Cooperative ordering of holes and spins inLa2NiO4.125

Abstract

${\mathrm{La}}_2$ ${\mathrm{NiO}}_{4.125}$ exhibits a cooperative ordering of dopant-induced holes and Ni spins below a transition temperature of 110 K. There is also an ordering of interstitial oxygens that occurs near room temperature. We present a comprehensive analysis of experimentally observed superlattice intensities in terms of structural models, justifying previous identifications. The model for the interstitial order involves a 3a×5b×5c unit cell with an ideal interstitial density of δ=2/15 per formula unit. In the spin and charge ordered state, the magnetic moments are sinusoidally modulated within the ${\mathrm{NiO}}_2$ planes. The moments point transverse to the modulation direction, with nearest neighbors antiparallel, and have a maximum amplitude that is >80% of that observed in undoped ${\mathrm{La}}_2$ ${\mathrm{NiO}}_4$. A corresponding modulation of atomic positions within ${\mathrm{NiO}}_2$ planes involves breathing-mode distortions consistent with a modulation of the charge density. The results are compared with theoretical models and with experimental observations on related systems.