  Understanding the magnetoresistance (MR) of a magnetic material forms the
basis for uncovering the orbital mechanisms and charge-spin interactions in the
system. Although the parent state of iron-based high-temperature
superconductors, including $BaFe_2As_2$, exhibits unusual electron transport
properties resulting from spin and charge correlations, there is still valuable
insight to be gained by understanding the in-plane MR effect due to twin
domains in the orthorhombic antiferromagnetic (AF) ordered state. Here, we
study the in-plane magnetoresistance anisotropy in detwinned $BaFe_2As_2$ and
compare the results to the non-magnetic Ni-doped sample. We find that in the
antiferromagnetically ordered state, $BaFe_2As_2$ exhibits anisotropic MR that
becomes large at low temperatures and high fields. Both transverse and
longitudinal MRs are highly anisotropic and dependent on the field and current
orientations. These results cannot be fully explained by calculations
considering only the anisotropic Fermi surface. Instead, the spin orientation
of the ordered moment also affects the MR effect, suggesting the presence of a
large charge-spin interaction in $BaFe_2As_2$ that is not present in the
Ni-doped material.
