Recently magnetization dynamics on the time scale of 100 fs has been observed. In this article, we explain this ultrafast spin dynamics by a microscopic 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 calculate the density of states as a crosscheck of our theory, where the typical satellite structure of nickel is present. Moreover, the magnetic moment of the monolayer is found to be 0.88 mu(B), which is also consistent with the experimental results. Finally, based on our Hamiltonian, we briefly discuss the origin of ferromagnetism in our model.