Gapless Magnetic Excitation In A Heavily Electron-Doped Antiferromagnetic Phase Of Lafeaso0.5d0.5

Magnetic excitations in a heavily electron-doped antiferromagnet, LaFeAsO0.5D0.5, have been investigated using powder inelastic neutron scattering. Unlike other parent compounds of the iron-based superconductors, the magnetic excitation gap in LaFeAsO0.5D0.5 was not detected down to the lowest measured temperature of 4 K. This result can be understood as a result of quasi-isotropy within the ab plane, which is consistent with the band calculation result that the d(xy) orbital plays the dominant role in the magnetism of LaFeAsO0.5D0.5. In addition, the intensities of the magnetic excitations in this phase are much stronger than those in nondoped LaFeAsO. Even in the paramagnetic phase, the magnetic excitation in LaFeAsO0.5D0.5 persists. These results corroborate recent studies showing that the electron doping enhances the localized nature in this system.