Magnetic correlations in La sub 2 NiO sub (4+delta).

We report elastic and inelastic neutron scattering experiments on single crystals of the layered perovskite La sub 2 NiO sub (4+delta). Three dimensional magnetic order sets in at T sub N = 70K for delta~=0.05. Strong two-dimensional magnetic correlations exist for T between T sub N and the tetragonal to orthorhombic transition temperature, T sub s ~ 240K. As T is increased through T sub s, from 228 to 246K, there is a rapid decrease in the 2-d correlation length, from more than 50angstroms to 10angstroms. The excitation spectra for the 3-d ordered state are characterized by relatively small (2-3 meV) anisotropy gaps, and large effective in-plane spin wave velocities which, like T sub N, depend strongly on delta: for delta = 0+-0.001, c > 0.2 eV- angstroms, while for delta = 0.05, c ~0.1eV- angstroms.