Superfluid Density and Field-Induced Magnetism in Ba(Fe1-xCox)2As2 and Sr(Fe1-xCox)2As2 Measured with Muon Spin Relaxation

TL;DR

This study uses muon spin rotation to investigate superfluid density and field-induced magnetism in doped BaFe2As2 and SrFe2As2, revealing a correlation between superfluid density and transition temperature, and detecting doping-dependent internal magnetic fields.

Contribution

First muon spin rotation measurements of these compounds showing how superfluid density and induced magnetism vary with doping and transition temperature.

Findings

Superfluid density correlates with T_C in doped compounds.

Field-induced internal magnetic fields are present across all doping levels.

Induced magnetic fields decrease with increased doping.

Abstract

We report muon spin rotation ($\mu$SR) measurements of single crystal Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$. From measurements of the magnetic field penetration depth $\lambda$ we find that for optimally- and over-doped samples, $1/\lambda(T\to 0)^2$ varies monotonically with the superconducting transition temperature T$_{\rm C}$. Within the superconducting state we observe a positive shift in the muon precession signal, likely indicating that the applied field induces an internal magnetic field. The size of the induced field decreases with increasing doping but is present for all Co concentrations studied.