Exchange interactions in the quasi-linear-chain antiferromagnetKFeS2

Abstract

We report magnetic susceptibility measurements on ${\mathrm{KFeS}}_{2}$ in the temperature range $4.2<T<300$ K. In addition to a curvature of the susceptibility curve $\ensuremath{\chi}(T)$ near 250 K, we also observe a small sharp maximum at 12.5 K characteristic of linear-chain antiferromagnetism in quasi-one-dimensional systems. The results were interpreted with use of the molecular-field approximation and the temperature-dependent Green's-function method as well as the refined molecular-field theory for the Heisenberg model. The interchain, and intrachain exchange interactions and their ratio ($\ensuremath{\eta}={10}^{\ensuremath{-}2}$) which measures the one dimensionality of the system could be estimated and compared with other results. We attribute the positive value of the paramagnetic Curie temperature $\ensuremath{\Theta}\ensuremath{\sim}+70$ K to a ferromagnetic exchange interaction ${J}_{2}$ between next-nearest neighbors along the chains, superimposed on the antiferromagnetic exchange ${J}_{1}$ between nearest neighbors. The best fit to the experimental data is obtained for $\frac{{J}_{1}}{{k}_{B}}\ensuremath{\sim}23.6$ K, $\frac{{J}_{2}}{{k}_{B}}=41.9$ K, and $\frac{{J}^{\ensuremath{'}}}{{k}_{B}}=3.3$ K, with ${J}^{\ensuremath{'}}$ being the interchain exchange constant. We suggest that the three-dimensional antiferromagnetism occurs at the N\'eel temperature ${T}_{N}\ensuremath{\sim}12.5$ K rather than at 250 K as is usually assumed.