Nonlinear magnon dynamics in antiferromagnetic Mn2Au driven by Terahertz Neel spin-orbit torque

Yannic Behovits

In antiferromagnets, strong exchange coupling leads to intrinsic terahertz (THz) magnon resonances, which have large potential for high-speed spin information processing. For CuMnAs and Mn2Au, switching of the Néel vector has been demonstrated by using pulsed electrical currents and free-space THz pulses [1-3]. The switching was attributed to the Néel spin-orbit torque (NSOT), which is proportional to the current [4]. However, the underlying spin dynamics have not been observed on ultrafast timescales.
Here, we employ a THz-pump magneto-optic-probe setup to investigate ultrafast dynamics of antiferromagnetic order induced by THz electromagnetic fields in Mn2Au. In our samples, the direction of the Néel vector was prealigned via a spin-flop transition in a high magnetic field (60 T) [5]. We observe a strongly damped oscillatory signal at 0.6 THz, whose amplitude is proportional to the driving THz electric field. Our observations are consistent with an NSOT-driven magnon mode.
Upon increasing the THz field strength to 0.65 MV/cm, a non-linear response emerges. By using a simple model, the signal can be related to a substantial deflection of the Néel vector from its equilibrium position. Based on our results, we can estimate important material-specific parameters and calculate THz pulse field strengths at which switching of the antiferromagnetic order of Mn2Au on picosecond timescales is achieved.

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