Exchange interactions in the quasi-linear-chain antiferromagnetKFeS2

We report magnetic susceptibility measurements on ${\mathrm{KFeS}}_2$ in the temperature range $4.2<T<\mathrm{}300\mathrm{}$ K. In addition to a curvature of the susceptibility curve $\chi \mathrm{}\left(T\right)\mathrm{}$ 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 ($\eta ={10}^{-2\mathrm{}}$) 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 $\Theta \sim +70\mathrm{}$ 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 $\frac{J_1}{k_B}\sim 23.6$ K, $\frac{J_2}{k_B}=41.9\mathrm{}$ K, and $\frac{J^{\prime }}{k_B}=3.3\mathrm{}$ K, with $J^{\prime }$ being the interchain exchange constant. We suggest that the three-dimensional antiferromagnetism occurs at the Néel temperature $T_N\sim 12.5$ K rather than at 250 K as is usually assumed.