Spin-Dependent Electron Dynamics in Front of a Ferromagnetic Surface

Exchange splitting and dynamics of image-potential states in front of a 3 monolayer iron film on Cu(100) have been studied with time-, energy-, and spin-resolved bichromatic two-photon photoemission. For the first image-potential state $n=1$ we observe an exchange splitting of $56\pm 10\mathrm{meV}$ and spin-dependent lifetimes of $16\pm 2\mathrm{fs}$ for majority-spin and of $11\pm 2\mathrm{fs}$ for minority-spin electrons, respectively. The time-resolved studies of both the population and the linewidth of image-potential states manifest that at the magnetic surface not only inelastic but also quasielastic scattering processes are spin dependent.

Figure 1

Energy-resolved 2PPE spectra at zero time delay between pump and probe pulse, spin integrated (▪) and spin resolved; the exchange splitting between the majority (▴) and minority (▿) components is clearly discernible. A schematic representation of bichromatic 2PPE is shown in the inset.

Figure 2

(a) Energy- and spin-resolved 2PPE spectra of the $n=1$ image-potential state for increasing pump-probe delay; the spectra are normalized on the spin-up channel at 0 fs; the numbers on the right indicate the multiplication factors of the intensity. (b) Time-resolved 2PPE spectrum of the $n=1$ image-potential state for majority (▴) and minority (▿) electrons and the cross correlation between pump and probe pulses (+). The fit (solid lines) was done employing Bloch equations.

Figure 3

Measured (symbols) and calculated (solid lines) linewidths of the majority (▴) and minority (▿) components of the first image-potential state as a function of pump-probe delay. Where the experimental uncertainty exceeds the symbol size error bars are indicated. In the inset the dispersing image-potential bands are shown schematically and possible elastic and inelastic scattering processes are indicated.