Femtosecond spin dynamics in the time domain

Recently magnetization dynamics on the time scale of 100 fs has been observ ed. In this article, we explain this ultrafast spin dynamics by a microscop ic many-body theory. Our theory yields a speed limit of the order of 10 fs for the spin dynamics in Ni. The fundamental mechanism is the dephasing of the initial excited states. We investigate the dephasing effects in detail as a function of the number of electronic states and K points. We also calc ulate the density of states as a crosscheck of our theory, where the typica l satellite structure of nickel is present. Moreover, the magnetic moment o f the monolayer is found to be 0.88 mu(B), which is also consistent with th e experimental results. Finally, based on our Hamiltonian, we briefly discu ss the origin of ferromagnetism in our model. (C) 1999 American Institute o f Physics. [S0021-8979(99)28208-4].