Magnetic excitations and exchange parameters of the nickel-chain compound PbMn2Ni6Te3 O18: Neutron scattering and density functional theory studies

We have investigated the quasi-one-dimensional Ni-chain compound PbMn2Ni6Te2O18 using theoretical DFT calculations, inelastic neutron scattering, and optical spectroscopy in order to understand the nature of magnetic exchange interactions. Our inelastic neutron scattering study at 5 K on a powder sample reveals two bands of magnetic excitations, the first near 8 meV and the second near 18 meV originating from the antiferromagnetic zone center near Q=1 Å. On the other hand, at 100 K (which is above TN=86 K) a broad diffuse scattering signal is observed indicating the presence of short range magnetic correlations. We have analyzed the magnetic excitations based on the Linear Spin Wave Theory (LSWT) and compared the experimentally estimated exchange parameters with the DFT calculations. Our analysis reveals that the value of the exchange parameter at the larger distance (d=3.654Å) J3=4.21(8) meV between Ni-Ni (from interchain) is the strongest amongst the allowed six exchange parameters, which suggests that this system is not really a quasi-one-dimensional and confirmed by the absence of a Haldane gap. We have also presented the electronic structure calculations. The spin-polarized partial density of states (DOS) projected onto the Mn-d and Ni-d orbitals reveals that the Ni-dx2-y2 contribution is dominant below the Fermi level in the spin-up and spin-down channel, while a minimal contribution from spin-up Mn states in the occupied region, suggesting a nearly high-spin state. The estimated Néel temperature, based on experimental exchange parameters is found to be in close agreement with the experimental value.