Femtosecond Electron and Spin Dynamics in $\mathrm{N}\mathrm{i}/\mathrm{W}(110)$ Films

The collapse of the magnetic exchange splitting in 7 monolayer thick epitaxial $\mathrm{N}\mathrm{i}/\mathrm{W}(110)$ films following a femtosecond laser pulse was measured using time-resolved photoemission spectroscopy. Ultrafast demagnetization during the laser induced hot electron cascade proceeds via spin-flip excitations with a relaxation time constant of $300\pm 70\mathrm{f}\mathrm{s}$. At longer times the electronic system cools down and the magnetization is finally reestablished with a time constant of $3.2\pm 0.2\mathrm{p}\mathrm{s}$.

Figure 1

Photoelectron spectra (dots and lines) taken with 6 eV probe laser pulses each preceded by a 1.5 eV pump laser pulse for pump-probe time delays of (a) $-1\mathrm{p}\mathrm{s}$, (b) 0 ps, (c) 0.1 ps, (d) 1 ps, and (e) 4 ps. The binding energy of the majority spin state relative to the Fermi level, $E_F$, is marked by vertical lines. The lower inset depicts the experimental setup. The upper inset shows spectra (a), (d), and (e) (symbols). Lines represent the spectra divided by the Fermi function for the obtained electronic temperatures of 343, 595, and 423 K, respectively. The assignment of the observed peaks to majority (spin up) and minority (spin down) states is indicated by arrows.

Figure 2

Log plots of the photoemission intensity above the Fermi level, $E_F$, for pump-probe time delays of (a) $-1\mathrm{p}\mathrm{s}$, (b) 0 ps, (c) 0.2 ps, (d) 0.4 ps, and (e) 0.8 ps. A linear behavior in this plot indicates that the electron distribution can be described by a Fermi function. The slope of this line corresponds to the temperature. The inset shows the number of excited electrons, $n/n_0$, obtained from the total photoemission intensity above $E_F$ (solid symbols) normalized to spectrum (a) and the mean electron energy, $⟨E⟩$, above $E_F$ (open symbols) vs time delay.

Figure 3

Binding energy of the majority spin state (diamonds) and electronic temperature (open squares) vs pump-probe time delay. Solid and open diamonds are photoemission data obtained for emission angles of $40°$ and $30°$, respectively. The binding energy was determined from a parabolic fit near the peak maxima to the raw data such as the ones shown in Fig. 1. The solid line represents the transient electron population of states 1.5 eV above $E_F$ (see Fig. 2). Dotted lines are exponential fits to the data.