Effect of residual stress on nematic domains in BaFe_2-xNi_xAs₂ studied by angular magnetoresistance

We have studied the angular magnetoresistance of iron pnictides BaFe_2-xNi_xAs₂, which shows clear 180 degree periodicity as fitted by a cosine function. In the x = 0.065 sample, the phase of the two-fold symmetry changes 90 degrees above the tetragonal-to-orthorhombic structural transition temperature T_s. Since the phase at low temperature is associated with the rotation of orthorhombic domains by magnetic field, we show that even vacuum grease can push the presence of orthorhombic domains at temperatures much higher than T_s. Our results suggest that residual stress may have significant effects in studying the nematic orders and its fluctuations in iron pnictides.