Muon spin rotation study of magnetism and superconductivity in BaFe2−xCoxAs2and Pr1−xSrxFeAsO

We present a muon spin rotation (μSR) study of the magnetic and superconducting properties of single crystals of electron-doped BaFe 2− x Co x As 2 with x = 0.08, 0.20 and 0.25 ( T c = 9, 25 and 20 K) and of polycrystalline hole-doped Pr 1− x Sr x FeAsO with x = 0 and 0.2 ( T c = 15 K). In the former series, we observe some interesting parallels with the electron-doped SmFeAsO 1− x F x 1111-type system (Drew et al 2009 Nat. Mater. 8 310). In particular, we obtain evidence that strongly disordered static magnetism coexists with superconductivity on a microscopic scale in underdoped samples and even at optimum doping there is a slowing down (or enhancement) of dynamic magnetic correlations below T c ≈25 K. In contrast, for the hole-doped Pr 1− x Sr x FeAsO samples we obtain evidence for a mesoscopic phase segregation into regions with nearly unperturbed antiferromagnetic (AF) order and others that are non-magnetic and most likely superconducting. The observed trend resembles the one that was previously reported for hole-doped Ba 1− x K x Fe 2 As 2 (Aczel et al 2008 Phys. Rev. B 78 214503, Park et al 2009 Phys. Rev. Lett. 102 117006) and thus might be a common property of the hole-doped systems.