title: Spin-state crossover model for the magnetism of iron pnictides.
abstract: 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 Fe(2+) 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.
authors.affiliation: Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany and Central European Institute of Technology, Masaryk University, Kotlářská 2, 61137 Brno, Czech Republic.
authors.forename: Jiří
authors.initials: J
authors.lastname: Chaloupka
authors.affiliation: Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany.
authors.forename: Giniyat
authors.initials: G
authors.lastname: Khaliullin
mesh_terms: D005296:Ferrous Compounds / Q000737:chemistry*; D060328:Magnetic Phenomena; D008280:Magnetics / Q000379:methods*; D008956:Models, Chemical; D013696:Temperature
chemical_list: D005296:Ferrous Compounds