Electron spin resonance in Eu-based iron pnictides

The phase diagrams of EuFe2-xCoxAs2 (0 <= x <= 0.4) and EuFe2As2-yPy (0 <= y <= 0.43) are investigated by Eu2+ electron spin resonance (ESR) in single crystals. From the temperature dependence of the linewidth Delta H(T) of the exchange narrowed ESR line, the spin-density wave (SDW) (T < T-SDW) and the normal metallic regime (T > T-SDW) are clearly distinguished. AtT > T-SDW the isotropic linear increase of the linewidth is driven by the Korringa relaxation which measures the conduction-electron density of states at the Fermi level. For T < T-SDW the anisotropy probes the local ligand field, while the coupling to the conduction electrons is strongly weakened. With increasing substitution of x or y the transition temperature T-SDW decreases linearly accompanied by a linear decrease of the Korringa-relaxation rate from 8 Oe/K at x = y = 0 down to 3 Oe/K at the onset of superconductivity. Forx > 0.2 andy > 0.3 it remains nearly constant. Comparative ESR measurements on single crystals of the Eu diluted SDWcompound Eu0.2Sr0.8Fe2As2 and superconducting (SC) Eu0.22Sr0.78Fe1.72Co0.28As2 corroborate the leading influence of the ligand field on the Eu2+ spin relaxation in the SDW regime as well as the Korringa relaxation in the normal metallic regime. A coherence peak is not detected in the latter compound below T-c = 21 K, which is in agreement with the expected complex anisotropic SC gap structure. In contrast, indications for phase coexistence and BCS-type superconductivity are found in EuFe2As1.57P0.43.