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The in-plane resistivity anisotropy has been measured for detwinned single crystals of Ba(Fe1−xNix)2As2 and Ba(Fe1−xCux)2As2. The data reveal a nonmonotonic doping dependence, similar to previous observations for Ba(Fe1−xCox)2As2. Magnetotransport measurements of the parent compound reveal a nonlinear Hall coefficient and a large linear term in the transverse magnetoresistance. Both effects are rapidly suppressed with chemical substitution over a similar compositional range as the onset of the large in-plane resistivity anisotropy. This suggests that the relatively small in-plane anisotropy of the parent compound in the spin-density wave state is due to the presence of an isotropic, high mobility pocket of the reconstructed Fermi surface. Progressive suppression of the contribution to the conductivity arising from this isotropic pocket with chemical substitution eventually reveals the underlying in-plane anisotropy associated with the remaining Fermi surface pockets.

We present results of angle-dependent magnetoresistance measurements and direct optical images of underdoped $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$ which reveal partial detwinning by action of a 14 T magnetic field. The relative change in the twin domain population for the given field is modest, of order $5--15\text{ }\mathrm{%}$. The associated field-induced hysteretic changes in the resistivity are up to 5% for intermediate Co concentrations, implying a large in-plane resistivity anisotropy in the broken-symmetry state. Based on the generic anisotropic susceptibility of collinear antiferromagnets, we infer a smaller resistivity along the antiferromagnetic ordering direction. The observation of field-induced motion of twin boundaries indicates a substantial magnetoelastic coupling in this material.