Unexpectedly marginal effect of electronic correlations on ultrafast demagnetization after femtosecond laser-pulse excitation

The treatment of ultrafast demagnetization after femtosecond laser-pulse excitation of a ferromagnetic film is usually done by a theory based on Fermi's golden rule which neglects the effects of electronic correlations. In the present paper the contribution of spin-flip electron-phonon scatterings to the ultrafast demagnetization of Ni is calculated by this theory and by the density-matrix theory in which the correlations are taken into account. The unexpected result is that the correlations which are essential for the ultrafast dynamics of nonmagnetic phenomena have only a marginal effect for the considered magnetic problem. From this point of view the use of Fermi's golden rule in all former papers on ultrafast demagnetization is justified.