Antiferromagnetic Order and Superconductivity in Sr4(Mg0.5-xTi0.5+x)2O6Fe2As2 with Electron Doping: 75As-NMR Study

We report an 75 As-NMR study on iron (Fe)-based superconductors with thick perovskite-type blocking layers Sr 4 (Mg 0.5- x Ti 0.5+ x ) 2 O 6 Fe 2 As 2 with x =0 and 0.2. We have found that antiferromagnetic (AFM) order takes place when x =0, and superconductivity (SC) emerges below T c =36 K when x =0.2. These results reveal that the Fe-pnictides with thick perovskite-type blocks also undergo an evolution from the AFM order to the SC by doping electron carriers into FeAs planes through the chemical substitution of Ti +4 ions for Mg +2 ions, analogous to the F-substitution in LaFeAsO compound. The normal-state property for x =0.2 unravels the suppression of AFM spin fluctuations at low energies, in contrast with the related Ca 4 Al 2 O 6 Fe 2 As 2 with T c =27 K, suggesting that the AFM spin fluctuations are not a unique factor for enhancing T c . A condition for optimizing SC should be addressed from the relation with the local tetrahedron structure of FeAs 4 .