Stripe Antiferromagnetism and Disorder in the Mott Insulator NaFe$_{1-x}$Cu$_{x}$As ($x \lesssim 0.5$)

TL;DR

This study reveals that heavily Cu-doped NaFe$_{1-x}$Cu$_{x}$As behaves as a Mott insulator with stripe antiferromagnetism and disorder caused by defects, with non-magnetic Cu inducing staggered magnetization at Fe sites.

Contribution

The paper provides detailed NMR analysis and numerical simulations showing how Cu doping induces magnetic order and disorder in NaFe$_{1-x}$Cu$_{x}$As, advancing understanding of Mott insulating behavior in this system.

Findings

NaFe$_{1-x}$Cu$_{x}$As exhibits stripe antiferromagnetism below a certain doping level.

Defects in magnetic stripes cause cluster spin-glass behavior.

Non-magnetic Cu dopants induce staggered magnetization at Fe sites.

Abstract

Neutron scattering measurements have demonstrated that the heavily Cu-doped NaFe$_{1-x}$Cu$_{x}$As compound behaves like a Mott insulator exhibiting both real space Fe-Cu stripes, as well as antiferromagnetism below a N\'eel temperature for $x\lesssim 0.5$. We have investigated evolution of structural and magnetic ordering using $^{23}$Na and $^{75}$As NMR for single crystals ($x$ = 0.39 and 0.48), confirming antiferromagnetism in the form of magnetic stripes. We show that end-chain defects in these stripes are the principal source of magnetic disorder and are responsible for cluster spin-glass transitions in both compounds, in the latter case coexistent with antiferromagnetism. Aided by our numerical simulation of the $^{75}$As spectra, we show that a staggered magnetization at the Fe sites is induced by non-magnetic Cu dopants.