A Novel Large Moment Antiferromagnetic Order in K_0.8Fe_1.6Se₂ Superconductor

The discovery of cuprate high T_C superconductors has inspired the search for unconventional superconductors in magnetic materials. A successful recipe has been to suppress long−range order in a magnetic parent compound by doping or high pressure to drive the material towards a quantum critical point. We report an exception to this rule in the recently discovered potassium iron selenide. The superconducting composition is identified as the iron vacancy ordered K_0.83(2)Fe_1.64(1)Se₂ with T_C above 30 K. A novel large moment 3.31 μ_B/Fe antiferromagnetic order that conforms to the tetragonal crystal symmetry has an unprecedentedly high ordering temperature T_N≈559 K for a bulk superconductor. Staggeringly polarized electronic density of states is thus suspected, which would stimulate further investigation into superconductivity in a strong spin-exchange field under new circumstances.