PhD thesis defense of Kamila Komędera

Magnetism of iron-based superconductors parent compound investigated by Mӧssbauer Spectroscopy

This dissertation presents the results obtained by means of the ⁵⁷Fe and ¹⁵¹Eu Mössbauer spectroscopy as far as magnetic order is considered in the parent compounds of the iron-based superconductors. Different types of magnetic structures have been characterized. For the FeAs and FeSb compounds magnetism of the 3d electrons has itinerant character with varying admixture of the spin-polarized covalent bonds. The hyperfine fields on iron are restricted to the a–b plane and are likely to form spirals propagating along the c-axis and incommensurate with the lattice period along this direction. Anisotropy is strongly direction dependent of the magnetically polarizable bonds. Another unusual feature is strong coupling between magnetism and lattice dynamics i.e. very strong phonon–magnon interaction. In magnetic order temperature lattice hardening occurs. For the ‘122’ (Eu_1-xCaxFe₂As₂) and ‘1111’ (PrFeAsO) iron-arsenic compounds, a longitudinal spin density wave (SDW) with complex shape develops. Furthermore, in this compounds the rare earth moments remain localized and they are weakly coupled to SDW. Spin structure of BaFe2Se3 compound is quite complex – antiferromagnetic spin-ladders have been identified. Presence of two groups of iron sites seen by the Mössbauer spectroscopy in the magnetically ordered state is likely to reflect presence of two spin ladders within this structure.