  High-resolution single crystal neutron diffraction measurements are presented
probing the magnetostructural response to uniaxial pressure in the iron
pnictide parent system BaFe2As2. Scattering data reveal a strain-activated,
anisotropic broadening of nuclear Bragg reflections, which increases upon
cooling below the resolvable onset of global orthorhombicity. This anisotropy
in lattice coherence continues to diverge until a lower temperature scale---the
first-order onset of antiferromagnetism---is reached. Our data suggest that
antiferromagnetism and strong magnetoelastic coupling drive the
strain-activated lattice response in this material and that the development of
anisotropic lattice coherence under strain is the physical origin for the
anomalous nematic anisotropy in this compound.
