THz-induced optical activity in the non-collinear antiferromagnet metal Fe_1/3NbS₂

The intercalated transition metal dichalcogenide Fe_1/3NbS₂ has received considerable attention due to its suitability for antiferromagnetic (AFM) spintronic devices. Recently, it has been demonstrated that the AFM order in Fe_1/3NbS₂ can be switched by applying DC pulses with remarkably low current densities. In this work, we use ultrafast THz-pump, near-infrared-probe spectroscopy to observe a transient change in the Kerr ellipticity in response to a strong THz pump pulse. We find that this change is peaked at the Nèel temperature (50K) and persists at even higher temperatures. We will discuss possible reasons for this behavior in terms of competing magnetic phases present in Fe_1/3NbS₂ and critical dynamics near the phase transition. Our results highlight the possibility of manipulating AFM ordering in metals at THz timescales.