Local measurement of the penetration depth in the pnictide superconductor Ba(Fe${}_{0.95}$Co${}_{0.05}$)${}_2$As${}_2$

We use magnetic force microscopy (MFM) and scanning SQUID susceptometry to measure the local superfluid density ${\rho }_s$ in Ba(Fe${}_{0.95}$Co${}_{0.05}$)${}_2$As${}_2$ single crystals from 0.4 K to the critical temperature $T_c=18.5$ K. We observe that the penetration depth $\lambda$ varies about ten times more slowly with temperature than previously published, with a dependence that can be well described by a clean two-band fully gapped model. We demonstrate that MFM can measure the important and hard-to-determine absolute value of $\lambda$, as well as obtain its temperature dependence and spatial homogeneity. We find ${\rho }_s$ to be uniform despite the highly disordered vortex pinning.