Multiple magnetic orders in LaFeAs1-xPxO uncover universality of iron-pnictide superconductors

Abstract

The iron-pnictide superconductors have generated tremendous excitement as the competition between magnetism and superconductivity has allowed unique in-roads towards elucidating a microscopic theory of unconventional high-temperature superconductivity. In addition to the stripe spin density wave ($C_{2M}^a$) phase observed in the parent compounds of all iron-pnictide superconductors, two novel magnetic orders have recently been discovered in different parent structures: an out-of-plane collinear double-Q ($C_{4M}^c$) structure in the hole-doped (Ca, Sr, Ba)_1-x(Na)_xFe₂As₂ and Ba_1-xK_xFe₂As₂ families, and a spin vortex crystal “hedgehog” ($C_{4M}^{ab}$) structure in the CaKFe₄As₄ family. Using neutron diffraction, we demonstrate that LaFeAs_1-xP_xO contains all three magnetic orders within a single-phase diagram as a function of substitution, all of which compete strongly with superconductivity. Our experimental observations combined with theoretical modeling demonstrate how the reduction in electronic correlations by chemical substitution results in larger Fermi surfaces and the sequential stabilization of multiple magnetic anisotropies. Our work presents a unified narrative for the competing magnetic and superconducting phases observed in various iron-pnictide systems with different crystal structures and chemistry.

The Fe-based superconductors are an ideal family of materials to investigate the mechanisms of unconventional superconductivity due to the co-existence of both magnetic and superconducting phases. Here, the authors experimentally demonstrate the presence of three magnetic phases in LaFeAs_1-xP_xO, which can be tuned as a function of phosphorus content, and discuss how the results connect to wider trends in magnetic and superconducting orders of the Fe-based superconductors.