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 ${\mathrm{Fe}}^{2+}$ ion by intersite electron hoppings, resulting in an effective local spin $S_{\mathrm{eff}}$. The moments $S_{\mathrm{eff}}$ 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 $S_{\mathrm{eff}}$ varies widely, $0\le m\le S_{\mathrm{eff}}$, but magnon spectra are universal and scale with $S_{\mathrm{eff}}$, resolving the puzzle of large but fluctuating Fe moments. Unusual temperature dependences of a local moment and spin susceptibility are also explained.