Successive transition from superconducting to antiferromagnetic phase in (Ca₆(Al, Ti)₄Oy)Fe₂As₂ studied via ⁷⁵As and ²⁷Al NMR

Abstract

An unusual successive phase transition from superconducting (SC) to antiferromagnetic (AF) phases was discovered via 75As and 27Al nuclear magnetic resonance (NMR) in (Fe2As2)(Ca6(Al, Ti)4Oy) with four (Al, Ti)O layers intercalated between FeAs planes. Although the spatially uniform AF ordering is clearly visible from 27Al spectra, the ordered moments are very small and the low-frequency fluctuation is much suppressed, contrary to existing pnictides with localized magnetic elements. Furthermore, the temperature (T) dependence of the fluctuation at both nuclei is very similar throughout the entire temperature range. These facts suggest that some hybridization between Ti and Fe orbitals induces a uniform electronic state within FeAs and (Al, Ti)O layers accompanied by the SC and AF transitions. The iron-based pnictide with Ti-doped blocking layers is the first high-Tc compound having metallic blocking layers.