  NaFeAs belongs to a class of Fe-based superconductors which parent compounds
show separated structural and magnetic transitions. Effects of the structural
transition on spin dynamics therefore can be investigated separately from the
magnetic transition. A plateau in dynamic spin response is observed in a
critical region around the structural transition temperature T_S. It is
interpreted as due to the stiffening of spin fluctuations along the in-plane
magnetic hard axis due to the dxz and dyz orbital ordering. The appearance of
anisotropic spin dynamics in the critical region above the T_S at T* offers a
dynamic magnetic scattering mechanism for anisotropic electronic properties in
the commonly referred "nematic phase".
