  Thin-film antiferromagnets (AFs) with Rashba spin-orbit coupling are
theoretically investigated. We demonstrate that the relativistic
Dzyaloshinskii-Moriya interaction (DMI) produces a large surface magnetization
and a boundary-driven twist state in the antiferromagnetic N\' eel vector. We
predict a magnetization on the order of $2.3\cdot 10^4$~A/m, which is
comparable to the magnetization of ferromagnetic semiconductors. Importantly,
the magnetization is characterized by ultra-fast terahertz dynamics and
provides new approaches for efficiently probing and controlling the spin
dynamics of AFs as well as detecting the antiferromagnetic DMI. Notably, the
magnetization does not lead to any stray magnetic fields except at corners
where weak magnetic monopole fields appear.
