Ultrafast Dynamics of Electrons and Magnons in Ferromagnetic Model Systems

Magnons are one of the carriers of angular momentum that are involved in the ultrafast magnetization dynamics in ferromagnets, but their contribution to the microscopic electronic dynamics and their interplay with other scattering processes that occur during ultrafast demagnetization has only been studied at the level of electronic lifetimes. This work presents an investigation of the whole electronic scattering dynamics during and after the ultrafast demagnetization process in itinerant ferromagnets. It presents a microscopic approach at the level of Boltzmann scattering integrals for the spin-dependent electron and magnon distributions in momentum space. We investigate electron-magnon scattering together with spin-orbit assisted spin-flip electron-electron scattering in a microscopic fashion and include interactions of electrons and magnons with phonons on a phenomenological relaxation-time level. We consider a ferromagnetic model system of two bands that are separated by a spin splitting. We study the scattering dynamics after electronic excitation. We analyze two types of excitation conditions, the first one is an excitation of electrons in one spin channel, either in majority or minority channel. The second one is a thermal excitation of both majority and minority electrons by using a hot Fermi-Dirac distribution at 2000 K. This excitation process is assumed to be instantaneous and to not change the spin polarization in order to mimic ultrafast optical excitation. The coupled dynamics of magnons, majority and minority electrons are then computed for both excitation conditions.