High-Energy Spin Dynamics in La_1.69Sr_0.31NiO₄

To test the prediction that the dispersion of the magnetic resonance in superconducting YBa₂Cu₃O_6+x is similar to magnons in an incommensurate antiferromagnet, we have mapped out the spin dynamics in a stripe-ordered nickelate, La_2-xSr_xNiO₄, with x≃0.31, using inelastic neutron scattering. We observe spin-wave excitations up to 80 meV emerging from the incommensurate magnetic peaks with a surprisingly large and almost isotropic spin velocity: ℏc_s∼0.32   eV Å. A comparison indicates that the inferred spin-excitation spectrum is not, by itself, an adequate model for the magnetic resonance feature of the superconductor.