Employing spin-, time-, and energy-resolved photoemission spectroscopy, we present the first study on the spin polarization of a single electronic state after ultrafast optical excitation. Our investigation concentrates on the majority-spin component of the $d$-band-derived Gd(0001) surface state $d_{z^2}^{\uparrow }$. While its binding energy shows a rapid Stoner-like shift by 90 meV with an exponential time constant of ${\tau }_E=0.6\pm 0.1\mathrm{ps}$, the $d_{z^2}^{\uparrow }$ spin polarization remains nearly constant within the first picoseconds and decays with ${\tau }_S=15\pm 8\mathrm{ps}$. This behavior is in clear contrast to the equilibrium phase transition, where the spin polarization vanishes at the Curie temperature.

• Received 2 July 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.207404