Coexistence of superconductivity and magnetism in FeSe1-x under pressure

An extended investigation of the electronic phase diagram of FeSe1-x up to pressures of p similar or equal to 2.4 GPa by means of ac and dc magnetization, zero-field muon spin rotation (ZF mu SR), and neutron diffraction is presented. ZF mu SR indicates that at pressures p >= 0.8 GPa static magnetic order occurs in FeSe1-x and occupies the full sample volume for p greater than or similar to 1.2 GPa. ac magnetization measurements reveal that the superconducting volume fraction stays close to 100% up to the highest pressure investigated. In addition, above p >= 1.2 GPa both the superconducting transition temperature T-c and the magnetic ordering temperature T-N increase simultaneously, and both superconductivity and magnetism are stabilized with increasing pressure. Calculations indicate only one possible muon stopping site in FeSe1-x, located on the line connecting the Se atoms along the c direction. Different magnetic structures are proposed and checked by combining the muon stopping calculations with a symmetry analysis, leading to a similar structure as in the LaFeAsO family of Fe-based superconductors. Furthermore, it is shown that the magnetic moment is pressure dependent and with a rather small value of mu approximate to 0.2 mu(B) at p similar or equal to 2.4 GPa.