  We analyze laser-induced ultrafast, spatially inhomogeneous magnetization
dynamics of epitaxial Co/Cu(001) films in a 0.4-10 nm thickness range with
time-resolved magnetization-induced second harmonic generation, which probes
femtosecond spin dynamics at the vacuum/Co and Co/Cu interfaces. The
interference of these two contributions makes the overall signal particularly
sensitive to differences in the transient magnetization redistribution between
the two interfaces, i.e. ultrafast magnetization profiles in the ferromagnetic
film. We find in films of up to 3 nm thickness a stronger demagnetization at
the surface, because the film thickness is smaller than the effective mean free
path of the spin current mediating the demagnetization, i.e. the difference
between the mean free paths of the majority and minority carriers. For film
thicknesses larger than 3 nm, the magnetization profile reverses, since
majority spins can escape into the conducting substrate only from the
interface-near region.
