NMR Investigation of the iron-based superconductors Ca4(Mg,Ti)3Fe2As2O8-y and Ca5(Sc,Ti)4Fe2As2O11-y

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⁷⁵As and ⁴⁵Sc NMR measurements unravel the electronic state for Fe-based superconductors with perovskite-type blocking layers Ca₄(Mg,Ti)₃Fe₂As₂O₈₋y (Tconset=47 K) and Ca₅(Sc,Ti)₄Fe₂As₂O₁₁₋y (Tconset=41 K). In Ca₅(Sc,Ti)₄Fe₂As₂O₁₁₋y, the nuclear spin relaxation rate 1/T₁ shows pseudogap behavior below 80 K, suggesting that the electronic state is similar to that of LaFeAs(O,F) system with moderate electron doping. The presence of the pseudogap behavior gives an interpretation that the hole-like band (so-called γ pocket) is located just below the Fermi level from the analogy to LaFeAs(O,F) system and the disappearance of the γ pocket yields the suppression of the low-energy spin fluctuations. On the other hand, in Ca₄(Mg,Ti)₃Fe₂As₂O₈₋y satisfying the structural optimal condition for higher Tc among the perovskite systems, the extrinsic contribution, which presumably originates in the Ti moment, is observed in 1/T₁T; however, the moderate temperature dependence of 1/T₁T appears by its suppression under high magnetic field. In both systems, the high Tc of 40 K is realized in the absence of the strong development of the low-energy spin fluctuations. The present results reveal that the structural optimization does not induce the strong development of the low-energy spin fluctuations. If we consider that superconductivity is mediated by spin fluctuations, the structural optimization is conjectured to provide a benefit to the development of the high-energy spin fluctuations irrespective to the low-energy part.