Ultrafast element- and depth-resolved magnetization dynamics probed by transverse magneto-optical Kerr effect spectroscopy in the soft x-ray range

We report on time- and angle-resolved transverse magneto-optical Kerr effect spectroscopy in the soft x-ray range that, by analysis via polarization-dependent magnetic scattering simulations, allows us to determine the spatiotemporal and element-specific evolution of femtosecond laser-induced spin dynamics in nanostructured magnetic materials. In a ferrimagnetic GdFe thin-film system, we correlate a reshaping spectrum of the magneto-optical Kerr signal to depth-dependent magnetization dynamics and disentangle contributions due to nonequilibrium electron transport and nanoscale heat diffusion on their intrinsic timescales. Our Letter provides a quantitative insight into light-driven spin dynamics occurring at buried interfaces of complex magnetic heterostructures, which can be tailored and functionalized for future optospintronic devices.

Figure 1

(a) Schematic of the $\vartheta \text{−}2\vartheta$ spectroscopy setup used for the angle- and time-resolved experiments. The inset shows the depth-dependent differential absorption of the $2.1\text{−}\mu \mathrm{m}$ pump pulse (incident from the left side onto the Ta layer) that drives the magnetization dynamics. (b) Static angle-resolved TMOKE asymmetry spectra (color scale) of the studied GdFe sample measured by soft x-ray pulses in the photon energy range across the Gd ${\mathrm{N}}_{5,4}$ resonance. (c) Simulation of the static angle-resolved TMOKE asymmetry spectra fitted to the experimental data in panel (b).

Figure 2

Transient TMOKE asymmetry spectra at $\vartheta ={20}^{\circ }$ as a function of incident excitation fluence and pump-probe delay. Each plot shows the equilibrium state (black) as well as a series of transient spectra (colored) recorded at different delays after excitation. The shaded areas indicate different photon energy intervals for each fluence (blue, green, and red) over which the asymmetry was integrated to obtain the time traces shown in Fig. 3. The arrows mark the time evolution of the main (P1) and neighboring (P2) asymmetry peaks.

Figure 3

(a) Transient shift of the main (P1) and its neighboring (P2) asymmetry peak upon 1.7 $\mathrm{mJ}/{\mathrm{cm}}^2$ excitation. (b) Integrated asymmetry as a function of excitation fluence and pump-probe delay. The colored data points for each fluence result from integrating over the accordingly colored photon energy intervals shown in Fig. 2. The inset compares the demagnetization dynamics and peak shifts during the first picosecond after excitation.

Figure 4

Determination of the magnetization depth profiles from the time-resolved TMOKE spectroscopy measurements. (a) Recorded transient asymmetry spectra (dots) and fitted simulations (lines) averaged over different time intervals as a function of the incident excitation fluence (black, blue, and green). The simulation was fitted to the experimental data by varying only the magnetization distribution within the GdFe layer. (b) Resulting magnetization depth profiles obtained for the respective excitation fluence and time interval.