Abstract
Inelastic neutron scattering is employed to investigate the impact of electronic nematic order on the
magnetic spectra of LaFeAsO and BaðFe0.953Co0.047
Þ
2As2. These materials are ideal to study the
paramagnetic-nematic state, since the nematic order, signaled by the tetragonal-to-orthorhombic transition
at TS, sets in well above the stripe antiferromagnetic ordering at TN. We find that the temperaturedependent
dynamic susceptibility displays an anomaly at TS followed by a sharp enhancement in the spinspin
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 or nematic
transition to magnetic fluctuations, and unveils the key role played by nematic order in promoting the longrange
stripe antiferromagnetic order in iron pnictides.
