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Abstract

The ultimate fate of the spin moment in ultrafast demagnetization is to appear as a gain in momentum of the lattice. However, the route by which it occurs remains to be conclusively demonstrated. Employing state-of-the-art time-dependent density functional theory we show that spin-orbit coupling at femtosecond timescales drives a transfer of spin angular momentum into orbital angular momentum, which via the Coulomb term transfers from this orbital angular momentum instantaneously to the lattice. The rate of change of angular and spin momenta have, respectively, predominantly extrinsic (the pulse duration) and intrinsic (spin-orbit coupling) timescales. This facilitates the design of pulses to clearly disentangle the physics of angular momentum redistribution. While experiments predominantly use bulk elemental Ni, we propose Co in thin film geometry with an experimentally realizable time resolution for ultrashort pulses will allow clear observation of the ultrafast transfer of momentum via orbital degrees of freedom to the lattice.