We report an inelastic neutron scattering study on the magnetic excitations of ${\mathrm{Pr}}_2{\mathrm{NiO}}_{4+\delta }$ ($\delta \sim 0.24\pm 0.01$) at $T=10$ K. Spin stripe ordering becomes pronounced below $\sim 220$ K with an incommensurability $\epsilon \approx 0.346$, and a strong influence of interstitial oxygen is identified on establishing a long-range spin stripe ordering. Apart from the Goldstone modes emerging from the magnetic satellites (${\mathbf{q}}_m$), multiple homologous modes are observed along the spin stripe modulation separated by $\mathrm{\Delta }{\mathbf{q}}_m\approx 0.076$ in reciprocal lattice units, which is interpreted by the internal periodicity of the long-range ordered discommensurated spin stripes.

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• Magnetic order
• Spin dynamics
• Stripes
• Strongly correlated systems
• Inelastic neutron scattering