The Heisenberg-Dirac intra-atomic exchange coupling is responsible for the formation of the atomic spin moment and thus the strongest interaction in magnetism. Therefore, it is generally assumed that intra-atomic exchange leads to a quasi-instantaneous aligning process in the magnetic moment dynamics of spins in separate, on-site atomic orbitals. Following ultrashort optical excitation of gadolinium metal, we concurrently record in photoemission the 4f magnetic linear dichroism and 5d exchange splitting. Their dynamics differ by one order of magnitude, with decay constants of 14 versus 0.8 ps, respectively. Spin dynamics simulations based on an orbital-resolved Heisenberg Hamiltonian combined with first-principles calculations explain the particular dynamics of 5d and 4f spin moments well, and corroborate that the 5d exchange splitting traces closely the 5d spin-moment dynamics. Thus gadolinium shows disparate dynamics of the localized 4f and the itinerant 5d spin moments, demonstrating a breakdown of their intra-atomic exchange alignment on a picosecond timescale.

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

Department of Physics and Astronomy Uppsala University PO Box 516 75120 Uppsala Sweden

Fachbereich Physik Freie Universität Berlin Arnimallee 14 14195 Berlin Germany

Fachbereich Physik Universität Konstanz 78457 Konstanz Germany

Max Born Institut Max Born Strasse 2a 12489 Berlin Germany

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

The role of ultrafast magnon generation in the magnetization dynamics of rare-earth metals