Femtosecond x-ray magnetic circular dichroism was used to study the time-dependent magnetic moment of 4f electrons in the ferromagnets Gd and Tb, which are known for their different spin-lattice coupling. We observe a two-step demagnetization with an ultrafast demagnetization time of 750 fs identical for both systems and slower times which differ sizeably with 40 ps for Gd and 8 ps for Tb. We conclude that spin-lattice coupling in the electronically excited state is enhanced up to 50 times compared to equilibrium.

After excitation by femtosecond laser pulses, Gd and Tb exhibit ultrafast demagnetization in two steps, with the time constant of the second step linked to the coupling strength of the 4f magnetic moments to the lattice. In time-resolved magneto-optical Kerr effect measurements of Gd 1−x Tb x alloys, we observe a decrease in this time constant from 33 to 9 ps with Tb content x increasing from 0 to 0.7. We explain this behavior by the stronger spin-lattice coupling of Tb compared to Gd, which increases the effective spin-lattice coupling in Gd 1−x Tb x with x. In contrast, the faster time constant of the first demagnetization step exhibits no dependence on x. Additional time-and element-resolved x-ray magnetic circular dichroism measurements show a two-step demagnetization of Gd and Tb in Gd 0.6 Tb 0.4 with an equivalent time scale of the second step but a different magnitude of demagnetization which persists for 15 ps. We explain this by an increased coupling of the Gd 4f magnetic moments to the lattice compared to pure Gd, via interatomic exchange coupling to the neighboring Tb 4f moments mediated by 5d electrons, which has limited efficiency and allows an estimation of a characteristic time scale of the interatomic exchange coupling. We assign the first demagnetization step to the dynamics of the laser-excited 5d electrons, while the second demagnetization step is dominated by the strength of spin-lattice coupling of the 4f electrons.

Ultrafast laser-induced demagnetization of Gd(0001) has been investigated by magneto-induced optical second harmonic generation and the magneto-optical Kerr effect which facilitate a comparison of surface and bulk dynamics. We observe pronounced differences in the transient changes of the surface and bulk sensitive magneto-optical signals which we attribute to transfer of optically excited, spin-polarized carriers between surface and bulk states of the Gd(0001) film. A fluence dependent analysis of the bulk magnetization dynamics results in a weak variation of the demagnetization time constant, which starts at about 700 fs and increases by 10% within a fluence variation up to 1 mJ/cm 2 . We compare these results with fluence dependent changes in the transient energy density calculated by the two temperature model. The determined characteristic times of excess energy transfer from the electron system to the lattice, which is mediated by e-ph scattering, range from 0.2 -0.6 ps. Such a more pronounced fluence dependent change in the characteristic time compared to the observed rather weakly varying demagnetization times suggests a more advanced description of the optically excited state than by the two-temperature model.