THz-driven ultrafast spin-lattice scattering in amorphous metallic ferromagnets

TL;DR

This study demonstrates that amorphous metallic ferromagnets exhibit ultrafast spin-lattice scattering driven by THz fields, leading to rapid demagnetization, which is faster than laser-induced effects and influenced by lattice disorder.

Contribution

It reveals the role of lattice disorder in amorphous ferromagnets in enabling ultrafast spin-lattice scattering and demagnetization under THz excitation, a novel insight into spin dynamics.

Findings

Ultrafast demagnetization observed only in amorphous CoFeB.

Spin-lattice scattering occurs on ~30 fs timescale, comparable to electronic scattering.

Lattice disorder enhances spin-lattice scattering, as shown by conductivity measurements.

Abstract

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.