Ultrafast Transport of Laser-Excited Spin-Polarized Carriers in $\mathrm{Au}/\mathrm{Fe}/\mathrm{MgO}(001)$

Hot carrier-induced spin dynamics is analyzed in epitaxial $\mathrm{Au}/\mathrm{Fe}/\mathrm{MgO}(001)$ by a time domain approach. We excite a spin current pulse in Fe by 35 fs laser pulses. The transient spin polarization, which is probed at the Au surface by optical second harmonic generation, changes its sign after a few hundred femtoseconds. This is explained by a competition of ballistic and diffusive propagation considering energy-dependent hot carrier relaxation rates. In addition, we observe the decay of the spin polarization within 1 ps, which is associated with the hot carrier spin relaxation time in Au.

Figure 1

Electron diffraction pattern of $\mathrm{Au}/\mathrm{Fe}/\mathrm{MgO}(001)$ (a), dark field scanning transmission electron microscope image of the $\mathrm{Fe}/\mathrm{Au}$ interface (b), and SH hysteresis loops at the Au surface before (c) and after (d) the excitation. The inset illustrates the pump-probe experiment.

Figure 2

Pump-induced variation of SH electronic contribution $\Delta$ (a),(b) and SH magnetic contrast $\rho \propto M$ (c),(d) measured at the Au surface of the $\mathrm{Au}/\mathrm{Fe}/\mathrm{MgO}(001)$ structure with the indicated $d_{\mathrm{Fe}}$ and $d_{\mathrm{Au}}$ for the pump fluence of $1\mathrm{mJ}/{\mathrm{cm}}^2$.

Figure 3

Calculated density of carriers excited in Fe vs their energy with respect to the Fermi level: Solid (dotted) curves correspond to HCs carrying negative (positive) SP. The dashed curve reproduces the HC lifetime from Ref. 26.