Coexistence of Magnetism and Superconductivity in the Iron-Based Compound Cs0.8(FeSe0.98)2

We report on muon-spin rotation and relaxation (mu SR), electrical resistivity, magnetization and differential scanning calorimetry measurements performed on a high-quality single crystal of Cs-0.8(FeSe0.98)(2). Whereas our transport and magnetization data confirm the bulk character of the superconducting state below T-c = 29.6(2) K, the mu SR data indicate that the system is magnetic below T-N = 478.5(3) K, where a first-order transition occurs. The first-order character of the magnetic transition is confirmed by differential scanning calorimetry data. Taken all together, these data indicate in Cs-0.8(FeSe0.98)(2) a microscopic coexistence between the superconducting phase and a strong magnetic phase. The observed T-N is the highest reported to date for a magnetic superconductor.