Successive transition from superconducting to antiferromagnetic phase in (Ca_6(Al, Ti)_4O_y)Fe_2As_2 studied via ^{75}As and ^{27}Al NMR

TL;DR

This study reveals a unique phase transition sequence from superconducting to antiferromagnetic states in a Ti-doped iron pnictide, highlighting hybridization effects and metallic blocking layers.

Contribution

It reports the first high-T_c iron pnictide with metallic blocking layers and details the unusual phase transition behavior observed via NMR.

Findings

Successive SC to AF phase transition observed.

Small ordered moments and suppressed fluctuations in AF phase.

Hybridization between Ti and Fe orbitals induces uniform electronic states.

Abstract

An unusual successive phase transition from superconducting (SC) to antiferromagnetic (AF) phases was discovered via ^{75}As and ^{27}Al nuclear magnetic resonance (NMR) in (Fe_2As_2)(Ca_6(Al, Ti)_4O_y) with four (Al, Ti)O layers intercalated between FeAs planes. Although the spatially-uniform AF ordering is clearly visible from ^{27}Al 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-T_c compound having metallic blocking layers.