  M\"ossbauer spectroscopy was used to probe the site specific information of
the $K_{0.84}Fe_{1.99}Se_2$ 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.
