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MA: Fachverband Magnetismus

MA 34: Focus Session: Spin-Phonon Coupling

MA 34.1: Invited Talk

Thursday, March 30, 2023, 09:30–10:00, HSZ 02

Polarized phonons carry angular momentum in ultrafast demagnetization — •Peter Baum — Universität Konstanz, Germany

Many laser-excited magnetic materials lose their magnetic order almost completely within femtosecond timescales, but where is the missing angular momentum in such a short time? Here we use ultrafast electron diffraction with THz-compressed electron pulses to reveal in nickel an almost instantaneous, long-lasting, non-equilibrium population of anisotropic high-frequency phonons with an anisotropy plane that is perpendicular to the direction of the initial magnetization. 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.