of the spin system before macroscopic sample rotation. The time that is needed for demagnetization is related to the time it takes to accelerate the atoms. These results provide an atomistic picture of the Einstein-de Haas effect and signify the general importance of polarized phonons for non-equilibrium dynamics and phase transitions.

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ISO 690TAUCHERT, Sonja R., Mikhail VOLKOV, Dominik EHBERGER, Daniel L. KAZENWADEL, Martin EVERS, Hannah LANGE, Andreas DONGES, Alexander BOOK, Ulrich NOWAK, Peter BAUM, 2022. Polarized phonons carry angular momentum in ultrafast demagnetization. In: Nature. Springer Nature. 2022, 602(7895), pp. 73-77. ISSN 0028-0836. eISSN 1476-4687. Available under: doi: 10.1038/s41586-021-04306-4
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@article{Tauchert2022Polar-56576,
  year={2022},
  doi={10.1038/s41586-021-04306-4},
  title={Polarized phonons carry angular momentum in ultrafast demagnetization},
  number={7895},
  volume={602},
  issn={0028-0836},
  journal={Nature},
  pages={73--77},
  author={Tauchert, Sonja R. and Volkov, Mikhail and Ehberger, Dominik and Kazenwadel, Daniel L. and Evers, Martin and Lange, Hannah and Donges, Andreas and Book, Alexander and Nowak, Ulrich and Baum, Peter}
}
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