Spin-state crossover model for the magnetism of iron pnictides
Jiří Chaloupka and Giniyat Khaliullin
Accepted
We propose a minimal model describing magnetic behavior of Fe-based superconductors. The key ingredient of the model is a dynamical mixing of quasi-degenerate spin states of 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, $0\leq m\leq S_\mathrm{eff}$, 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.