Terahertz Néel spin-orbit torques drive nonlinear magnon dynamics in antiferromagnetic Mn2Au.

Antiferromagnets have large potential for ultrafast coherent switching of magnetic order with minimum heat dissipation. In materials such as Mn₂Au and CuMnAs, electric rather than magnetic fields may control antiferromagnetic order by Néel spin-orbit torques (NSOTs). However, these torques have not yet been observed on ultrafast time scales. 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⁻¹ s⁻¹. At incident terahertz electric fields above 500 kV cm⁻¹, we find pronounced nonlinear dynamics with massive Néel-vector deflections by as much as 30°. Our data are in excellent agreement with a micromagnetic model. It indicates that fully coherent Néel-vector switching by 90° within 1 ps is within close reach. Néel spin-orbit torques can occur in antiferromagnets with broken inversion symmetry, such as Mn₂Au, and have garnered significant interest recently, as they allow for the electrical control of the antiferromagnetic ordering. Here, Behovits et al. apply intense terahertz electric fields to Mn₂Au and observe the deflection of the Néel vector on ultrafast time scales due to Néel spin-orbit torques. [ABSTRACT FROM AUTHOR]