Ultrafast Spin Dynamics of Ferromagnetic Thin Films Observed by fs Spin-Resolved Two-Photon Photoemission

Using a ``pump and probe'' technique, the time scale of spin relaxation effects in ferromagnetic thin films is investigated. Samples are excited by a 3 eV ``pump'' laser pulse $(\ensuremath{\tau}\ensuremath{\approx}170\mathrm{fs})$, and electrons are photoemitted by a 6 eV ``probe'' pulse, after delays ranging up to 1 ns. The spin polarization of the photoelectrons is measured using a Mott detector. The time dependence of the spin polarization of thin Ni films on Ag is explained in a local magnetic moment picture with two excitation processes. Stoner excitations are responsible for the reduction of the observed spin polarization on a time scale of 1 ps and phonon-magnon scattering leads to the loss of long range magnetic order on a time scale of 500 ps.