Neutron-diffraction study of stripe order in ${\mathbf{La}}_2{\mathrm{NiO}}_{4+\delta }$ with $\delta =$$\frac{2}{15}$

We report a detailed neutron-scattering study of the ordering of spins and holes in oxygen-doped ${\mathrm{La}}_2$NiO${}_{4.133}.$ The single-crystal sample exhibits the same oxygen-interstitial order but better defined charge-stripe order than that studied previously in crystals with $\delta =0.125\mathrm{}$. In particular, charge order is observed up to a temperature at least twice that of the magnetic transition, $T_m=110.5\mathrm{}$ K. On cooling through $T_m,$ the wave vector $\epsilon$, equal to half the charge-stripe density within an NiO${}_2$ layer, jumps discontinuously from $\frac{1}{3}$ 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 $T>\mathrm{}{T}_m,$ with a shift towards Ni centering at $T<\mathrm{}{T}_m.$ Inferences concerning the competing interactions responsible for the temperature dependence of $\epsilon$ and the localization of charge within the stripes are discussed.