Role of Spin-Flip Exchange Scattering for Hot-Electron Lifetimes in Cobalt

The spin-dependent lifetimes of hot electrons in fcc Co films were studied by spin- and time-resolved two-photon photoemission. Even for excitation energies close to the Fermi level, we find almost identical lifetimes for majority and minority electrons. This result contradicts ab initio theories predicting 5 to 10 times longer lifetimes for the majority electrons in $3d$ ferromagnets. We provide direct experimental evidence that this discrepancy is caused by the dominance of exchange scattering in inelastic electron decay, in combination with the excitation of secondary electrons. The latter are inherent for all real materials and devices.

Figure 1

Spin- and energy-resolved spectra, recorded at zero delay between laser pulses. Hot electrons and the $n=1$ IPS are excited in parallel by reverted pump-probe processes as shown in the excitation scheme; for clarity the hot-electron signal is multiplied by 30 at small energies.

Figure 2

Time-resolved measurements of hot majority (green filled triangles) and minority (red, empty triangles) electrons at different $E-E_F$. Optical Bloch equations were used to fit the experimental data (black solid lines) and to distinguish the different contributions: hot electrons (dashed lines), off-resonant IPS (light solid lines), and $S_{\downarrow }$ (light dashed lines).

Figure 3

Top panel: Lifetimes ${\tau }_{\uparrow }$ and ${\tau }_{\downarrow }$: present work (left), between 0.28 and 0.68 eV, and experimental results from Ref. 11 (including statistical error only) with fits from Ref. 8. Bottom panel: lifetime ratio ${\tau }_{\uparrow }/{\tau }_{\downarrow }$.

Figure 4

Scheme of (a) direct and (b),(c) exchange scattering. (d) Time-resolved measurements at $E-E_F=0.34\mathrm{eV}$: For the clean Co(001) surface (filled symbols) the long-living component in the majority channel is visible which originates from $S_{\downarrow }$. A quenching of $S_{\downarrow }$ by dosing 1 L of oxygen (open symbols) leads to similar behavior for majority and minority electrons. Inset: relative contribution (peak area) of $S_{\downarrow }$ to the majority electron counts.