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MA: Fachverband Magnetismus

MA 7: Poster Magnetism I

MA 7.12: Poster

Montag, 9. März 2026, 09:30–12:30, P2

Fluence dependency of ultrafast-demagnetization in Gadolinium and Terbium — •Timo Dully, Christian Strüber, and Martin Weinelt — Freie Universität Berlin

The magnetic moment in rare-earth metals is primarily determined by the 4f orbitals. Upon optical pumping, the 4f spin polarization decays on markedly different timescales in Gd and Tb of 14 ps and 0.4 ps, respectively. Earlier studies attributed this difference to the stronger magnetocrystalline anisotropy of Tb relative to Gd [1].

Optical excitation of the 5d6s electrons transfers energy to the phonon system; the 4f orbitals are strongly localized and tightly coupled to the lattice. Recent measurements, however, have revealed direct 5d - 4f electron scattering upon optical excitation of Tb, offering an alternative explanation for the differing response times. The first 4f multiplet excitation in Tb requires 0.26 eV, whereas the corresponding excitation energy in Gd is 4.1 eV. Thus hot electrons in the 5d6s valence bands excite 4f electrons via inelastic electron-electron scattering in Tb but not in Gd [2] This should lead to an ultrafast equilibration of 5d and 4f electron temperatures in Tb.

Motivated by these findings, we investigate the fluence dependence of the valence-electron temperature. Using a tr-ARPES setup with 1300 nm pump and XUV probe pulses, we determine the electron-temperature dependence on pump fluence.

[1] B. Frietsch et al., Sci. Adv. 6, eabb1601 (2020)

[2] N. Thielemann-Kühn et al.,Sci. Adv. 10, eadk9522 (2024)

Keywords: Ultrafast Magnetism; Gadolinium and Terbium; tr-ARPES; Fluence dependence; 4f orbitals