Ultrafast demagnetization of rare-earth metals is distinct from that of 3d ferromagnets, as rare-earth magnetism is dominated by localized 4f electrons that cannot be directly excited by an optical laser pulse. Their demagnetization must involve excitation of magnons, driven either through exchange coupling between the 5d6s-itinerant and 4f-localized electrons or by coupling of 4f spins to lattice excitations. Here, we disentangle the ultrafast dynamics of 5d6s and 4f magnetic moments in terbium metal by time-resolved photoemission spectroscopy. We show that the demagnetization time of the Tb 4f magnetic moments of 400 fs is set by 4f spin-lattice coupling. This is experimentally evidenced by a comparison to ferromagnetic gadolinium and supported by orbital-resolved spin dynamics simulations. Our findings establish coupling of the 4f spins to the lattice via the orbital momentum as an essential mechanism driving magnetization dynamics via ultrafast magnon generation in technically relevant materials with strong magnetic anisotropy.

Charles University Faculty of Mathematics and Physics DCMP Ke Karlovu 5 CZ 12116 Prague 2 Czech Republic

Department of Physics and Astronomy Uppsala University P O Box 516 S 75120 Uppsala Sweden

Fachbereich Physik Freie Universität Berlin Arnimallee 14 14195 Berlin Germany

Fachbereich Physik Universität Konstanz 78457 Konstanz Germany

IT4Innovations Czech National Supercomputing Centre VSB Technical University Ostrava 17 listopadu 2172 15 CZ 708 00 Ostrava Czech Republic

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Optical control of 4f orbital state in rare-earth metals