Spin excitations in a K_0.84Fe_1.99Se₂ superconductor as studied by Mössbauer spectroscopy

Mössbauer spectroscopy was used to probe the site-specific information of a K_0.84Fe_1.99Se₂ superconductor. A spin excitation gap, ΔE ≈5.5 meV, is observed by analyzing the temperature dependence of the hyperfine magnetic field (HMF) at the iron site within the spin wave theory. Using the simple model suggested in the literature, the temperature dependence of the HMF is well reproduced, suggesting that, below room temperature, the alkali metal intercalated 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.