57Fe Mossbauer study of magnetic ordering in superconducting K0.80Fe1.76Se2.00 single crystals

The magnetic ordering of superconducting single crystals of K0.80Fe1.76Se2.00 has been studied between 10 and 550 Kusing Fe-57 Mossbauer spectroscopy. Despite being superconducting below T-sc similar to 30 K, the iron sublattice in K0.80Fe1.76Se2.00 clearly exhibits magnetic order from well below T-sc to its Neel temperature of T-N = 532 +/- 2 K. The iron moments are ordered almost parallel to the crystal c axis. The order collapses rapidly above 500 K and the accompanying growth of a paramagnetic component suggests that the magnetic transition may be first order, which may explain the unusual temperature dependence reported in recent neutron diffraction studies.