Cooperative order in the weakly magnetic domain of LaFeAsO1-xFx near the doping phase boundary

Spatial phase separation into mesoscopic domains of non-magnetic [superconducting (SC) below Tc ≅ 25.5 K] and two kinds of magnetic phases, one showing disordered spin density wave (d-SDW) order and another associated with glassy weak magnetism (WM), are observed below ∼100 K by muon spin rotation (μSR) in LaFeAsO1−xFx for x = 0.057(3), which is near the boundary of these phases on the doping phase diagram. In contrast to the competing order observed in the regular SDW phase of Ba(Fe1−xCox)2As2, the WM domain exhibits cooperative coupling of superconducting and magnetic order parameters as inferred from strong diamagnetism and the associated negative shift of μSR frequency just below Tc. (author)