Magnetic Correlations in $\mathrmLa_2\mathrmNi\mathrmO_4+$δ$$
We report neutron-scattering experiments on single crystals of the layered perovskite La2NiO4+Î´. Three-dimensional magnetic order sets in at TN=70 K for Î´â0.05. Strong two-dimensional magnetic correlations exist for T between TN and the tetragonal-to-orthorhombic transition temperature, Tsâ240 K. As T is increased through Ts, the 2D correlation length decreases from more than 50 to 10 Ã. The excitation spectra for the 3D ordered state are characterized by large effective in-plane spin-wave velocities which, like TN, depend strongly on Î´. For Î´=0Â±0.001, câ³0.2 eV-Ã, while for Î´=0.05, câ0.1 eV-Ã.