$\mu$SR Studies of Electron-doped 122 Pnictide Superconductors

Single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ were studied using Transverse field (TF)-$\mu$SR. We measured the penetration depth in the superconducting state by fitting the $\mu$SR spectra to a microscopic model for a range of applied magnetic fields and dopings. We have fit the temperature dependence of the superfluid density to a phenomenological two-gap model. We find that the low-temperature superfluid density varies roughly linearly with superconducting T$_C$, both of which decrease with increasing Co substitution above x = 0.06.