Hot-Electron-Driven Enhancement of Spin-Lattice Coupling in Gd and Tb $4f$ Ferromagnets Observed by Femtosecond X-Ray Magnetic Circular Dichroism

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.

Figure 1

Orbital wave-function distributions within an hcp unit cell for $L$, $m=0$ and 3. The $m=3$ nonspherical distribution of Tb couples to the ion cores via single ion anisotropy, which is absent for the spherical $m=0$ state of Gd.

Figure 2

(a) X-ray transmission at the $M_5$ absorption edges of Gd and Tb films recorded for opposite magnetization direction (black and gray lines) with 10 ps circularly polarized x-ray pulses. (b) XMCD signals of Gd and Tb before and 200 ps after laser excitation (solid and dotted lines).

Figure 3

Time-dependent XMCD signals for Gd (top) and Tb (bottom) measured by 10 ps x-ray probe and 50 fs laser pump pulses. Solid lines indicate fits to the data. The inset depicts Gd data in a smaller time window with the actual time-resolution of 16 ps indicated. The biexponential fit (solid line) highlights the two-step demagnetization process and the dashed line indicates the behavior expected for an instantaneous first step.

Figure 4

Time-dependent XMCD signals for Gd (top) and Tb (bottom) measured with fs x-ray pulses. Note the different time intervals. Solid lines depict biexponential fits determined by simultaneously fitting the fs and ps time-resolved data (cf. solid lines in Fig. 3). A single exponential (dash-dotted line) for Tb yields an unsatisfactory fit.