Spin excitations in K${}_{0.84}$Fe${}_{1.99}$Se${}_2$ superconductor as studied by M\"ossbauer spectroscopy

M"ossbauer spectroscopy was used to probe the site specific information of the $K_{0.84}F{e}_{1.99}S{e}_2$ superconductor. Possibility of coexistence of superconductivity and magnetism is discussed. A spin excitation gap, $\Delta E \approx$5,meV, is observed by analyzing the temperature dependence of the hyperfine magnetic field (HMF) at the iron site within the spin wave theory. Using a simple model suggested in the literature, the temperature dependence of the HMF is well reproduced, suggesting that, below room temperature, the iron-selenide superconductors can be regarded as ferromagnetically coupled spin blocks that interact with each other antiferromagnetically to form the observed checkerboard-like magnetic structure.