Spin nematicity in the local-moment Fe-chalcogenide KFe_0.8Ag_1.2Te₂

Iron-based superconductors appear in the vicinity of a nematic quantum critical point, and due to the presence of spin, orbital and charge degrees of freedom, nature of the nematic order has been the focus of research in recent years. One route to address this question is to look for clues in related materials that exhibit similar physics. In semiconducting KFe_0.8Ag_1.2Te₂, an analogue of the archetypical iron pnictide BaFe₂As₂, we found simultaneous stripe-type magnetic order and nematic order below T_S,N=35 K, with striking similarities between the two systems. Above T_S,N, a sizable spin anisotropy develops under a small strain and increase upon cooling towards T_S,N, indicative of a divergent nematic susceptibility. Since the magnetic susceptibility of KFe_0.8Ag_1.2Te₂ is well described by the Curie-Weiss law with S≈1, it is a local-moment magnet with orbital degeneracy. The similarities between nematic orders in KFe_0.8Ag_1.2Te₂ and BaFe₂As₂ suggest our findings to be relevant for iron-based superconductors, and establish KFe_0.8Ag_1.2Te₂ as a model system to investigate electronic nematicity in the localized limit.