Fig. 1: Crystal structure and Optical pump-THz probe measurements of Fe_nGeTe₂ (n = 3, 5) films.

a Crystal structure as seen from two different points of view for Fe₃GeTe₂ (F3GT) and Fe₅GeTe₂ (F5GT). For simplicity, the Fe I atoms are represented only near the Te top plane. In reality, they are distributed near both Te layers, forming two partially occupied sub-lattices^15,27. b Real part of optical conductivity for the four measured films, as extracted through a multi-layer stacking model. The pink dotted line identifies the 800 nm (12,500 cm⁻¹) pump wavelength. c Optical pump-THz probe spectroscopy setup. An initial seed laser (800 nm, 30 fs, 7 mJ) is split twice to create a probe THz signal through optical rectification in a ZnTe crystal (0.5 mm), a pump signal at variable wavelength through an optical parametric amplifier and difference frequency generation (OPA+DFG) system, and a final optical signal to probe the THz electric field through an electro-optical (EO) detection in a second ZnTe crystal (0.3 mm). d Pump-probe data at 300 K and 5 mJ/cm² for the four measured films over an extended time scale. The discontinuous curves overlaid to the experimental data represent the best fits according to Eq. (1) and a bi-exponential type-II demagnetization, respectively for the 15 ML F3GT and F5GT films. The best-fit parameters are indicated in the inset. e Light-induced demagnetization THz emission from a 15 ML F5GT film.