Polarized phonons carry angular momentum in ultrafast demagnetization

• Published in: Nature (London) Vol. 602; no. 7895; pp. 73 - 77
• Main Authors: Tauchert, S. R., Volkov, M., Ehberger, D., Kazenwadel, D., Evers, M., Lange, H., Donges, A., Book, A., Kreuzpaintner, W., Nowak, U., Baum, P.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 03.02.2022; Nature Publishing Group
• Subjects: 140/125; 639/301/119/997; 639/624/400/584; Angular momentum; Anisotropy; Asymmetry; Circular polarization; Demagnetization; Electron diffraction; Equilibrium; Experiments; Femtosecond pulsed lasers; Humanities and Social Sciences; Information processing; Lasers; Magnetic fields; Magnetization; multidisciplinary; Nickel; Phase transitions; Phonons; Science; Science (multidisciplinary); Science and technology; Spintronics; Symmetry
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-021-04306-4

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 timescales 1 . This phenomenon is widespread 2 – 7 and offers opportunities for rapid information processing 8 – 11 or ultrafast spintronics at frequencies approaching those of light 8 , 9 , 12 . Consequently, the physics of ultrafast demagnetization is central to modern materials research 1 – 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. Ultrafast electron diffraction is used here to reveal in nickel an almost instantaneous, long-lasting population of anisotropic phonons with angular momentum.