High-energy magnetic excitations in lightly oxygen-doped lanthanum nickel oxides

Abstract

We investigated magnetic excitations in lightly oxygen doped La2NiO4+δ (δ = 0.02 and 0.11) by inelastic neutron scattering over a wide energy range. It is known that two types of two-dimensional (2D) antiferromagnetic (AF) excitations coexist in these systems, i.e., well-defined excitations with effective exchange interactions suppressed by hole doping in the lower energy region (impurity mode) and higher energy excitations with a steep dispersion (host mode). However, details of these excitations, especially the host mode, has not been well studied so far due to the lack of information in the higher energy region. Our results reveal that host modes can be characterized by a simple 2D AF spin-wave model with the same exchange energy as that of the stoichiometric system in the δ = 0.02 phase. In the δ = 0.11 phase, although considerable broadening was observed, the upper band limit of the host modes was on the same level as that in the stoichiometric system, which implies that the magnitude of intrinsic exchange interaction is not affected by doping. The observed results can help understand the microscopic nature of doped holes in this system and other layered-transition metal oxides.