Fig. 1. Comparison of 5d6s and 4f spin dynamics in Gadolinium and Terbium.

Upper panels: Orbital-resolved spin model. The yellow arrows represent the energy flow from the laser-excited electrons into the lattice (G_ep) and to the 5d and 4f spin systems. Note the different 4f spin–to–lattice couplings α_4f in (A) Tb (J = L + S = 6, L = 3) and (B) Gd (J = S = 7/2, L = 0). In contrast, inter- and intra-atomic exchange constants (J_ij and J_intra) are of comparable magnitude. Lower panels: Illustration of 5d6s and 4f spin dynamics about 1 ps after laser excitation. While in (B), the 4f spins (yellow arrows) are strongly excited by lattice motions and tilted with respect to M_z, in (A), they remain cold and aligned along the magnetization direction M_z. The 5d6s spins (red arrows) are additionally coupled to the optically excited valence electrons α_5d and thus quiver around the 4f moments.