  Terahertz emission spectroscopy of ultrathin multilayers of magnetic and
heavy metals has recently attracted much interest. This method not only
provides fundamental insights into photoinduced spin transport and spin-orbit
interaction at highest frequencies but has also paved the way to applications
such as efficient and ultrabroadband emitters of terahertz electromagnetic
radiation. So far, predominantly standard ferromagnetic materials have been
exploited. Here, by introducing a suitable figure of merit, we systematically
compare the strength of terahertz emission from X/Pt bilayers with X being a
complex ferro-, ferri- and antiferromagnetic metal, that is, dysprosium cobalt
(DyCo$_5$), gadolinium iron (Gd$_{24}$Fe$_{76}$), Magnetite (Fe$_3$O$_4$) and
iron rhodium (FeRh). We find that the performance in terms of spin-current
generation not only depends on the spin polarization of the magnet's conduction
electrons but also on the specific interface conditions, thereby suggesting
terahertz emission spectroscopy to be a highly surface-sensitive technique. In
general, our results are relevant for all applications that rely on the optical
generation of ultrafast spin currents in spintronic metallic multilayers.
