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The magnetic moment enigma in Fe-based high temperature superconductors

The determination of the most appropriate starting point for the theoretical description of Fe-based materials hosting high temperature superconductivity remains among the most important unsolved problem in this relatively new field. Most of the work to date has focused on the pnictides, with LaFeAsO, BaFe2As2 and LiFeAs being representative parent compounds of three families known as 1111, 122 and 111, respectively. This Topic Review examines recent progress in this area, with particular emphasis on the implication of experimental data which have provided evidence for the presence of electron itinerancy and the detection of local spin moments. In light of the results presented, the necessity of a theoretical framework contemplating the presence and the interplay between itinerant electrons and large spin moments is discussed. It is argued that the physics at the heart of the macroscopic properties of pnictides Fe-based high temperature superconductors appears to be far more complex and interesting than initially predicted.