Mapping spin-wave dispersions in stripe-ordered La_2−xSr_xNiO₄ (x=0.275, 0.333)

Using the MAPS spectrometer at the ISIS spallation source, we have measured the magnetic excitations of single-crystal samples of stripe-ordered La_2−xSr_xNiO₄ with x=0.333 and 0.275. The full two-dimensional spin-wave dispersions were obtained using incident energies of 60 and 160 meV. To analyze the excitations, we have evaluated a spin-only Hamiltonian describing diagonal, site-centered stripes in the linear spin-wave approximation. Besides the superexchange energy J within antiferromagnetic domains, we have considered effective exchange couplings J₁ and J₂ across a charge stripe coupling second-neighbor Ni sites along Ni-O bond directions and along the plaquette diagonal, respectively. From least-squares fits of the model to the measurements on the x=1∕3 sample at T=10 K, we find that the dispersions are well described by a model using just J and J₁, but not J and J₂. Consistent with an analysis of previous measurements, we find that J is about 90% of the superexchange energy of undoped La₂NiO₄ and J₁∕J≈0.5. The excitations observed for x=0.275 are surprisingly similar to those for x=1∕3, despite the differing magnetic-ordering wave vectors; the main difference is a broadening of the excitations for x=0.275. For both samples, we find that one spin-wave branch has a gap of ∼20 meV, confirming a previous observation for x=1∕3. We discuss the possible origin of this gap.