Muon-spin-rotation study of the magnetic structure in the tetragonal antiferromagnetic state of weakly underdoped Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $

TL;DR

This study uses muon spin rotation to investigate the magnetic structure transition in underdoped Ba$_{1-x}$K$_x$Fe$_2$As$_2$, revealing an inhomogeneous double-Q magnetic state with c-axis moments and zero magnetic moments on half of the Fe sites.

Contribution

First muon spin rotation analysis identifying the magnetic structure of the tetragonal antiferromagnetic state in this compound.

Findings

Identified the magnetic structure as inhomogeneous double-Q with c-axis moments.

Detected changes in magnetic field magnitude and orientation at muon sites.

Confirmed vanishing magnetic moments on half of the Fe sites.

Abstract

With muon spin rotation ($ \mu $SR) we studied the transition between the orthorhombic antiferromagnetic (o-AF) and the tetragonal antiferromagnetic (t-AF) states of a weakly underdoped Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $ single crystal. We observed some characteristic changes of the magnitude and the orientation of the magnetic field at the muon site which, due to the fairly high point symmetry of the latter, allow us to identify the magnetic structure of the t-AF state. It is the so-called, inhomogeneous double-$\mathbf{Q}$ magnetic structure with $ c $-axis oriented moments which has a vanishing magnetic moment on half of the Fe sites.