Regensburg 2010 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 39: SC: Iron-Based Superconductors - 1111

TT 39.4: Talk

Friday, March 26, 2010, 11:00–11:15, H19

High field ESR spectroscopy on (Gd,La)OFeAs superconductors — •A. Alfonsov¹, F. Murányi², V. Kataev¹, N. Leps¹, R. Klingeler¹, A. Kondrat¹, C. Hess¹, S. Wurmehl¹, J. Werner¹, G. Behr¹, and B. Büchner¹ — ¹IFW Dresden, D-01171 Dresden, Germany — ²Universität Zürich, CH-8057 Zürich, Schweiz

In the present work we have studied polycrystalline samples of the (Gd,La)O_1−xF_xFeAs superconductor with different levels of fluorine and gadolinium doping by means of high field/high frequency electron spin resonance (HF-ESR) spectroscopy. The Gd ESR signal is found to be sensitive to the magnetic phase transition from the paramagnetic to the spin density wave (SDW) state occurring in the parent (Gd,La)OFeAs compounds at temperatures T_SDW∼ 130−150K. In addition, in case of LaO_1−xF_xFeAs with 5% Gd doping there is a clear indication of the suppression of the magnetic order in the samples with the superconducting ground state. The GdOFeAs samples reveal an antiferromagnetic coupling between Gd and Fe planes which reflects in the splitting and shift of Gd ESR line below T_SDW∼ 130K. Surprisingly, the 15% and 17% fluorine doped GdOFeAs samples, with T_c=20K and T_c=45K respectively, show the low-T ESR response similar to the undoped sample which indicates that the SDW is not completely suppressed in the SC samples. We compare HF-ESR data with results of transport measurements on these samples and discuss a possible contribution of magnetic rare-earths to the interplay between magnetism of the FeAs planes and superconductivity which evolves upon the fluorine doping.