Spin state and exchange in the quasi-one-dimensional antiferromagnet KFeS2

Abstract

We report the optical spectra and single crystal magnetic susceptibility of the one-dimensional antiferromagnet KFeS2. Measurements have been carried out to ascertain the spin state of Fe3+ and the nature of the magnetic interactions in this compound. The optical spectra and magnetic susceptibility could be consistently interpreted using a S = 1/2 spin ground state for the Fe3+ ion. The features in the optical spectra have been assigned to transitions within the d-electron manifold of the Fe3+ ion, and analysed in the strong field limit of the ligand field theory. The high temperature isotropic magnetic susceptibility is typical of a low-dimensional system and exhibits a broad maximum at ∼565 K. The susceptibility shows a well defined transition to a three dimensionally ordered antiferromagnetic state at TN = 250 K. The intra and interchain exchange constants, J and J′, have been evaluated from the experimental susceptibilities using the relationship between these quantities, and χmax, Tmax, and TN for a spin 1/2 one-dimensional chain. The values are J = -440·71 K, and J′ = 53·94 K. Using these values of J and J′, the susceptibility of a spin 1/2 Heisenberg chain was calculated. A non-interacting spin wave model was used below TN. The susceptibility in the paramagnetic region was calculated from the theoretical curves for an infinite S = 1/2 chain. The calculated susceptibility compares well with the experimental data of KFeS2. Further support for a one-dimensional spin 1/2 model comes from the fact that the calculated perpendicular susceptibility at 0K (2·75 × 10-4emu/mol) evaluated considering the zero point reduction in magnetization from spin wave theory is close to the projected value (2·7 × 10-4emu/mol) obtained from the experimental data.