Mossbauer Study of Superconductors LaFeO_0.88F_0.12As

Abstract

This study continues our investigation of high-temperature superconductors of LaFeO(1-x)F (x) As type initiated in our previous paper (J. Supercond. Nov. Magnetism 27, 1825, [1]), where the high-temperature Kondo effect had been observed in the sample with x = 0.15. We proceed with exploring the properties of the LaFeO (0.88) F As-0.12 sample, applying the X-ray diffraction (XRD) method, resistivity measurement, and the magnetic and Mossbauer measurements at the temperature range of 4.2-300 K. We observe the emergence of high-temperature Kondo effect at the temperatures between 30 and 50 K. At the same time, as opposed to the case x=0.15, we reveal the transformation of the high-temperature Kondo effect to the high-temperature superconductivity at the temperature near 25 K. The Mossbauer spectra of the sample obtained at different temperatures represent a single line, lying near zero relative velocity and do not exhibit any visible alterations. Concurrently, the application of an external magnetic field of 5 T to the sample at 5 K induces the emergence of a hyperfine magnetic splitting with the effective magnetic field on resonant nuclei H (ef) = 45.5 kOe. We suppose that the appearance of a magnetic ordering in the sample occurs due to the alignment of magnetic moments of impurities that confirms our hypothesis about a strong coupling of electron's spins with the magnetic moments of impurities in iron-containing superconductors.