Spin dynamics in stripe-orderedLa5/3Sr1/3NiO4

Polarized- and unpolarized-neutron inelastic scattering has been used to measure the spin excitations in the striped spin-charge-ordered phase of ${\mathrm{La}}_{5/3}{\mathrm{Sr}}_{1/3}{\mathrm{NiO}}_4.$ Above 30 meV, sharp magnetic modes are observed characteristic of a static stripe lattice. This part of the energy spectrum is described well by a linear spin-wave model with intrastripe and interstripe exchange interactions between neighboring Ni spins given by $J=15\mathrm{}\pm 1.5\mathrm{meV}$ and $J^{\prime }=7.5\mathrm{}\pm 1.5\mathrm{meV},$ respectively. A pronounced suppression and broadening of the magnetic fluctuations in a band between 10 meV and 25 meV is suggestive of coupling to collective motions of the stripe domain walls.