  Inelastic neutron scattering was employed to investigate the impact of
electronic nematic order on the magnetic spectra of LaFeAsO and
Ba(Fe$_{0.953}$Co$_{0.047}$)$_{2}$As$_{2}$. These materials are ideal to study
the paramagnetic-nematic state, since the nematic order, signaled by the
tetragonal-to-orthorhombic transition at $T_{{\rm S}}$, sets in well above the
stripe antiferromagnetic ordering at $T_{{\rm N}}$. We find that the
temperature-dependent dynamic susceptibility displays an anomaly at $T_{{\rm
S}}$ followed by a sharp enhancement in the spin-spin correlation length,
revealing a strong feedback effect of nematic order on the low-energy magnetic
spectrum. Our findings can be consistently described by a model that attributes
the structural/nematic transition to magnetic fluctuations, and unveils the key
role played by nematic order in promoting the long-range stripe
antiferromagnetic order in iron pnictides.
