ESR spectroscopy on (Gd,La)O1-xFxFeAs superconductors

We present results on electron spin resonance (ESR) spectroscopy of polycrystalline samples of the (Gd,La)O1-xFxFeAs superconductor with different levels of fluorine and gadolinium doping. The ESR signal of a small amount of Gd spins doped to the parent compound LaOFeAs is sensitive to the structural and in particular to the magnetic phase transition occurring in this material at temperatures ∝130-150 K. Fluorine doping suppresses both transitions and leads to superconductivity. Correspondingly, the Gd ESR response shows no signatures of the magnetic order in the FeAs planes of samples with a superconducting ground state. In the concentrated compound GdOFeAs the Gd ESR response is sensitive to the magnetism of the FeAs planes, too. Doping of this material with ∝ 15 % of fluorine yields superconductivity with Tc∼21 K. Surprisingly, Gd ESR gives clear indications of the enhancement of (quasi)-static magnetic correlations in the superconducting samples which set in below ∝ 80 K and continue to develop even in the superconducting state. We compare ESR data with results of thermodynamic and transport measurements on these samples and discuss a possible role of magnetic rare-earths for the magnetism of the FeAs-planes in which the superconductivity evolves upon the fluorine doping.