Single crystals of Eu Ca FeAs ([Formula: see text]) are grown using the high-temperature solution-growth method employing FeAs self-flux. Structural and chemical analysis indicates that these crystals are homogeneous and their lattice parameters exhibit a gradual monotonic decrease with increasing Ca concentration. Detailed magnetic, specific heat and resistivity data were used to construct a phase diagram which depicts the evolution of the structural/spin-density-wave transition at T , and of the antiferromagnetic (AFM) ordering temperature of the Eu moments at T. We found out that while T decreases monotonically from 19.1 K (for x  =  0) to below 2 K (for [Formula: see text]), T remains almost constant up to x  =  x and decreases steadily for higher values of x. Annealing at low temperatures for several days leads to enhancement of T and T by a few kelvin and sharpened the anomalies associated with these transitions. However, annealing did not change the variation of T and T across the series. The observation that T is almost constant until the long-range AFM ordering of the Eu moments gets destroyed, suggests a subtle interrelationship between the Eu and Fe magnetic sublattices.

We investigate the magnetic pair-breaking due to Mn impurities in the optimally electron doped Sr(Fe0.88Co0.12)2As2 superconductor to deduce the symmetry of the superconducting order parameter. Experiments on the as-grown crystals reveled a Tc suppression rate of ∼30 mK/µΩcm, which is in close agreement with similarly slower values of Tc suppression rates reported previously for various transition metal impurities, both, magnetic and non-magnetic, in several structurally analogous iron-based superconductors. However, careful annealing of these crystals at low temperature for longer durations reveals new information crucial to the determination of the pairing symmetry. We found that the crystallographic defects are a significant source of pair-breaking in the as-grown crystals. We first establish that these defects are point-like by showing that their sole effect on electrical transport is to add a temperature independent scattering term that shifts the whole ρ vs. T curves rigidly up. The Tc suppression rate due to these point-like defects is slow, ≤ 35 mK/µΩcm. On the other hand, Tc suppression rate due to magnetic pair-breaking is estimated to be faster than 325 mK/µΩcm. A slower pair-breaking rate (measured in mK/µΩcm) than expected due to non-magnetic crystallographic defects, together with a faster pair-breaking rate due to magnetic impurities disfavors a sign-changing s+−-wave and argues in the favor of a non-sign-changing s++-wave state in the optimally electron doped SrFe2As2 superconductor.

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