Magnetic correlations and their dependence on excess oxygen inLa2NiO4+δ

We report results of elastic- and inelastic-neutron-scattering studies of three single crystals of the layered perovskite ${\mathrm{La}}_2$ ${\mathrm{NiO}}_{4+\mathrm{\delta }}$, with δ=0.00, 0.067, and 0.077, as well as neutron powder diffraction on a material with an oxygen excess δ≊0.065. The magnetic correlations are highly sensitive to the oxygen content, and three-dimensional antiferromagnetic order sets in at ${\mathit{T}}_{\mathit{N}}$≥300 K, and ${\mathit{T}}_{\mathit{N}}$=68, and 48 K for the single crystals, respectively, and at ${\mathit{T}}_{\mathit{N}}$=74 K for the powder sample. The crystal with δ=0.067 was studied in detail to characterize the tetragonal to orthorhombic (${\mathit{T}}_{\mathit{s}}$=232–240 K) and antiferromagnetic (${\mathit{T}}_{\mathit{N}}$=68 K) transitions. The order parameter η(T) for the orthorhombic distortion shows a power-law dependence on temperature with the exponent β=0.21 and hysteresis, suggesting that this transition is less than second order. Two-dimensional critical scattering is observed around the antiferromagnetic phase transition. The magnetic excitation spectra in the three-dimensional ordered state are characterized by small anisotropy gaps between ∼0 and 3 meV, and large effective in-plane spin-wave velocities, which decrease with increasing δ. For δ=0.077, c≊130 meV Å; and for δ=0.00, c≥300 meV Å.