Superfluid density and field-induced magnetism in Ba(Fe1−xCox)2As2 and Sr(Fe1−xCox)2As2 measured with muon spin relaxation

We report muon spin rotation (μSR) measurements of single-crystal Ba(Fe1−xCox)2As2 and Sr(Fe1−xCox)2As2. From measurements of the magnetic field penetration depth λ we find that for optimally and overdoped samples, 1/λ(T→0)2 varies monotonically with the superconducting transition temperature TC. Within the superconducting state we observe a positive shift in the muon precession signal, likely indicating that the applied field induces an internal magnetic field. The size of the induced field decreases with increasing doping but is present for all Co concentrations studied.