Pressure-induced tetragonal magnetic order in FeSe*

We present a microscopic study of magnetism and lattice distortion on FeSe single crystals by high-energy x-ray diffraction and time-domain Mössbauer spectroscopy over wide temperature and pressure ranges. The magneto-structural ground state can be tuned by pressure from a paramagnetic state with an orthorhombic lattice distortion through a strongly coupled magnetically ordered and orthorhombic state to a magnetically ordered state without an orthorhombic lattice distortion. Close to the reported maximum of the superconducting critical temperature, the orthorhombic distortion suddenly disappears and FeSe remains tetragonal down to the lowest temperature measured. Analysis of the structural and magnetic order parameters suggests an independent origin of the structural and magnetic ordering phenomena, and their cooperative coupling leads to the similarity with the canonical phase diagram of iron pnictides.