  We use single-cycle THz fields and the femtosecond magneto-optical Kerr
effect to respectively excite and probe the magnetization dynamics in two
thin-film ferromagnets with different lattice structure: crystalline Fe and
amorphous CoFeB. We observe Landau-Lifshitz-torque magnetization dynamics of
comparable magnitude in both systems, but only the amorphous sample shows
ultrafast demagnetization caused by the spin-lattice depolarization of the
THz-induced ultrafast spin current. Quantitative modelling shows that such
spin-lattice scattering events occur on similar time scales than the
conventional spin conserving electronic scattering ($\sim30$ fs). This is
significantly faster that optical laser-induced demagnetization. THz
conductivity measurements point towards the influence of lattice disorder in
amorphous CoFeB as the driving force for enhanced spin-lattice scattering.
