Muons and magnets

Abstract:

The discovery of high-temperature superconductivity in a layered iron arsenide has led to an intensive search to optimize the superconducting properties of iron-based superconductors by changing the chemical composition of the spacer layer between adjacent anionic iron arsenide layers. Superconductivity has been found in iron arsenides with cationic spacer layers consisting of metal ions (for example, Li(+), Na(+), K(+), Ba(2+)) or PbO- or perovskite-type oxide layers, and also in Fe(1.01)Se (ref. 8) with neutral layers similar in structure to those found in the iron arsenides and no spacer layer. Here we demonstrate the synthesis of Li(x)(NH(2))(y)(NH(3))(1-y)Fe(2)Se(2) (x~0.6; y~0.2), with lithium ions, lithium amide and ammonia acting as the spacer layer between FeSe layers, which exhibits superconductivity at 43(1) K, higher than in any FeSe-derived compound reported so far. We have determined the crystal structure using neutron powder diffraction and used magnetometry and muon-spin rotation data to determine the superconducting properties. This new synthetic route opens up the possibility of further exploitation of related molecular intercalations in this and other systems to greatly optimize the superconducting properties in this family.

Abstract:

The series SrFe 1-xCo xO 3 evolves from spiral antiferromagnetic order to long-range ferromagnetism on increasing Co doping. In the Fe-rich region below the onset of ferromagnetism (x ≤ 0.20), there exists a number of disordered magnetic ground states. Here we present a detailed study of the composition SrFe 0.90Co 0.10O 3, which we find to display quasicanonical spin-glass behavior. The analysis of its freezing transition by means of muon spin relaxation (μSR) spectroscopy and ac and dc susceptibility suggests that the system constitutes a new experimental realization of the three-dimensional Heisenberg spin-glass model with weak random anisotropy. The derived critical exponents are consistent with predictions by Kawamura's chiral driven freezing scenario with a bimodal exchange distribution. © 2012 American Physical Society.