In-plane anisotropic magnetoresistance in detwinned BaFe_2-xNi_xAs_2 (x = 0, 0.6)

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.