Electron spin resonance in iron pnictides

We report on electron spin resonance studies in Eu based 122-superconductors where the Eu$^{2+}$ ions serve as a probe of the normal and superconducting state. In polycrystalline Eu$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ the spin-lattice relaxation rate $1/T_1^{\rm ESR}$ obtained from the ESR linewidth exhibits a Korringa-like linear increase with increasing temperature above $T_{\rm c}$ evidencing a normal Fermi-liquid behavior. Below $T_{\rm c}$ the spin lattice relaxation rate $1/T_1^{\rm ESR}$ follows a $T^{1.5}$-behavior without any appearance of a coherence peak. In superconducting EuFe$_2$As$_{1.8}$P$_{0.2}$ single crystals we find a similar Korringa slope in the normal state and observe anisotropic spectra for measuring with the external field parallel and perpendicular to the $c$-axis. In addition, we will discuss the ESR properties of selected systems from the 1111 and 11 families.