Coexistence of Superconductivity and Magnetism in FeSe_1-x under Pressure

An extended investigation of the electronic phase diagram of FeSe${}_{1-x}$ up to pressures of $p\simeq 2.4$,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 $p\ge 0.8$,GPa static magnetic order occurs in FeSe${}_{1-x}$ and occupies the full sample volume for $p\gtrsim 1.2$,GPa. ac magnetization measurements reveal that the superconducting volume fraction stays close to 100% up to the highest pressure investigated. In addition, above $p\ge 1.2$,GPa both the superconducting transition temperature $T_{\mathrm{c}}$ and the magnetic ordering temperature $T_{\mathrm{N}}$ increase simultaneously, and both superconductivity and magnetism are stabilized with increasing pressure. Calculations indicate only one possible muon stopping site in FeSe${}_{1-x}$, 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 $\mu \approx 0.2{\mu }_B$ at $p\simeq 2.4$,GPa.