  An antiferromagnet emits spin currents when time-reversal symmetry is broken.
This is typically achieved by applying an external magnetic field below and
above the spin-flop transition or by optical pumping. In this work we apply
optical pump-THz emission spectroscopy to study picosecond spin pumping from
metallic FeRh as a function of temperature. Intriguingly we find that in the
low-temperature antiferromagnetic phase the laser pulse induces a large and
coherent spin pumping, while not crossing into the ferromagnetic phase. With
temperature and magnetic field dependent measurements combined with atomistic
spin dynamics simulations we show that the antiferromagnetic spin-lattice is
destabilised by the combined action of optical pumping and picosecond
spin-biasing by the conduction electron population, which results in spin
accumulation. We propose that the amplitude of the effect is inherent to the
nature of FeRh, particularly the Rh atoms and their high spin susceptibility.
We believe that the principles shown here could be used to produce more
effective spin current emitters. Our results also corroborate the work of
others showing that the magnetic phase transition begins on a very fast
picosecond timescale, but this timescale is often hidden by measurements which
are confounded by the slower domain dynamics.
