Terahertz switching of antiferromagnetic order by N\'eel spin-orbit torques
Ultrafast electric manipulation of magnetic order in solids is critical for the development of future terahertz data processing. A fascinating concept for such high-speed operation is offered in metallic antiferromagnets by N\'eel spin-orbit torque. It should allow one to coherently rotate the ordered spins by simply applying an electric current of suitable amplitude and polarity. However, such switching has been severely hampered by competing heat-induced effects, and it has not yet been achieved on the intrinsically ultrafast time scales of antiferromagnets. Here, we report robust, direction-controlled and non-thermal rotation of the N\'eel vector $\mathbf{L}$ by $\pm$90{\deg} at room temperature in the antiferromagnet Mn$_2$Au driven by phase-locked terahertz current pulses. All observed features are consistent with ultrafast N\'eel spin-orbit torque: First, nonlinear optical imaging reveals that the terahertz current direction sets the final orientation of $\mathbf{L}$ in the absence of any bias field for at least two months. Second, transient optical birefringence shows that the switching proceeds ultrafast in less than 15 picoseconds. Finally, atomistic spin-dynamics simulations reproduce the observed dynamics and confirm the minor role of thermal effects. While the switching is already one order of magnitude faster than in ferromagnets at comparable dissipated energy, our simulations predict routes toward switching times and energies which are another order of magnitude lower. Our approach can be transferred to electric-field-driven switching in many more antiferromagnets, including magnetoelectric insulators. The engineering of spin torques, resonance frequencies and read-out mechanisms provides an exciting pathway toward on-chip applications of terahertz antiferromagnetic spin-orbitronics.