Origin of the dynamics of the spin state in undoped BaFe2As2: Mössbauer studies.

Fe-As based superconductors provide a good system for understanding the relationship between magnetism and superconductivity. Considerable efforts have been expended in understanding the magnetic behavior of the parent compound, BaFe2As2. However, it had not been realized that traces of adsorbed O2 by the material bring about drastic changes in its magnetic behavior. O2 is known to trap electrons, forming O2(-). Guided by this discovery, we observe in the absence of O2 a dynamic transition between intermediate and low spin states of Fe and hysteresis effects, and in the presence of O2 trapping of the magnetic state. These observations are likely to have a bearing on the role of magnetism in superconductivity.