Local Magnetic Inhomogeneities in Lightly Doped BaFe$_2$As$_2$

TL;DR

This study uses $^{75}$As NMR to investigate how Ni doping affects magnetic order and inhomogeneities in BaFe$_2$As$_2$, revealing local amplitude variations near dopants.

Contribution

It demonstrates that local magnetic inhomogeneities in lightly doped BaFe$_2$As$_2$ can be explained by a model of commensurate order with local amplitude variations.

Findings

Spin lattice relaxation rate diverges at $T_N$ with 3D critical behavior.

Spectra broaden inhomogeneously with doping.

Ordered moments vary locally near dopants.

Abstract

We report $^{75}$As NMR measurements in BaFe$_2$As$_2$ doped with Ni. Like Co, Ni doping suppresses the antiferromagnetic and structural phase transitions and gives rise to superconductivity for sufficiently large Ni doping. The spin lattice relaxation rate diverges at $T_N$, with a critical exponent consistent with 3D ordering of local moments. In the ordered state the spectra quickly broaden inhomogeneously with doping. We extract the average size of the ordered moment as a function of doping, and show that a model in which the order remains commensurate but with local amplitude variations in the vicinity of the dopant fully explains our observations.