All-optical magnetometric characterization of the exchange-coupled antiferromagnet/ferromagnet system Mn2Au|Ni80Fe20 by terahertz spin-orbit torques

Abstract

Antiferromagnetic materials have great potential for spintronic applications at terahertz (THz) frequencies. However, in contrast to ferromagnets, experimental investigations of antiferromagnets are often challenging due to a lack of straightforward external control of the Néel vector 𝑳 and suitable probes. Here, we study an AFM|FM stack consisting of an antiferromagnetic metal layer (AFM) of the novel material Mn2⁢Au and a ferromagnetic metal layer (FM) of Ni80⁢Fe20. To characterize the AFM|FM stack as a function of the quasistatic 𝑩ext, we perform quasistatic as well as THz-pump magneto-optic probe experiments. Our results demonstrate that AFM and FM are exchange-coupled, and that 𝑳 of the AFM can be set by the simple application of an external in-plane magnetic field 𝑩ext. We identify ultrafast signal components that can consistently be explained by the in-plane antiferromagnetic magnon mode excited by fieldlike Néel spin-orbit torque (NSOT). Remarkably, we find that the 𝑩ext- and THz-pump-induced changes in the optical response of the sample are dominated exclusively by the spin degrees of freedom of AFM. Nonmagnetic signal contributions are minor. We fully calibrate the magnetic circular and magnetic linear optical birefringence of AFM and extract the efficiency of the NSOT. Finally, by selective excitation of domains with different orientations of 𝑳, we are able to determine the relative volume fraction of 0∘, 90∘, 180∘ and 270∘ domains during the quasistatic reversal of 𝑳 by 𝑩ext. Our insights are an important prerequisite for future studies of ultrafast coherent switching of spins by THz NSOT and show that THz-pump magneto-optic-probe experiments are a powerful tool to characterize magnetic properties of antiferromagnets.