Mapping spin-wave dispersions in stripe-ordered La2-xSrxNiO4 (x=0.275, 0.333) -: art. no. 064437

Using the MAPS spectrometer at the ISIS spallation source, we have measured the magnetic excitations of single-crystal samples of stripe-ordered La2-xSrxNiO4 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 J(1) and J(2) 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 J(1), but not J and J(2). Consistent with an analysis of previous measurements, we find that J is about 90% of the superexchange energy of undoped La2NiO4 and J(1)/J approximate to 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 similar to 20 meV, confirming a previous observation for x=1/3. We discuss the possible origin of this gap.