Spin-State Crossover Model for the Magnetism of Iron Pnictides

We propose a minimal model describing magnetic behavior of Fe-based superconductors. The key ingredient of the model is a dynamical mixing of quasidegenerate spin states of Fe2+ ion by intersite electron hoppings, resulting in an effective local spin S-eff. The moments S-eff tend to form singlet pairs and may condense into a spin nematic phase due to the emergent biquadratic exchange couplings. The long-range ordered part m of S-eff varies widely, 0 <= m <= S-eff, but magnon spectra are universal and scale with S-eff, resolving the puzzle of large but fluctuating Fe moments. Unusual temperature dependences of a local moment and spin susceptibility are also explained.