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: NaFeAs and BaFeAs. Despite their very different values of the ordered magnetic moment and Néel temperatures, , 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 , with no signatures of a precursor gap at temperatures between the orthorhombic and magnetic phase transitions in NaFeAs. We also find a relatively weak dispersion of the spin-wave gap in BaFeAs along the out-of-plane momentum component, . At the magnetic zone boundary (), 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, , as compared to previous estimates based on fitting the high-energy spin-wave dispersion to a Heisenberg-type model.
- Received 31 March 2012
DOI:https://doi.org/10.1103/PhysRevB.86.024437
©2012 American Physical Society