  Spin-polarized currents play a key role in spintronics. Recently, it has been
found that antiferromagnets with a non-spin-degenerate band structure can
efficiently spin-polarize electric currents, even though their net
magnetization is zero. Among the antiferromagnetic metals with magnetic space
group symmetry supporting this functionality, the noncollinear
antiferromagnetic antiperovskites ANMn$_3$ (A = Ga, Ni, Sn, and Pt) are
especially promising. This is due to their high N\'eel temperatures and a good
lattice match to perovskite oxide substrates, offering possibilities of high
structural quality heterostructures based on these materials. Here, we
investigate the spin polarization of antiferromagnetic ANMn$_3$ metals using
first-principles density functional theory calculations. We find that the spin
polarization of the longitudinal currents in these materials is comparable to
that in widely used ferromagnetic metals, and thus can be exploited in magnetic
tunnel junctions and spin transfer torque devices. Moreover, for certain film
growth directions, the out-of-plane transverse spin currents with a giant
charge-to-spin conversion efficiency can be achieved, implying that the
ANMn$_3$ antiperovskites can be used as efficient spin sources. These
properties make ANMn$_3$ compounds promising for application in spintronics.
