Magnetism driven by anion vacancies in superconducting α-FeSe1-x

To study the microscopic electronic and magnetic interactions in the substoichiometric iron chalcogenide FeSe1-x which is observed to superconduct at x approximate to 1/8 up to T-c = 27 K, we use first principles methods to study the Se vacancy in this nearly magnetic FeSe system. The vacancy forms a ferrimagnetic cluster of eight Fe atoms, which for the ordered x = 1/8 alloy leads to half-metallic conduction. Similar magnetic clusters are obtained for FeTe1-x and for BaFe2As2 with an As vacancy although neither of these are half-metallic. Based on fixed spin-density results, we suggest the low-energy excitations in FeSe1-x are antiparamagnonlike with short correlation length.