  Interfaces between heavy metals (HMs) and antiferromagnetic insulators (AFIs)
have recently become highly investigated and debated systems in the effort to
create spintronic devices able to function at terahertz frequencies. Such
heterostructures have great technological potential because AFIs can generate
sub-picosecond spin currents which the HMs can convert into charge signals. In
this work we demonstrate an optically induced picosecond spin transfer at the
interface between AFIs and Pt using time-resolved THz emission spectroscopy. We
select two antiferromagnets in the same family of fluoride cubic perovskites,
KCoF3 and KNiF3, whose magnon frequencies at the centre of the Brillouin zone
differ by an order of magnitude. By studying their behaviour with temperature
we correlate changes in the spin transfer efficiency across the interface to
the opening of a gap in the magnon density of states below the N\'eel
temperature. Our observations are reproduced in a model based on the spin
exchange between the localized electrons in the antiferromagnet and the free
electrons in Pt. These results constitute an important step in the rigorous
investigation and understanding of the physics of AFIs/HMs interfaces on the
ultrafast timescale.
