In-plane spin excitation anisotropy in the paramagnetic state of NaFeAs

We use unpolarized and polarized inelastic neutron scattering to study low-energy spin excitations in NaFeAs, which exhibits a tetragonal-to- orthorhombic lattice distortion at T_s≈58 K followed by a collinear antiferromagnetic (AF) order below T_N≈45 K. In the AF ordered state (T<T_N), spin waves are entirely c-axis polarized below ∼10 meV, exhibiting a gap of ∼4 meV at the AF zone center and disperse to ∼7 meV near the c-axis AF zone boundary. On warming to the paramagnetic state with orthorhombic lattice distortion (T_N<T<T _s), spin excitations become anisotropic within the FeAs plane. Upon further warming to the paramagnetic tetragonal state (T>T_s), spin excitations become more isotropic. Since similar magnetic anisotropy is also observed in the paramagnetic tetragonal phase of superconducting BaFe _1.904Ni_0.096As₂, our results suggest that the spin excitation anisotropy in superconducting iron pnictides originates from similar anisotropy already present in their parent compounds.