Muon spin rotation study of magnetism and superconductivity in BaFe2-xCoxAs2 and Pr1-xSrxFeAsO

We present a muon spin rotation (mu SR) study of the magnetic and superconducting properties of single crystals of electron-doped BaFe2-xCoxAs2 with x = 0(08, 0.20 and 0.25 (T-c = 9, 25 and 20 K) and of polycrystalline hole-doped Pr1-xSrxFeAsO with x = 0 and 0.2 (T-c = 15 K). In the former series, we observe some interesting parallels with the electron-doped SmFeAsO1-xFx 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 approximate to 25 K. In contrast, for the hole-doped Pr1-xSrxFeAsO samples we obtain evidence for a mesoscopic phase segregation into regions with nearly unperturbed antiferromagnetic (AF) order and others that are nonmagnetic and most likely superconducting. The observed trend resembles the one that was previously reported for hole-doped Ba1-xKxFe2As2 (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.