Pressure Induced Static Magnetic Order in Superconducting FeSe₁-x

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We report on a detailed investigation of the electronic phase diagram of FeSe₁-x under pressures up to 1.4GPa by means of AC magnetization and muon-spin rotation. At a pressure ≃0.8GPa the non-magnetic and superconducting FeSe₁-x enters a region where long range static magnetic order is realized above Tc and bulk superconductivity coexists and competes on short length scales with the magnetic order below Tc. For even higher pressures an enhancement of both the magnetic and the superconducting transition temperatures as well as of the corresponding order parameters is observed. These exceptional properties make FeSe1-x to be one of the most interesting superconducting systems investigated extensively at present.