  Polarized- and unpolarized-neutron scattering measurements of the spin
excitation spectrum in the stripe-ordered phase of La2NiO4+d (d = 0.11) are
presented. At low energies, the magnetic spectral weight is found to shift
anomalously towards the two-dimensional antiferromagnetic wave vector, similar
to the low-energy dispersions observed in cuprate superconductors. While
spin-wave spectra in stripe phases can exhibit an apparent inward dispersion,
we find that the peak shifts measured here cannot be accounted for by this
effect. Possible extensions of the model are discussed.
