Muon spin rotation study of magnetism and superconductivity in BaFe₂₋<i>_x</i>Co<i>_x</i>As₂and Pr₁₋<i>_x</i>Sr<i>_x</i>FeAsO

Abstract

We present a muon spin rotation (\\muSR) study of the magnetic and\nsuperconducting properties of single crystals of electron-doped BaFe2-xCoxAs2\nwith x=0.08, 0.20, and 0.25 (Tc=9, 25 and 20K) and of polycrystalline\nhole-doped Pr1-xSrxFeAsO with x=0 and 0.2 (Tc=15 K). In the former series we\nobserve some interesting parallels with the electron doped SmFeAsO1-xFx\n1111-type system [A.J. Drew et al., to appear in Nature Materials 2009 and\narXiv:0807.4876]. In particular, we obtain evidence that strongly disordered\nstatic magnetism coexists with superconductivity on a microscopic scale in\nunderdoped samples and even at optimum doping there is a slowing down (or\nenhancement) of dynamic magnetic correlations below Tc\\approx25K. To the\ncontrary, for the hole-doped Pr1-xSrxFeAsO samples we obtain evidence for a\nmesoscopic phase segregation into regions with nearly unperturbed AF order and\nothers that are non magnetic and most likely superconducting. The observed\ntrend resembles the one that was previously reported for hole-doped\nBa1-xKxFe2As2 [A.A. Aczel et al., Phys. Rev. B 78, 214503 (2008); J.T. Park et\nal., arXiv:0811.2224] and thus seems to be fairly common in these hole doped\nsystems.\n