Antiferromagnets have large potential for ultrafast coherent switching of magnetic order with minimum heat dissipation. In novel materials such as Mn2Au and CuMnAs, electric rather than magnetic fields may control antiferromagnetic order by N\'eel spin-orbit torques (NSOTs), which have, however, not been observed on ultrafast time scales yet. Here, we excite Mn2Au thin films with phase-locked single-cycle terahertz electromagnetic pulses and monitor the spin response with femtosecond magneto-optic probes. We observe signals whose symmetry, dynamics, terahertz-field scaling and dependence on sample structure are fully consistent with a uniform in-plane antiferromagnetic magnon driven by field-like terahertz NSOTs with a torkance of (150±50) cm2/A s. At incident terahertz electric fields above 500 kV/cm, we find pronounced nonlinear dynamics with massive N\'eel-vector deflections by as much as 30{\deg}. Our data are in excellent agreement with a micromagnetic model which indicates that fully coherent N\'eel-vector switching by 90{\deg} within 1 ps is within close reach.
@article{arxiv.2305.03368,
title = {Nonlinear terahertz N\'eel spin-orbit torques in antiferromagnetic Mn$_2$Au},
author = {Yannic Behovits and Alexander L. Chekhov and Stanislav Yu. Bodnar and Oliver Gueckstock and Sonka Reimers and Tom S. Seifert and Martin Wolf and Olena Gomonay and Mathias Kläui and Martin Jourdan and Tobias Kampfrath},
journal= {arXiv preprint arXiv:2305.03368},
year = {2023}
}