Femtosecond Electron and Spin Dynamics in Gd(0001) Studied by Time-Resolved Photoemission and Magneto-optics

Femtosecond electron and spin dynamics of the Gd(0001) surface are investigated by time-resolved photoemission and second harmonic generation. Upon optical excitation the spin polarization of the surface state is reduced by half while its exchange splitting remains nearly unchanged. Electron-magnon interaction is proposed to facilitate electron-spin-flip scattering among spin-mixed surface and bulk states, which provides a mechanism for ultrafast demagnetization.

Figure 1

Time-dependent odd second harmonic field representing the spin polarization of the Gd(0001) surface state. Lines depict the incoherent contribution for different relative pump fluences (maximum absorbed fluence $\approx 1\mathrm{mJ}/{\mathrm{cm}}^2$) after subtraction of the oscillatory part which is generated by a coherent phonon at the surface [25].

Figure 2

Top: Normalized photoelectron spectrum at 300 fs (circles) with fit (solid line) and distribution function $f(E,t)$ (dashed line) on a logarithmic scale and divided by $f$ (triangles) on a linear scale at right. The inset sketches the TRPE experiment. Bottom: Photoelectron spectra at different delays are displayed in color (fits in black) and are offset vertically.

Figure 3

(a) Energy positions of the occupied and unoccupied surface state components as a function of delay time with their energetic separation ${\Delta }_{\mathrm{ex}}$ given in the inset. The error bars account for systematic deviations of the fitting procedure. (b) Transient energy density ${\epsilon }_{\mathrm{el}}$ extracted from TRPE spectra (circles) representing the electron system. The solid line is a simulation following Ref. 26. The inset depicts the population of $S^{\uparrow }$ normalized to the equilibrium value at 100 K.

Figure 4

Majority and minority electronic band structure of Gd(0001) based on Ref. 13; areas represent bulk and lines surface states. The individual panels illustrate (a) optically excited electron-hole pairs, (b) exemplary $e\mathrm{\text{−}}e$ scattering events, (c),(d) electron-spin-flip scattering among spin-mixed states (dashed lines for the surface and hatched area for the bulk).