  Critical current density was studied in single crystals of
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ for the values of $x$ spanning the entire doping
phase diagram. A noticeable enhancement was found for slightly underdoped
crystals with the peak at $x = 0.058$. Using a combination of polarized-light
imaging, x-ray diffraction and magnetic measurements we associate this behavior
with the intrinsic pinning on structural domains in the orthorhombic phase.
Domain walls extend throughout the sample thickness in the direction of
vortices and act as extended pinning centers. With the increasing $x$ domain
structure becomes more intertwined and fine due to a decrease of the
orthorhombic distortion. This results in the energy landscape with maze-like
spatial modulations favorable for pinning. This finding shows that iron-based
pnictide superconductors, characterized by high values of the transition
temperature, high upper critical fields, and low anisotropy may intrinsically
have relatively high critical current densities.

Superconductivity;Strongly Correlated Electrons