Inelastic neutron scattering study of stripe and overdoped checkerboard ordering in layered nickel oxide Nd_2-xSr_xNiO₄

We have investigated magnetic excitations in both the stripe (x < 0.5) and overdoped checkerboard (CB) (x ≥ 0.5) regions in Nd_2-xSr_xNiO₄ (x = 0.33, 0.60, and 0.70) samples by inelastic neutron scattering. The magnetic excitations of Nd_1.67Sr_0.33NiO₄ in the stripe region can be described by the linear spin wave theory with the same exchange constants as estimated for La_5/3Sr_1/3NiO₄. In strong contrast to the well-defined spin wave excitations in the stripe region, the CBordered x = 0.60 and 0.70 samples show nondispersive broad magnetic excitation spectra. The broad feature of the excitation spectra in the CB region appears to be independent of the hole concentration up to x = 0.7. We discuss the origin of the broad excitation spectra and the robustness of the CB order above x > 0.5.