Magnetic Correlations inLa2NiO4+δ

Abstract

We report neutron-scattering experiments on single crystals of the layered perovskite ${\mathrm{La}}_2\mathrm{Ni}{\mathrm{O}}_{4+\delta }$. Three-dimensional magnetic order sets in at $T_N=70\mathrm{}$ K for $\delta \cong 0.05$. Strong two-dimensional magnetic correlations exist for $T$ between $T_N$ and the tetragonal-to-orthorhombic transition temperature, $T_s\approx 240$ K. As $T$ is increased through $T_s$, the 2D correlation length decreases from more than 50 to 10 Å. The excitation spectra for the 3D ordered state are characterized by large effective in-plane spin-wave velocities which, like $T_N$, depend strongly on $\delta$. For $\delta =0\mathrm{}\pm 0.001$, $c\gtrsim 0.2$ eV-Å, while for $\delta =0.05\mathrm{}$, $c\approx 0.1$ eV-Å.