Time-dependent ab initio insight to the ultrafast demagnetization mechanism

Laser induced ultrafast demagnetization process has a great potential in future spintronic devices. Ever since its discovery, this subject has been studied intensively. However, the fundamental physics is still not well understood. There are several long-standing problems regarding the demagnetization mechanism. One major issue is the remarkable underestimation in ab initio simulation. At the same intensity of laser, the theoretical demagnetization rate can be ten times smaller than the experimental observation. Here, we present an ultrafast real-time time-dependent density functional theory method, together with atomic Landau-Lifshitz-Gilbert model, to investigate this problem. Our results show that one important reason for the underestimation is the missing of initial spin disorder, which can significantly enhance the demagnetization to the experimentally observed rate. This spin disorder connects the electronic structure theory with the phenomenological three-temperature model. We have also systematically studied the roles of various interactions which are heavily debated. In particular, we found that electron-electron interaction and spin-orbit interaction play extremely important roles, while electron-phonon and light-spin interactions are not essential.