Saturation of the ultrafast laser-induced demagnetization in nickel

We study the loss of magneto-optic contrast in nickel, in response to excitation by a short laser pulse, as a function of excitation density. Our measurements in the picosecond and subpicosecond regime show saturation of the contrast loss at high excitation densities. We conclude that spin relaxation is incomplete on this time scale, and explain the loss of magneto-optic contrast and its saturation in terms of a collapse of the Stoner gap due to band filling effects.