Room temperature antiferromagnetic order in superconducting X_yFe_{2-x}Se_ 2, (X= Rb, K): a powder neutron diffraction study

TL;DR

This study reveals that superconducting Rb- and K-doped FeSe compounds exhibit room temperature antiferromagnetic order with specific magnetic structures, confirmed by neutron diffraction, and identifies the magnetic irreducible representations fitting the data.

Contribution

The paper provides the first detailed neutron diffraction analysis of room temperature magnetic order in superconducting FeSe-based compounds, identifying specific magnetic irreducible representations and their associated structures.

Findings

Room temperature antiferromagnetic order observed in superconducting FeSe compounds.

Identified magnetic irreducible representations tau_2 and tau_7 fitting the neutron data.

Quantified magnetic moments on iron sites, around 2.1 to 2.6 μ_B.

Abstract

Magnetic and crystal structures of superconducting X yFe 2-xSe 2 (X= Rb and K with Tc=31.5 K and 29.5 K) have been studied by neutron powder diffraction at room temperature. Both crystals show ordered iron vacancy pattern and the crystal structure is well described in the I4/m space group with the lattice constants a=8.799, c=14.576 and a=8.730, c=14.115 A, and the refined stoichiometry x=0.30(1), y=0.83(2) and x=0.34(1), y=0.83(1) for Rb- and K-crystals respectively. The structure contains one fully occupied iron position and one almost empty vacancy position. Assuming that the iron moment is ordered only on the fully occupied site we have sorted out all eight irreducible representations (irreps) for the propagation vector k=0 and have found that irreps tau_2 and tau_7 well fit the experimental data with the moments along c-axis. The moment amplitudes amounted to 2.15(3) mu_B, 2.55(3) mu_B for tau_2 and 2.08(6) mu_B, 2.57(3) mu_B for tau_7 for Rb- and K-crystals respectively. Irrep tau_2 corresponds to the Shubnikov group I4/m' and gives a constant moment antiferromagnetic configuration, whereas tau_7 does not have Shubnikov counterpart and allows two different magnetic moments in the structure.