Angular dependence of magneto-transport properties in Ba$_{1-x}$K$_{x}$(FeAs)$_{2}$ and Ba (Fe$_{1-x}$Co$_{x})_{2}$As$_{2 }$single crystals

We find unexpectedly sharp minima in the angular dependence of the flux-flow resistance of a series of Ba$_{1-x}$K$_{x}$(FeAs)$_{2}$ and Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2 }$crystals when the magnetic field is applied parallel to the FeAs-planes. These minima are too sharp to be accounted for by effective mass anisotropy. Furthermore, since the c-axis coherence length is substantially larger than the FeAs layer spacing, intrinsic pinning mechanisms are ruled out. However, high-resolution cross-sectional SEM reveal the presence of plate-like inclusions of 50 - 100 nm thickness, which in a natural way can account for the enhanced pinning for H $\vert \vert $ ab. On crystals that have been irradiated with heavy ions along the c-axis we observe additional resistance minima for H $\vert \vert $ c, indicating the correlated nature of the irradiation induced defects.