Similar zone-center gaps in the low-energy spin-wave spectra of Na1−δFeAs and BaFe2As2

Abstract

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${}_{1\ensuremath{-}\ensuremath{\delta}}$FeAs and BaFe${}_{2}$As${}_{2}$. Despite their very different values of the ordered magnetic moment and N\'eel temperatures, ${T}_{\mathrm{N}}$, 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 ${T}_{\mathrm{N}}$, with no signatures of a precursor gap at temperatures between the orthorhombic and magnetic phase transitions in Na${}_{1\ensuremath{-}\ensuremath{\delta}}$FeAs. We also find a relatively weak dispersion of the spin-wave gap in BaFe${}_{2}$As${}_{2}$ 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 $\ensuremath{\sim}$20 meV, which implies a much smaller value of the effective out-of-plane exchange interaction, ${J}_{\mathrm{c}}$, as compared to previous estimates based on fitting the high-energy spin-wave dispersion to a Heisenberg-type model.