Uniaxial pressure effect on structural and magnetic phase transitions in NaFeAs and its comparison with as-grown and annealed BaFe₂As ₂

We use neutron scattering to study the effect of uniaxial pressure on the tetragonal-to-orthorhombic structural (T_s) and paramagnetic-to- antiferromagnetic (T_N) phase transitions in NaFeAs and compare the outcome with similar measurements on as-grown and annealed BaFe ₂As₂. In previous work on as-grown BaFe₂As ₂, uniaxial pressure necessary to detwin the sample was found to induce a significant increase in zero pressure T_N and T_s. However, we find that similar uniaxial pressure used to detwin NaFeAs and annealed BaFe₂As₂ has a very small effect on their T _N and T_s. Since transport measurements on these samples still reveal resistivity anisotropy above T_N and T_s, we conclude that such anisotropy can not be due to uniaxial strain-induced T _N and T_s shifts, but must arise from intrinsic electronic anisotropy in these materials.