Figure 3: Dynamics of spin and orbital momenta for Co and Tb in Co_0,74Tb_0,26.

(a) Time-resolved XMCD at the Co L₃ (filled red circles), Co L₂ (empty red circles), Tb M₅ (filled black circles) and Tb M₄ edges (empty black circles) measured on a 15-nm Co_0,74Tb_0,26 alloy film. The XMCD values at negative time delays match the XMCD magnitude recorded at the Co L₃, Co L₂, Tb M₅ and Tb M₄ edges during quantitative static XMCD measurements²⁷. The pump fluence used during our experiment was adjusted to 12 mJ cm⁻². (b) Ultrafast dynamics of the spin and orbital momenta S^Co(t) (black circles) and L^Co(t) (red circles) for Co extracted by applying the sum rules. The continuous lines are the simulations of S^Co(t) (black line) and L^Co(t) (blue line; see Methods) with characteristic thermalization times of τ^Co_therm=180±40 fs for both momenta. The dynamics of the ratio L^Co (t)/S^Co(t) (green circle) shows that the orbital momentum decays more than the spin momentum during the ultrafast demagnetization. (c) Ultrafast dynamics of the spin and orbital momenta −S^Tb(t) (black circles) and −L^Tb(t) (red circles) for Tb. The continuous lines are the simulations of S^Tb(t) (black line) and L^Tb(t) (red line; see Methods) with characteristic thermalization times of τ^Tb_therm=280±40 fs for both momenta. The simulations of Lⁱ(t) and Sⁱ(t) have been obtained using a nonlinear least square fitting procedure. The extracted error bar on τⁱ_therm is the s.d., which is given as an output parameter of the fitting procedure. The error bars obtained for Lⁱ(t) and Sⁱ(t) as shown in b and c are given by the s.d. of the experimental data with respect to the fitting functions (see Methods).