Microstructural magnetic phases in superconducting FeTe_0.65Se_0.35

Abstract

In this paper, we address a number of outstanding issues concerning the\nnature and the role of magnetic inhomogenities in the iron chalcogenide system\nFeTe1-xSex and their correlation with superconductivity in this system. We\nreport morphology of superconducting single crystals of FeTe0.65Se0.35 studied\nwith transmission electron microscopy, high angle annular dark field scanning\ntransmission electron microscopy and their magnetic and superconducting\nproperties characterized with magnetization, specific heat and magnetic\nresonance spectroscopy. Our data demonstrate a presence of nanometre scale\nhexagonal regions coexisting with tetragonal host lattice, a chemical disorder\ndemonstrating non homogeneous distribution of host atoms in the crystal\nlattice, as well as hundreds-of-nanometres-long iron-deficient bands. From\nmagnetic data and ferromagnetic resonance temperature dependence, we attribute\nmagnetic phases in Fe-Te-Se to Fe3O4 inclusions and to hexagonal symmetry\nnanometre scale regions with structure of Fe7Se8 type. Our results suggest that\nnonhomogeneous distribution of host atoms might be an intrinsic feature of\nsuperconducting Fe-Te-Se chalcogenides and we find a surprising correlation\nindicating that faster grown crystal of inferior crystallographic properties is\na better superconductor.\n