Ultrafast Demagnetization After Femtosecond Laser Pulses: Transfer of Angular Momentum from the Electronic System to Magnetoelastic Spin-Phonon Modes

When exciting a ferromagnetic film with a femtosecond optical laser pulse, then there is a partial demagnetization within a few hundred fs. This means that the angular momentum of the electronic system is transferred to the lattice. This transfer is calculated by describing the lattice degrees of freedom as spin-phonon eigenmodes which have a sharp angular momentum. Possible other lattice degrees of freedom are mentioned but not considered explicitly. For Ni and Fe the calculated amount of transfer of angular momentum to the spin-phonon eigenmodes is rather small.