Antiferromagnetic order in CaK(Fe[1-x]Ni[x])4As4 and its interplay with superconductivity

TL;DR

This study reveals that CaK(Fe[1-x]Ni[x])4As4 exhibits a non-collinear hedgehog spin-vortex magnetic order that coexists and competes with superconductivity, highlighting complex magnetic interactions in iron arsenide superconductors.

Contribution

It reports the discovery of a non-collinear spin-vortex magnetic order in CaK(Fe[1-x]Ni[x])4As4 and its interplay with superconductivity, expanding understanding of magnetic structures in 1144 compounds.

Findings

Presence of a hedgehog spin-vortex crystal magnetic order.

Coexistence and competition between magnetic order and superconductivity.

Suppression of magnetic moments with increasing superconductivity.

Abstract

The magnetic order in CaK(Fe[1-x]Ni[x])4As4 (1144) single crystals (x = 0.051 and 0.033) has been studied by neutron diffraction. We observe magnetic Bragg peaks associated to the same propagation vectors as found for the collinear stripe antiferromagnetic (AFM) order in the related BaFe2As2 (122) compound. The AFM state in 1144 preserves tetragonal symmetry and only a commensurate, non-collinear structure with a hedgehog spin-vortex crystal (SVC) arrangement in the Fe plane and simple AFM stacking along the c direction is consistent with our observations. The SVC order is promoted by the reduced symmetry in the FeAs layer in the 1144 structure. The long-range SVC order coexists with superconductivity, however, similar to the doped 122 compounds, the ordered magnetic moment is gradually suppressed with the developing superconducting order parameter. This supports the notion that both collinear and non-collinear magnetism and superconductivity are competing for the same electrons coupled by Fermi surface nesting in iron arsenide superconductors.