Frustrated spin order in FeSe*

Magnetism in the structurally simplest iron based superconductor, FeSe, poses a challenge. Unlike in the related iron pnictide compounds, no magnetic order was found down to lowest temperatures in FeSe. However, recent theoretical and experimental studies point towards frustration quenching long range order. Here we demonstrate that inelastic light scattering can distinguish between itinerant and localized magnetism. We find the Raman response from FeSe to be more similar to localized systems such as cuprates. Together with simulations of a spin-1 Heisenberg model this furnishes evidence that FeSe hosts an almost fully frustrated system of essentially localized moments.