Similar zone-center gaps in the low-energy spin-wave spectra of Na _1-δFeAs and BaFe ₂As ₂

We report results of inelastic-neutron-scattering measurements of low-energy spin-wave excitations in two structurally distinct families of iron-pnictide parent compounds: Na _1-δFeAs and BaFe ₂As ₂. Despite their very different values of the ordered magnetic moment and Néel temperatures, T _N, in the antiferromagnetic state both compounds exhibit similar spin gaps of the order of 10 meV at the magnetic Brillouin-zone center. The gap opens sharply below T _N, with no signatures of a precursor gap at temperatures between the orthorhombic and magnetic phase transitions in Na _1-δFeAs. We also find a relatively weak dispersion of the spin-wave gap in BaFe ₂As ₂ along the out-of-plane momentum component, q _z. At the magnetic zone boundary (q _z=0), spin excitations in the ordered state persist down to ∼20 meV, which implies a much smaller value of the effective out-of-plane exchange interaction, J _c, as compared to previous estimates based on fitting the high-energy spin-wave dispersion to a Heisenberg-type model.