Neutron-diffraction study of stripe order in La2NiO4+δ with δ=2/15

We report a detailed neutron-scattering study of the ordering of spins and holes in oxygen-doped La2NiO4.133. The single-crystal sample exhibits the same oxygen-interstitial order but better defined charge-stripe order than that studied previously in crystals with delta = 0.125, In particular, charge order is observed up to a temperature at least twice that of the magnetic transition, T-m = 110.5 K. On cooling through T-m, the wave vector epsilon, equal to half the charge-stripe density within an NiO2 layer, jumps discontinuously from 1/3 to 0.2944. It continues to decrease with further cooling, showing several lock-in transitions on the way down to low temperature. To explain the observed lock-ins, a model is proposed in which each charge stripe is centered on either a row of Ni or a row of O ions. The model is shown to be consistent with the l dependence of the magnetic peak intensities and with the relative intensities of the higher-order magnetic satellites, Analysis of the latter also provides evidence that the magnetic domain walls (charge stripes are relatively narrow, In combination with a recent study of magnetic-field-induced effects, we find that the charge stripes are all O centered at T> T-m, with a shift towards Ni centering at T< T-m. Inferences concerning the competing intel actions responsible for the temperature dependence of epsilon and the localization of charge within the stripes are discussed.