We report on the synthesis of large single crystals of a new FeSe layer superconductor Cs(0.8)(FeSe(0.98))(2). X-ray powder diffraction, neutron powder diffraction and magnetization measurements have been used to compare the crystal structure and the magnetic properties of Cs(0.8)(FeSe(0.98))(2) with those of the recently discovered potassium intercalated system K(x)Fe(2)Se(2). The new compound, Cs(0.8)(FeSe(0.98))(2), shows a slightly lower superconducting transition temperature (T(c) = 27.4 K) in comparison to 29.5 in (K(0.8)(FeSe(0.98))(2)). The volume of the crystal unit cell increases by replacing K by Cs-the c parameter grows from 14.1353(13) to 15.2846(11) Å. For the alkali metal intercalated layered compounds known so far, (K(0.8)Fe(2)Se(2) and Cs(0.8)(FeSe(0.98))(2)), the T(c) dependence on the anion height (distance between Fe layers and Se layers) was found to be analogous to those reported for As-containing Fe superconductors and Fe(Se(1 - x)Ch(x)), where Ch = Te, S.

We present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy and muon-spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of $0.25(5){\mu }_B$ at the iron site below $T_N=138\mathrm{K}$, well separated from a structural phase transition at $T_S=156\mathrm{K}$. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below $T_N$ are reproduced.

We report on a detailed investigation of the electronic phase diagram of ${\mathrm{FeSe}}_{1-x}$ under pressures up to 1.4 GPa by means of ac magnetization and muon-spin rotation. At a pressure $\simeq 0.8\mathrm{GPa}$ the nonmagnetic and superconducting ${\mathrm{FeSe}}_{1-x}$ enters a region where static magnetic order is realized above $T_c$ and bulk superconductivity coexists and competes on short length scales with the magnetic order below $T_c$. 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 ${\mathrm{FeSe}}_{1-x}$ to be one of the most interesting superconducting systems investigated extensively at present.