Figure 1

$\mathrm{TF}\mathrm{\text{−}}\mu \mathrm{SR}$ line shape of ${\mathrm{BaFe}}_{1.82}{\mathrm{Co}}_{0.18}{\mathrm{As}}_2$ at $H=0.5\mathrm{T}$ and $T=3.9\mathrm{K}$ (green circles). (a) The red curve is the Fourier transform of a fit in the time domain assuming Eq. (2). In addition to the indicated values of $\tilde{\lambda }$ and $\tilde{\xi }$, the fit yields $\sigma =0.265\mu {\mathrm{s}}^{-1}$ and a PM shift of 8.6 G. (b) Fourier transform of a fit that assumes the model of field-induced AFM order described in the main text (red curve). The fit yields $\sigma =0.251\mu {\mathrm{s}}^{-1}$ and a PM shift of 9.2 G. Other fit parameters are shown in Fig. 3.Reuse & Permissions

Figure 2

(a) Envelopes of $\mathrm{TF}\mathrm{\text{−}}\mu \mathrm{SR}$ spectra of ${\mathrm{BaFe}}_{1.82}{\mathrm{Co}}_{0.18}{\mathrm{As}}_2$ in the normal state at $T=23\mathrm{K}$. The solid curves are fits to a single exponential relaxation function $G(t)=\exp (-\Lambda t)$, yielding $\Lambda =0.081\pm 0.003\mu {\mathrm{s}}^{-1}$ and $\Lambda =0.119\pm 0.003\mu {\mathrm{s}}^{-1}$ at $H=0.02\mathrm{T}$ and $H=0.5\mathrm{T}$, respectively. (b) $\mathrm{TF}\mathrm{\text{−}}\mu \mathrm{SR}$ line shapes of ${\mathrm{BaFe}}_{1.82}{\mathrm{Co}}_{0.18}{\mathrm{As}}_2$ below $T_c$ at $H=0.02\mathrm{T}$. The dashed vertical line corresponds to $H$.Reuse & Permissions

Figure 3

Results of fits of $\mathrm{TF}\mathrm{\text{−}}\mu \mathrm{SR}$ time spectra of ${\mathrm{BaFe}}_{1.82}{\mathrm{Co}}_{0.18}{\mathrm{As}}_2$ at $H=0.5\mathrm{T}$, assuming the model of magnetic order described in the main text. Temperature dependence of (a) $1/{\tilde{\lambda }}^2$, (b) the depolarization rates $\sigma$ (Gaussian) and $\Lambda$ (exponential), (c) $B_{\mathrm{AFM}}$ and the ratio ${\xi }_{\mathrm{AFM}}/\tilde{\xi }$. Also shown in (a) are results of fits without magnetic order but with $\tilde{\xi }$ fixed to be 5 Å (blue circles).Reuse & Permissions

Figure 4

(a) Radial distribution function (RDF) of ${\mathrm{BaFe}}_{1.81}{\mathrm{Co}}_{0.19}{\mathrm{As}}_2$ at $H=0.5\mathrm{T}$ from Ref. 20 (dashed curve) and of the disordered VL shown in the lower right generated by molecular dynamics (MD). Note 5000 vortices were used in the MD simulation. The horizontal scale is normalized with respect to the intervortex spacing $a=691\AA$ of the perfect hexagonal VL. (b) Theoretical simulations of the $\mathrm{TF}\mathrm{\text{−}}\mu \mathrm{SR}$ line shape of the perfect VL (black curve) and of the disordered VL (red curve) corresponding to the RDF shown in (a). The green circles are the line shape of the perfect VL convoluted by a Gaussian distribution of fields, corresponding to $\sigma =1.9\mu {\mathrm{s}}^{-1}$. All three simulations assume $\tilde{\lambda }=2000\AA$ and $\tilde{\xi }=50\AA$. (c) Same Gaussian-broadened line shape in (b) and a Gaussian-broadened ideal line shape with $\tilde{\lambda }=1600\AA$, $\tilde{\xi }=100\AA$, and $\sigma =1.8\mu {\mathrm{s}}^{-1}$. The heights of the line shapes in (b) and (c) are normalized with respect to the height $n_{\max }(B)$ of the ideal line shape.Reuse & Permissions