RKKY coupled local moment magnetism in NaFe$_{1-x}$Cu$_{x}$As

TL;DR

This study provides direct experimental evidence of RKKY interaction in NaFe$_{1-x}$Cu$_{x}$As using high-resolution NMR and simulations, revealing coexistence of local and itinerant magnetism in lightly Cu-doped pnictides.

Contribution

The paper demonstrates the presence of RKKY interaction in a pnictide system through combined NMR measurements and simulations, which was previously lacking evidence.

Findings

Identification of RKKY oscillations via NMR satellite resonances

Evidence of coexistence of local and itinerant magnetism

Signature of an inhomogeneous antiferromagnetic phase

Abstract

A central question in a large class of strongly correlated electron systems, including heavy fermion compounds and iron pnictides, is the identification of different phases and their origins. It has been shown that the antiferromagnetic (AFM) phase in some heavy fermion compounds is induced by Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between localized moments, and that the competition between this interaction and Kondo effect is responsible for quantum criticality. However, conclusive experimental evidence of the RKKY interaction in pnictides is lacking. Here, using high resolution $^{23}$Na NMR measurements on lightly Cu-doped metallic single crystals of NaFe$_{1-x}$Cu$_{x}$As ~($x \approx 0.01$) and numerical simulation, we show direct evidence of the RKKY interaction in this pnictide system. Aided by computer simulation, we identify the $^{23}$Na NMR satellite resonances with the RKKY oscillations of spin polarization at Fe sites. Our $^{23}$Na spin-lattice and spin-spin relaxation data exhibits signature of an itinerant and inhomogeneous AFM phase in this system, accompanied by a simulation of Cu-induced perturbation to the ordered moments on the Fe sites. Our NMR results indicate coexistence of local and itinerant magnetism in lightly Cu-doped NaFe$_{1-x}$Cu$_{x}$As.