ULTRAFAST SPIN DYNAMICS OF FERROMAGNETIC THIN-FILMS OBSERVED BY FS SPIN-RESOLVED 2-PHOTON PHOTOEMISSION

Using a ''pump and probe'' technique, the time scale of spin relaxatio n effects in ferromagnetic thin films is investigated. Samples are exc ited by a 3 eV ''pump'' laser pulse (tau approximate to 170 fs), and e lectrons are photoemitted by a 6 eV ''probe'' pulse, after delays rang ing up to 1 ns. The spin polarization of the photoelectrons is measure d using a Matt 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 magnet ic order on a time scale of 500 ps.