Neutron diffraction study of stripe order in La(2)NiO(4+d) with d=2/15

TL;DR

This neutron diffraction study reveals detailed charge and spin stripe ordering in oxygen-doped La2NiO4+d, showing temperature-dependent lock-in transitions and a shift from O-centered to Ni-centered charge stripes.

Contribution

The paper provides the first detailed neutron scattering analysis of stripe order in La2NiO4+d with d=2/15, including a model explaining lock-in transitions and charge stripe centering.

Findings

Charge order persists up to twice the magnetic transition temperature.

Wave vector  shows a discontinuous jump at T_m and lock-in transitions.

Charge stripes are initially O-centered and shift towards Ni-centering at lower temperatures.

Abstract

We report a detailed neutron scattering study of the ordering of spins and holes in oxygen-doped 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 d = 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 NiO(2) 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 interactions responsible for the the temperature dependence of \epsilon and the localization of charge within the stripes are discussed.