Accessing Ultrafast Demagnetization Rates of Ferrimagnetic Thin Films through THz Emission Spectroscopy

We present a comprehensive characterization of the optically induced sub-picosecond demagnetization of GdFeCo thin films, done via a rigorous frequency-domain analysis of their THz emission. We extract the temperature and chemical composition dependences of the Gd and FeCo contributions to the measured THz pulses and map them to their demagnetization rates, finding in some instances a significant Gd component that has been neglected in previous studies. We detect a strong suppression and a change in the polarity of the THz signal, features that we ascribe to equilibrium properties of the samples, i.e., out-of-plane magnetic anisotropy and magnetization compensation. Furthermore, our measurements show that the observed dynamics are also affected by the antiferromagnetic interaction between the Gd and FeCo sublattices which varies with temperature and concentration. Our methodology is readily applicable to disentangle the THz emission from other ferrimagnets and can thus contribute to our understanding of the ultrafast response of these technologically relevant materials.