  Experimental observations of the ultrafast (less than 50 fs) demagnetization
of Ni have so far defied theoretical explanations particularly since its
spin-flipping time is much less than that resulting from spin-orbit and
electron-lattice interactions. Through the application of an approach that
benefits from spin-flip time-dependent density-functional theory and dynamical
mean-field theory, we show that proper inclusion of electron correlations and
memory (time-dependence of electron-electron interaction) effects leads to
demagnetization at the femtosecond scale, in good agreement with experimental
observations. Furthermore, our calculations reveal that this ultrafast
demagnetization results mainly from spin-flip transitions from occupied to
unoccupied orbitals implying a dynamical reduction of exchange splitting. These
conclusions are found to be valid for a wide range of laser pulse amplitudes.
They also pave the way for ab initio investigations of ultrafast charge and
spin dynamics in a variety of quantum materials in which electron correlations
may play a definitive role.
