Spin dynamics of the spin chain antiferromagnet RbFeS$_2$

We report transport and inelastic neutron scattering studies on electronic properties and spin dynamics of the quasi-one-dimensional spin chain antiferromagnet RbFeS$_2$. An antiferromagnetic phase transition at $T_N\approx195$ K and dispersive spin waves with a spin gap of 5 meV are observed. By modeling the spin excitation spectra using linear spin wave theory, intra and inter-chain exchange interactions are found to be $SJ_1=100(5)$ meV and $SJ_3=0.9(3)$ meV, respectively, together with a small single-ion anisotropy of $SD_{zz}=0.04(1)$ meV. Comparison with previous results for other materials in the same class of Fe$^{3+}$ spin chain systems reveals that although the magnetic order sizes show significant variation from 1.8 to 3.0$\mu_B$ within the family of materials, the exchange interactions $SJ$ are nevertheless quite similar, analogous to the iron pnictide superconductors where both localized and delocalized electrons contribute to the spin dynamics.