Abstract
We report a neutron-scattering study on Pr2−xSrxNiO4 (x≈0.5) with magnetic incommensurability ε=0.461 at 10 K, showing direct evidence for out-of-plane magnetic excitations caused by a non-negligible interlayer spin coupling (J⊥). The magnetic spectrum is fully represented by linear spin wave theory considering an unconventional body-centered-like stacking of discommensurated spin stripes. A large easy-plane-type single-ion anisotropy (≈1.5 meV) has been found to be responsible for a finite-energy (≈13 meV) Goldstone mode at the magnetic zone center. Our results validate the presence of a sizable out-of-plane interaction resulting in an anisotropic three-dimensional nature of spin dynamics in Pr2−xSrxNiO4 near the half-doped region.