Neutron diffraction measurements are presented measuring the responses of both magnetic and structural order parameters of parent and lightly Co-doped Ba(${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$)${}_2{\mathrm{As}}_2$ under the application of uniaxial pressure. We find that the uniaxial pressure induces a thermal shift in the onset of antiferromagnetic order that grows as a percentage of $T_N$ as Co doping is increased and the superconducting phase is approached. Additionally, as uniaxial pressure is increased within parent and lightly doped Ba(${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$)${}_2{\mathrm{As}}_2$ on the first-order side of the tricritical point, we observe a decoupling between the onsets of the orthorhombic structural distortion and antiferromagnetism. Our findings place needed constraints on models exploring the nematic susceptibility of the bilayer pnictides in the tetragonal, paramagnetic regime.

• Received 10 April 2014
• Revised 23 May 2014

DOI:https://doi.org/10.1103/PhysRevB.89.214404