Itinerant electrons, local moments, and magnetic correlations in the pnictide superconductors CeFeAsO1-xFx and Sr(Fe1-xCox)2As2

A direct and element-specific measurement of the local Fe spin moment has been provided by analyzing the Fe 3s core level photoemission spectra in the parent and optimally doped CeFeAsO1-xFx (x = 0, 0.11) and Sr(Fe1-xCox)(2)As-2 (x = 0, 0.10) pnictides. The rapid time scales of the photoemission process allowed the detection of large local spin moments fluctuating on a 10(-15) s time scale in the paramagnetic, antiferromagnetic, and superconducting phases, indicative of the occurrence of ubiquitous strong Hund's magnetic correlations. The magnitude of the spin moment is found to vary significantly among different families, 1.3 mu(B) in CeFeAsO and 2.1 mu(B) in SrFe2As2. Surprisingly, the spin moment is found to decrease considerably in the optimally doped samples, 0.9 mu(B) in CeFeAsO0.89F0.11 and 1.3 mu(B) in Sr(Fe0.9Co0.1)(2)As-2. The strong variation of the spin moment against doping and material type indicates that the spin moments and the motion of itinerant electrons are influenced reciprocally in a self-consistent fashion, reflecting the strong competition between the antiferromagnetic superexchange interaction among the spin moments and the kinetic energy gain of the itinerant electrons in the presence of a strong Hund's coupling. By describing the evolution of the magnetic correlations concomitant with the appearance of superconductivity, these results constitute a fundamental step toward attaining a correct description of the microscopic mechanisms shaping the electronic properties in the pnictides, including magnetism and high-temperature superconductivity.