Dependence of the absolute value of the penetration depth in mathrm{mathbf{{(Ba_(1-x)K_x)Fe₂As₂}}} on doping

We report magnetic force microscopy (MFM) measurements on the iron-based superconductor $\mathrm{Ba_{1-x}K_xFe_2As_2}$. By measuring locally the Meissner repulsion with the magnetic MFM tip, we determine the absolute value of the in-plane magnetic penetration depth ($\lambda_{ab}$) in underdoped, optimally-doped, and overdoped samples. The results suggest an abrupt increase of $\lambda_{ab}$ as doping is increased from $x_\mathrm{opt}$, which is potentially related to the presence of a quantum critical point. The response of superconducting vortices to magnetic forces exerted by the MFM tip for $x=0.19$ and $0.58$ is compatible with previously observed structural symmetries at those doping levels.