Energy- and crystal momentum-resolved study of laser-induced femtosecond magnetism

When a femtosecond (fs) laser pulse strikes a ferromagnet, it demagnetizes the sample within a few hundred fs but its underlying mechanism has remained elusive for over a decade. Here a possible microscopic picture is revealed through an energy- and crystal momentum-resolved first-principles investigation, first by locating the optimal excitation-energy window for the maximal magnetization change and then mapping out every magnetic contribution from each crystal momentum k point along the high-symmetry lines within the Brillouin zone. We find that not all the k points contribute evenly, where a few momentum k points show a much stronger magnetic-moment change than others. In ferromagnetic nickel, less than 50% of the k points contribute over 90% of the magnetization change. By closely examining the transition-matrix elements and spin-moment change associated with those k points, we further find the reduction in the dynamical magnetic moment is directly connected with these transition-moments and spin-moment changes between band states.