The influence of the structural transition on magnetic fluctuations in NaFeAs

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".